In the magical underworld of Victorian London, Tessa Gray has at last found safety with the Shadowhunters. But that safety proves fleeting when rogue forces in the Clave plot to see her protector, Charlotte, replaced as head of the Institute. If Charlotte loses her position, Tessa will be out on the street, and easy prey for the mysterious Magister, who wants to use Tessa's powers for his own dark ends.
With the help of the handsome, self-destructive Will and the fiercely devoted Jem, Tessa discovers that the Magister's war on the Shadowhunters is deeply personal. He blames them for a long-ago tragedy that shattered his life.
To unravel the secrets of the past, the trio journeys from mist-shrouded Yorkshire to a manor house that holds untold horrors, from the slums of London to an enchanted ballroom where Tessa discovers that the truth of her parentage is more sinister than she had imagined. When they encounter a clockwork demon bearing a warning for Will, they realize that the Magister himself knows their every move, and that one of their own has betrayed them.
Tessa finds her heart drawn more and more to Jem, but her longing for Will, despite his dark moods, continues to unsettle her. But something is changing in Will; the wall he has built around himself is crumbling. Could finding the Magister free Will from his secrets, and give Tessa the answers about who she is and what she was born to do?
As their dangerous search for the Magister and the truth leads the friends into peril, Tessa learns that when love and lies are mixed, they can corrupt even the purest heart.
Will Herondale, who is on an errand for Magnus Bane, pays the ghost Old Molly at the Cross Bones Graveyard a visit to pick up Foraii demonic powder. Before Will leaves, he asks her if there is a potion that could put an end to being in love, which Molly misinterprets as him using it to end someone else's love for him and scoffs at the request.
Later, Tessa and Jem are making their way to the Council Chamber for a Council meeting, which is held to discuss the events of the night Mortmain infiltrated the Institute. Jem stops for sometime and shows her Poet's Corner in Westminster Abbey. Will catches up with the two there, and they all make their way to the Chamber together. There, Tessa and Charlotte are asked once again to testify about the events of the that night, which they do. The Consul listens and then prepares to give Charlotte her sentence. But, before he can finish, Benedict Lightwood stands up and challenges Charlotte's command of the London Institute, citing that Charlotte isn't fit to be head of an Institute as a woman who is so easily misled by their enemies such as Mortmain and Nate Gray. The Consul, as a family friend of the Fairchilds, as well as being the person who appointed Charlotte as head of the Institute himself, then proposes a bargain: Charlotte will be given two weeks to find the Magister, or at least, have strong evidence to indicate where he is. If she fails, then Benedict's challenge may go forward and the Institute shall fall under his leadership. Also, due to the London Institute's loss of servants during Mortmain's invasion, Gabriel and Gideon, Benedict's two sons, are instructed to train Tessa, Sophie and Jessamine so as to prepare them to defend themselves and others in the case that they're put in this position again.
The Institute inhabitants immediately get to work by looking for any information on Mortmain. While speculating the reasons for Mortmain's attacks, Jem has an idea to look in the Reparation archives to find out if Mortmain's plans against the Nephilim are personal to as a result of him having tried to go through the system, only for it to fail him. When Jem leaves to pass the idea on to Charlotte, Tessa accompanies him, not wanting to be left alone with Will. She and Will then have a hostile conversation, before Will leaves.
For some unknown reason, Will has been searching for a specific demon for which he hires the warlock, Magnus Bane. Will ends up at Magnus' apartment—Camille Belcourt's residence—where Magnus summons another demon, Thammuz, in the hopes that it will be the demon Will has been looking for. Will confirms it is the wrong one, and Magnus sends it away.
Sophie walks toward Jessamine's room, but when she hears Tessa and Jem coming, she hides, wanting to avoid seeing them together. She felt slightly jealous of the way Jem looked at Tessa but brushes it off. Once the pair has passed, Sophie is about to leave when she notices Jessamine, dressed as a man and exiting her room while acting suspiciously.
The next day, Jem, accompanying Tessa and Sophie, both dressed in Shadowhunter gear, meet the Lightwood brothers, where they learn that Jessamine has claimed to have a headache and thus will not be participating in training. Gideon and Gabriel Lightwood introduce Jem, Tessa and Sophie to their new servants, Cyril Tanner—their late former servant Thomas' brother—and Bridget Daly.
Gabriel, in an attempt to goad Jem, Tessa and Sophie, asks Cyril to fight with him to show the girls what a trained servant is able to do with the right training. This upsets Sophie, as Thomas was her dear friend. Gabriel easily beats Cyril, but is surprised when pinned by Bridget. Jem leaves, and the girls spend the day training with Gabriel and Gideon, with Tessa being tutored by the former, and Sophie being tutored by the latter.
At dinner, Charlotte is not present, having gone to the Silent City to check on the Reparation log book. The group makes small talk, until Charlotte dashes in holding a parchment, claiming to have found something about Axel Mortmain. It is revealed that Axel was adopted by two warlocks, John Thaddeus Shade and Anne Evelyn Shade. However, they were both killed by Shadowhunters for possessing the Book of the White, as well as dabbling in dark magic by using the spells within the Book to work on binding souls to automatons. Sometime in 1825 Mortmain filed a Reparation at the York Institute, headed by Aloysius Starkweather. However, it had apparently been rejected and the only way to find out was by talking to the Head of the York Institute himself. Charlotte is not thrilled with the idea of contacting Aloysius, because he and her father had a falling out years ago and the latter still holds a grudge. However, she does end up writing to him to request information regarding Mortmain.
After dinner, whilst Charlotte writes the letter to Aloysius, Tessa goes to the library in search of books to read. While searching, Will approaches her, and they have a conversation about books. He discusses how in A Tale of Two Cities, Sydney Carton's sacrifice is one of despair, because he is lost without his love, Lucie Manette, but knows that if she were with him it would only degrade her; clearly suggesting that he himself is Sydney, while Tessa is Lucie.
The next day, as expected by Charlotte, Aloysius Starkweather has denied her access to the information she wants, so the residents of the Institute ultimately decide, with Tessa's approval, that Tessa, Will, and Jem will travel to the York Institute to retrieve something that belongs to Aloysius so she can Change into him and sort through his memories of what happened with Mortmain and the Shades. In the meantime, Tessa and Sophie continue to be trained by the Lightwood brothers, and Sophie confesses to Tessa that she has an issue with Bridget, both because she sings depressing songs of death and beheading, and also because she's not as amiable as the former cook, Agatha, who was Sophie's dear friend. Later that evening, Tessa receives a gift at her door, a book - Vathek by William Beckford. On the front cover, Will has written a funny poem, which makes her laugh.
The next morning, Tessa, Jem and Will depart to York from Kings Cross station. Whilst on the train, they discuss what to tell Aloysius regarding the reason for their visit. At the end, they decide that, since Tessa has no Marks and will be unable to claim to be a Shadowhunter, she will instead pretend to be Jem's mundane fiancée who is about to Ascend and wants to see more of the different Institutes and the life of a Shadowhunter. Upon arriving in York, Aloysius's driver meets the three, and they make their way to the Institute.
At the York Institute, Aloysius looks startles to see Tessa and feels as if he recognizes her, but later turns into his own stony self. Jem, Tessa and Will have an awful time, as the Institute is nearly empty, Aloysius is grumpy toward them, and the food is terrible. However, Aloysius surprisingly decides to give the three the information that they wanted, including the address of an old house that was previously inhabited by Mortmain. Aloysius had been a part of the group that captured and killed the Shades. As this incident had taken place before the Accords had been formed, Mortmain's plea for compensation had been rejected. Aloysius then goes on to show them the spoils room—a room full of mummified bodies, skulls, faerie wings, eyeballs and jars of blood—all of which were harvested from the bodies of the Downworlders that the Starkweathers have killed. Tessa is so overwhelmed with disgust at how some Shadowhunters treated her fellow Downworlders that she faints and is taken straight to bed.
That night Tessa has a nightmare and cries out, so Will runs in to comfort her. Unfortunately, Tessa has not forgiven Will for the cruel words he uttered whilst on the roof on the day of Agatha and Thomas' funeral, and asks Will to leave, when he tries to kiss her, not wanting to keep repeating the cycle of kindness and cruelty. The next morning Jem, Tessa, and Will all three decide to go check out the house previously occupied by Mortmain, known as Ravenscar Manor. When they arrive at the house, they notice a girl walking into the building. Will is astonished to realize that the girl is his young sister, Cecily, whom Tessa had heard Will cry out for when he was ill.
Will, seeing Cecily, makes a mad dash to the house in an attempt to see her and find out if his entire family is there, but Jem stops him and points out an automaton that has been following them for their entire trip. Will then rolls down a hill and runs toward the automaton, presumably to finish it off. Jem follows Will, and Tessa runs after Jem, but since she's wearing a long dress and impractical shoes, she ends up losing sight of the boys and is suddenly accosted by the automaton.
Tessa quickly picks up a tree branch and uses the parrying lessons she learned from Gabriel to defend herself, and Will comes in just in time to cut the automaton down. The automaton gives them a warning from the Magister to leave his past buried, or the Herondales will pay the price. Will, Tessa, and Jem then return to catch the train back to London. During their journey home, Tessa confesses to Jem and Will that she retrieved one of Aloysius' buttons while they stayed at the York Institute so that she may Change into him, should it be necessary to get more information from him about the deaths of Mortmain's parents. Jem blurts out how he believed that the Carstairs family ring which he had given to her, as a part of their charade, looked good on her, Tessa feels some sort of attachment with him while Will turns visibly colder.
When the train arrives at Kings Cross station, Will dashes out of the train, without sending a word to anyone, and runs to Lady Belcourt's home—who is still on holiday—to see Magnus again. Once there, Will requests that Magnus open a portal to the demon realms so that he can find and kill the demon who cursed him. When Magnus refuses to do so, Will finally tells him the story about why he's searching for a demon.
As a child, Will is looking through his father's old things, when he finds a pyxis and opens it. A demon emerges from the box and strikes at him, when his older sister, Ella, bursts in with a seraph blade in an attempt to fight the demon off. Before he leaves, the demon curses Will, promising, "All who love you will die. Their love will be their destruction. It may take moments, it may take years, but any who look upon you with love will die of it, unless you remove yourself from them forever. And I shall begin it with her." By the next morning, Ella had died. Will, horrified and filled with grief, and sure that his curse caused his sister's death, fled his home and joined the Shadowhunters to prevent his family from the same fate that Ella had suffered.
He then explains to Magnus that since his arrival at the Institute of London, he's been pushing people away because of this curse; however, he's in love with Tessa now, and is tired of living a lie, pretending to be a man who drinks and involves himself with whores to keep people from loving him, even though all he does during the night is wander around in the streets to convince them of his folly. Magnus then asks why he allows Jem's affections, to which Will replies that Jem is dying anyway, and since Ella's death was swift, perhaps Jem will get a good death from his affiliation with Will. He calls Jem his "only sin". Will subsequently requests that Magnus open a portal to the demon world and send Will there, to which Magnus again refuses, causing Will to leave the house angrily.
Back at the Institute, Charlotte and the residents are all worried as to where Will has gone. Charlotte talks about the time when Will's parents came to beg for Will to return; after his parents finally left, Will asked Charlotte for one thing—to be notified if they should die. Since then, Charlotte has had Ragnor Fell keep tabs on the Herondales. She informs Jem and Tessa that Will's father had lost their house gambling for years after Will joined the Shadowhunters, and after some time Ragnor could no longer find any trace of the Herondales. Charlotte then decides that the best solution regarding Mortmain is for Tessa to Change into Aloysius and see if there's anything more they can learn about him. However, when Tessa does attempt the transformation into Aloysius, she realizes that he's no longer mentally competent, and has incredible difficulty catching any of his memories, only getting flashes of bits and pieces of his thoughts. When she Changes back to herself, she notices the face of Mrs. Dark in the courtyard, but when she checks again, the figure is gone.
The next morning, Tessa is at breakfast, where Will is still absent. During her training with the Lightwood brothers, Gabriel and Tessa have an argument, where Gabriel is impatient about something and snaps at her. She in turn asks him why he hates Will so much, which causes Gabriel to storm out. Tessa also notices Gideon paying a lot of attention, perhaps more than appropriate, towards Sophie. She also records that Sophie is quite comfortable around him as well. When Tessa goes to lunch, she sees Ragnor Fell at the table. Charlotte, Tessa and Ragnor discuss their current problems with regards to Mortmain and the Herondales. Ragnor tells Tessa of Edmund Herondale's past and Charlotte pays Ragnor to go to York and keep an eye on the family for her while they stay at Ravenscar manor.
When Tessa returns to her room, she is informed by Sophie that a letter has arrived for her. Tessa reads the letter from Magnus Bane, informing her that Will is very troubled, and that she should make certain that he is okay. Worried, Tessa rushes over to Jem's room, and shows him the letter, not knowing what to make of it. Jem and Tessa run to Will's room to investigate, where Tessa discovers that Will had taken the letters that she had written to her brother during her horrid stay at the Dark House. Jem searches through Will's room and finds a family dagger of Will's to use to track him, as well as a receipt for the yin fen that Will buys for Jem so he doesn't have to do it himself; Jem is also shocked and confused to learn that Will has been paying much more for the drug than Jem was led to believe it cost, and Will was in fact charging him a lot less, a fact which softens Tessa's opinion of Will. Using the dagger and a tracking rune, Jem tracks down Will to a Downworlder drug den in the East End of London.
In the carriage, Jem and Tessa have a conversation, and Jem tells Tessa that she's beautiful, though he says it in Mandarin, so she does not know what exactly he has said. They arrive at the Downworlder's drug den, filled mostly with ifrits, where they find Will, laying on a bunk, obviously high on warlock drugs. Jem is very upset with Will and they drag him outside. Will is not in his senses and Jem hits him across the face before he returns to pay Will's debt. When they arrive back at the Institute, Jem runs from the carriage into the Institute and disappears. Later that evening, Tessa hears the screeching of violin strings in the hall, like a person screaming in awful pain. She walks into Jem's room, and sees him playing violently. When Tessa asks him what the matter is, Jem breaks down and reveals that he's angry that Will has taken pleasure using drugs for fun, when he knows that Jem has to take them in order to stay alive, even while the yin fen slowly kills him anyway.
Tessa tries to calm him down, and sees that Jem is running a fever, and puts her hand on his head to check. Jem flinches away. Jem then reveals that he feels that Tessa has never seen him as a man, but always as a patient; when she looks at him, it's to see how much yin fen he's taken. Caught up in the fierce discussion, they eventually begin kissing passionately for some time, until Tessa accidentally knocks a pillow off the bed, which knocks Jem's box of yin fen onto the floor, spilling the drug all over the carpet. Jem then asks Tessa to go back to her room, while he cleans, as he's visibly embarrassed by it. As she slips back to her room, she notices through the crack of her door that Jessamine is sneaking out of the Institute, dressed as a man once again.
The next morning, Will announces aloud to those sitting at the breakfast table that he was at "an opium den" the previous night. Tessa jumps in to tell them that it wasn't really an opium den, but a den that trades in magic powders, but Will cuts her off and informs everyone that while at the so-called "opium den," he witnessed many werewolves buying yin fen in high quantities; he overheard a werewolf say that the drug allowed them to work all night for the Magister. This catches the attention of Charlotte, and she immediately sends for Woolsey Scott—the leader of the London werewolf pack—to inform him of this newest development and inquire about the behavior of his pack. While Charlotte goes to send a letter to Woolsey, Will insists on accompanying Tessa to her training with the intention of annoying the Lightwood brothers. During the session, Gabriel talks loudly about how Charlotte is inept, and that when his father runs the Institute, everything will be much better.
When both Sophie and Tessa defend Charlotte, Gabriel then tells them that Charlotte's father reported Gabriel's uncle, Silas Pangborn for what he called a small infraction of Covenant Law, and claims the shame led to his uncle committing suicide. He specifically states, that the Fairchilds only care about the Law, and goes as far as to say that it's Charlotte's father's actions that caused his own mother to kill herself, as well. When he brings up the idea that Henry doesn't love Charlotte because no one could love her, Sophie slaps him across the face. Gabriel is so angry that he storms out of the Institute and leaves in the Lightwoods' carriage without his brother Gideon. Tessa rushes to apologize to him, but Gabriel has already left. Will follows Tessa out of the training room, and they have a discussion about how inconsiderate Will has been to Jem, especially regarding his recent drug use. Will appears to be truly sorry for what he's done to Jem, and tells Tessa that even though people think he doesn't care about the opinions of others, in reality, he cares deeply enough that it drives him mad. Their conversation is interrupted by Woolsey Scott and Charlotte, who enter the room.
Back in the training room, Sophie and Gideon are alone, where they discuss what happens, after the end of their training. Gideon seems smitten by Sophie, and asks if he could see her again, even after their lessons.
Woolsey, Charlotte, Tessa and Will discuss the situation with Mortmain and the werewolves he is employing and drugging with yin fen. Woolsey informs them that the werewolves assisting Mortmain are not part of his pack, and because of this, he has no control over them. Disappointed, Charlotte asks Woolsey to keep his eyes out for new information, just in case. Woolsey then mentions that with Mortmain buying so much yin fen, there may be a shortage coming soon. At this revelation, Will bolts from the room to buy more of the drug for Jem, just in case they are faced with the situation where the drug is unavailable.
Ragnor Fell sends a letter to Charlotte informing her of the situation of the Herondales. In the letter, he tells Charlotte that he managed to speak with Cecily, who claimed that Will's desertion led to her father's drinking and gambling problem, which in turn led them to become destitute. Ragnor also notes the cleverness of Mortmain, and equates that power over the Herondales would mean power over Will, and thus, indirectly, control over the London Institute. Charlotte burns the letter to destroy the evidence of her breaking Covenant Law, and shortly afterward, Jem approaches her in her study and asks her about the veracity of Gabriel's story regarding the Fairchilds. Charlotte denies Gabriel's account, and claims it couldn't have been true, because Gabriel's grandfather had asked Charlotte's father to assist him in writing Gabriel's uncle Silas' eulogy.
Will is hurrying back from purchasing more yin fen. Will arrives at Jem's room, but notices him pacing outside Tessa's bedroom; Jem claims he was hoping to apologize to her for keeping her up with his violin playing. Jem invites Will into his room, where Will apologizes for his inconsiderate actions. Jem, unable to hold a grudge or a temper, quickly and easily forgives him.
Tessa, meanwhile, cannot sleep, because she fears that Jem thinks poorly of her because of how unrestrained she had acted the night when she and Jem had kissed. Sophie then knocks on her door, extremely anxious and fearful, and shows her an invitation to a masked ball addressed to Jessamine that she had found in Jessamine's room, which is accompanied by a personal handwritten note from Tessa's brother, Nate Gray. Sophie confesses to Tessa that when she was cleaning Jessamine's coat, she discovered the letter in a pocket and was horrified when she read it and realized who it was from. When Jessamine came into the room and saw Sophie with the letter, Jessamine was so furious, that Sophie reacted instinctively and knocked her out with a heavy hand mirror, and, afraid Jessamine would wake up, subsequently tied her to the bed to prevent her from running away. Tessa and Sophie agree that they must tell someone, and since Sophie is too scared to inform neither Charlotte out of fear of losing her job, nor Jem, whom she's afraid of scaring with her aggressive reaction, the girls are only left with Will. Will, Tessa, and Sophie convene in Jessamine's room, where Will and Tessa agree that they must attend the masked ball, which was suspiciously being thrown by Benedict Lightwood, in hope of discovering the full extent of Jessamine's relationship with Nate, as well as what information Nate may have. Will is also curious why Benedict Lightwood would host a party that involved individuals such as Nate, especially such a party to which other Shadowhunters did not seem to be invited. Tessa then Changes into Jessamine, and Will escorts her to the ball, which was being held at Chiswick Manor.
During the carriage ride to the ball, Will tells Tessa why the Lightwoods hate him; when Will was twelve, Tatiana Blackthorn was madly in love with him, and Will sought to stop her from loving him. He then discovered her diary, and read aloud the contents at the London Institute's annual Christmas party. This infuriated the Lightwood brother and reduced Tatiana to tears. Gabriel and he had a fight which resulted in him breaking the former's hand. Some time later, Tessa, curious about her lineage, asks Will about whether it is possible to have a half-demon, half-Shadowhunter child, but Will states that it is impossible, as all products of a union between a demon and a Shadowhunter are stillborn at birth.
At the party, Nate dances with Tessa who is disguised as Jessamine, and reveals a lot of information. The Magister had planned to implicate Tessa by hiding the Book of the White in her bedroom, which Jessamine had effectively done. They observe a demon lady getting intimate with Benedict. Nate states that it was quite a normal occurrence. He calls her his "wife," which confuses and disturbs Tessa. Needing time to process the information she has just learned, Tessa, as Jessamine, asks Nate to fetch her a lemonade, which she drinks. Before anything else happens, Nate is called away by an urgent letter, supposedly sent from the Magister. A faerie named Hyacinth approaches her and asks if she is Elizabeth Gray's daughter, Tessa. Hyacinth talks of her mother as if they were known acquaintances. Will then appears beside her, and informs her that it was he who sent the letter, to create a diversion so as to get Tessa away from her brother for a moment. He drinks up whatever was left of her drink and they then leave the ballroom and go out onto the balcony for some fresh air. Distressed, Tessa has already changed back into herself. Once outside, they ending up engaging in passionate kissing, despite being intruded once. Finally, Magnus Bane arrives on the balcony and informs the two that the lemonades have warlock powder mixed in them, a substance that lowers the inhibitions of the drinkers.
Magnus also informs them that since Camille's footmen, Archer, recognized Will at the party, it's likely that others have or will as well, and suggests that they all leave. The three of them head toward the carriage, but are stopped on their way when Will notices a blue-skinned demon exiting the party and chases after him. Tessa and Magnus end up going back to the Institute in the carriage without Will, after Magnus assures her that Will, as a Shadowhunter, is bound to hunt and kill demons, and that he will be perfectly safe. Magnus himself, however, is suspicious of Will's reaction but does not say so.
Tessa arrives back at the London Institute, and insists to Sophie that, now that they have learnt a lot of valuable information at the party, they must alert Charlotte of Jessamine's betrayal. While Sophie nervously leaves to fetch Charlotte, Jessamine awakens and quickly remembers the events that led to her being tied to the bed. Jessamine swears that Nate truly loves her, though Tessa reminds Jessamine that he is her brother, and that she knows better than anyone that he loves no one but himself. Jessamine is furious at Tessa and proclaims her to be jealous, before she shouts that she hopes Mortmain kills all of the Shadowhunters.
Charlotte walks in just as Jessie has shouted at Tessa, and is so furious and full of wrath at Jessamine's betrayal that she informs them all that she has called Brother Enoch to the Institute. Brother Enoch follows her into the room holding the Mortal Sword, which immediately frightens Jessamine. Brother Enoch uses the sword to untie Jessamine before he places the Mortal Sword in her hands. Tessa is afraid they're going to torture her, but Sophie assures her quietly that the Sword forces Nephilim to tell the truth. Charlotte proceeds to question Jessamine mercilessly, and Jessamine, unable to lie purposely, reveals that she met Nate only a few times, and that he'd proposed after their second meeting. She then reveals that they'd been married, and that Benedict Lightwood is helping Mortmain because he's being blackmailed by him, though she doesn't know the reason behind it. Tessa angrily asks her what she knows about Tessa and her biological family and Jessamine confesses that Nate told her that Tessa's father was a demon, and that her mother was a Shadowhunter. She also reveals that Mortmain wanted them to report everything to the Clave, as he seemingly had a plan. Jessamine is then asked where Mortmain is hiding, but she is unable to give a straight answer and Brother Enoch reveals that Jessamine has had a magical block put in her mind to prevent her from telling secrets, presumably by Mortmain, and requests that Jessamine be taken away with him to the Silent City so that the Silent Brothers can undo the block. After the two of them leave, Charlotte, overwhelmed, bursts into tears.
While Jessamine is being questioned, Will has returned to the Institute from the ball and enters Magnus' house, telling him that he was bitten by the blue-demon he chased, the demon who cursed Will five years ago; when the demon bit Will, he accidentally lost a tooth in the process, and Will begs Magnus to use the tooth to summon the demon. Before he can demand further, he becomes ill due to the poison from his bite and passes out at the apartment. Magnus casts a painkilling spell and puts Will on the couch to rest and heal.
A short while later, Camille Belcourt returns home from her long holiday after helping the Shadowhunters to implicate Alexei de Quincey, which initially delights Magnus, but when he confesses that he knows that Camille was actually in St. Petersburg with a human lover, and not in Paris as she had told him, Camille brushes off the dalliance. She goes so far as to suggest that if Magnus does not allow her her "diversions," she will break up with him. Magnus is so hurt that he effectively breaks up with her, and, in an effort to make Camille jealous, states that he and Will have begun a relationship. Magnus awakens Will to demonstrate his point, and kisses him on the mouth, however when later inquired by Will, denies it, stating that Will was hallucinating due to the after effects of the poison leaving his system. Camille is disgusted that Magnus would consort amongst Nephilim, and demands that he leave her home. After being thrown out, Magnus, happy that Will helped him without even knowing he did it, promises that he will try to summon the demon using the tooth, before sending the latter off to return to the Institute.
The next morning, the rest of the Institute's residents learn of Jessamine's betrayal. They discuss what course of action they should take regarding the information they had procured about Benedict Lightwood. They all conclude that it is best to keep quiet for now, since they only have limited time before their trial period is up, and until then, they will continue to investigate more into the matter. Jem suggests that he and Tessa go visit Jessamine at the Silent City, to ask her to help them by writing to Nate and requesting a meeting. Their plan is that Tessa will once again meet with Nate while Changed into Jessamine, and they will capture him and do whatever they need to do to get him to spill Mortmain's plans, even if they have to torture him to do it. While waiting for the Silent Brothers' carriage, Tessa corners Will and makes him promise not to speak to anyone of the events that occurred on the balcony, and though he jokes at first, Will eventually swears on the Angel that he will not say a word. They part on the note, that whatever happened was merely the effect of the magical powder. Tessa and Jem then enter the carriage and travel to the Silent City.
Meanwhile, Sophie meets up with Gideon in Regent's Park. Gideon seems excited for the picnic, however, she gently turns him down. Presuming that she was angry with him due to the events of the previous night, he justifies himself, stating that his father had forcefully dragged him to the Ball in Chiswick Manor and he was just as horrified to notice his father associating with demons, openly. He continues telling her that while he was in the Madrid Institute, he realized how flawed his father's teachings were; demons were not supposed to be engaged with undercover of the darkness either. They walk down the River Thames and part after some time.
Upon Jem and Tessa's arrival, Jessamine initially resists, but they are successful in convincing her to help them out. They instruct her to write a note to Nate with the promise that, should she genuinely help them catch Nate and Mortmain, she will be shown leniency by the Clave. Jem and Tessa then return to the Institute to prepare their plan.
The rest of the members of the Institute begin preparing for the evening meeting. Will and Jem are worried about leaving the Institute unguarded yet again, but just before they leave, Gideon arrives to train Sophie. Charlotte asks Gideon to protect and watch over the Institute while the rest of them are gone for the evening, to which he agrees. A letter then arrives from Nate, accepting Jessamine's request to meet at sundown in a tea storage room on Mincing Lane. Everyone agrees to the following plan: Henry, Charlotte, Will & Jem will arrive before Nate does and hide in the storage facility; Tessa will dress as a man, just as Jessamine does when she meets Nate, Change into Jessamine, and walk to the storage room to meet him. She would closely be followed by Cyril.
Just before Tessa is to leave, she remembers that she has left her clockwork angel necklace behind, and although she knows she cannot wear it, she wants to keep it in her pocket, knowing how many times it has already saved her life. However, as she proceeds to do so, she notices Sophie and Gideon in a tender moment in the hall outside of Jessamine's bedroom, and chooses to simply go without the angel out of fear of interrupting them. When Charlotte, Jem, Will and Henry arrive at the warehouse they find a werewolf dying of yin fen. The werewolf demands for more of the drug, and before they can question him, he dies. Nate arrives, compelling them to hide. Charlotte is overcome with nausea and rushes down the staircase.
Tessa now posing as Jessamine, arrives at the storage room, and calls out for Nate. Nate greets her and she demands him for more information. Nate assures her that he will answer her questions and beckons for a kiss, but before they can do it, he informs her that he, in fact, knows that she is Tessa in disguise. He then tells her that he'd known since he had received that false letter during the masquerade ball.
He goes on to tell Tessa that he is not her biological brother, as she originally thought, but actually her cousin; he is the son of Harriet Moore, Tessa's aunt. He narrates how Harriet had engaged in intimacy before her fiancé died. Elizabeth had adopted him as her son and saved her sister's reputation from getting ruined. Nate then shows Tessa a gigantic automaton, who has an unconscious Charlotte clutched in its clawed hand. Nate commands the automaton to set Charlotte down and crush her with its feet. Henry suddenly appears and fights the automaton with barely contained rage. before he is injured.
A fight ensues, among the Shadowhunters who come to her aid and the automaton. When she finds that they are losing Tessa manages to control the automaton by Changing into Nate and commanding the automaton to hold the real Nate. While he is subdued Will manages to attach a new invention of Henry's—designed to disrupt the automaton's mechanisms and cause it to explode—to the automaton. Will then dives upon Tessa, who has transformed back to herself, and shields her with his body just as the automaton explodes.
In the explosion, Nate is fatally wounded by a shrapnel, while Tessa suffers minor injuries and Will is grievously wounded, having taken the brunt of the shrapnel while shielding her. Tessa, noticing neither her own injuries, nor Will's, runs to Nate and tries desperately to save him, even despite what he's done, but he dies after telling her to always wear the clockwork angel. After Nate's death, Tessa realizes that everyone has gathered around Will, and feels guilty as she runs over to him to find that he is bleeding profusely. When Jem explains the extent of Will's injuries and points out her own wound on her head, Tessa becomes overwhelmed by shock and faints on the floor of the warehouse.
Tessa is brought back to the Institute, where she is put in the infirmary with Will. She awakens briefly to overhear Charlotte speaking with Brother Enoch. Afterward, Tessa floats in an out of consciousness and the next time Tessa awakens is to witness Brother Enoch using tweezers to pick the pieces of shrapnel out of Will's back, while Jem sits at his bedside and holds Will's hand for comfort. She then quickly falls asleep again, and finds herself talking to Aunt Harriet's soul. Harriet feels guilty, stating that if she had accepted Nate as her son, perhaps he might not have betrayed the. She then tells Tessa to return back to where she belongs.
She later finds herself in the form of a soul, and floats near the ceiling. She overhears Charlotte telling Jem at her and Will's bedside that she's sure she will ultimately lose the Institute to Benedict Lightwood, after not realizing that Jessamine was a spy until it was too late, and indirectly causing Nate's death before they could glean any information about Mortmain from him.
Jem asks her what Henry thinks about it, but Charlotte confesses that she didn't have the heart to tell him what she fears, since he already blames himself for Will and Tessa getting hurt as a result of his invention. Sophie then calls Charlotte out of the room, leaving Jem to say a heartfelt goodbye to Tessa before he leaves. Tessa is then transported to the Silent City where sees Jessamine crying in her cell. Jessamine tries to use her wedding ring to carve her initials into the stone wall, but the stone breaks under the pressure, revealing that it wasn't a real diamond; this revelation makes Jessamine even more upset. The next time she wakes up, Will is at her bedside with a lemon tea, and after she worries about his injuries, he promises her that he's okay. Tessa embraces Will, but begins to feel lightheaded, and as a result of her confusion, believes that she's dreaming this encounter with Will. She loses consciousness again.
Will travels to meet Magnus after receiving a note from him. While he walks he thinks about Tessa, and hopes that after tonight he will no longer have to push her away. When Will arrives at the address he is surprised when Woolsey Scott opens the door instead of a footman. Magnus appears behind Woolsey and invites Will in, explaining to Woolsey that he needs the use of a room to summon the demon Marbas. Woolsey allows him the use of his late brother Ralf's room.
Magnus leads Will to the room, and begins the summoning, using the demon tooth Will recovered. The process is successful, and Magnus indicates to Will to take over. Will questions the demon, Marbas, asking whether it remembers him. The demon recognizes him after a while, recalling the curse he inflicted and happily repeats it. Will, visibly upset, begs the demon to lift it, saying five years is long enough for his suffering. Marbas chuckles and refuses, saying he was stuck in the Pyxis for twenty years, and that perhaps Will should be cursed for just as long.
Will is about to throw himself at Marbas when Magnus interrupts, saying that something about the story strikes him as odd. He says that any demon trapped as long as Marbas in a Pyxis would be extremely weak when released, and unable to cast a curse as strong and subtle as the one he claims to have cast on Will. Marbas disagrees, but Magnus warns him, using his father as an incentive, not to lie to him. Marbas reluctantly admits the curse was false, saying that Will's sister Ella died because she was struck by his stinger, and not because of a curse. Will falls to his knees, shocked, and Magnus calls Marbas a blue-skinned bastard before destroying him with fire.
Magnus tries to comfort Will, saying he still has a chance to make things right and assuring him that Tessa, despite Will's belief otherwise, does seem to like him ,judging by what Magnus himself saw on the balcony. Will gets up and leaves, while Magnus watches him, hoping Tessa wouldn't break his heart.
In the morning Tessa, who had fallen asleep in the living room, is awoken by Sophie. Sophie tells her Charlotte sent her to take Tessa back to her room. In Tessa's room, she tells Sophie that she saw her with Gideon, trying to warn her against him because of his family's association with Mortmain. Tessa implies that Gideon should not be trusted either and ends up upsetting Sophie, who storms out of her room.
In her office, Charlotte begins composing a letter of resignation for her position. Henry walks in and sees the letter over her shoulder, and is shocked. He and Charlotte have a much avoided conversation regarding the terms of their marriage, where they figure out that they have both been mistaken in their beliefs of why they married each other. They reveal their love for each other and share a kiss.
Will barges in looking for Tessa, and after noticing and reading the letter vehemently opposes her decision. He proposes that they present the theory of demons associating with Benedict Lightwood in order to win their case. He asks Charlotte to call a meeting to discuss new plans. He leaves to gather the others, but before he does he lets Henry and Charlotte know how grateful he is for everything they have done for him, leaving them confused about the newfound gentleness in his character.
Tessa is still in her room, upset over Sophie, when her door opens, and Jem enters. He begins to speak, saying what Tessa already knows; the deadline to find Mortmain is almost upon them, and it is likely Charlotte will lose the Institute. He seems confident the Lightwoods would let him and Will stay, but he knows Benedict would never allow her to remain, being a Downworlder. Tessa tells him that he was not responsible for her well-being and thanks him. Jem is visibly agitated and on prompting confesses that he loves her. He finally asks Tessa to marry him.
Tessa is shocked, saying he will be kicked out of the Clave, but Jem quickly explains that in a case such as hers, the Clave will have to consider their predicament for at least three months, while they can still remain engaged. Tessa is skeptical, believing that Jem is just being kind, but he assures her that he loves her, admitting that he has for a while. He gives her his mother's jade pendant as a sign of his love, and she agrees to marry him. Sophie knocks on the door to let them know about the meeting.
When everyone arrives Charlotte begins talking about the deadline, and says that, with Nate dead, they have no choice but to report Benedict to the Clave, even though they know the Magister wants them to. Charlotte announces she will go to the Consul and resign, but try to convince him that Benedict is unfit to run the Institute. Sophie loudly protests this, saying she will be playing into Mortmain's hands, and when Charlotte asks her how she knows, Sophie reluctantly reveals that she has been stepping out with Gideon Lightwood, who heard his father saying that he hoped Charlotte would go to the Clave.
Sophie also reveals that Benedict has demon pox and Mortmain has been sending him medicine, which is why he is working with him. They all believe her, as they know Gideon is in love with her, and wouldn't lie. Will is ecstatic that his theory was proven correct and sings a song. Tessa and the others see a visible change in his temperament.
Jem rushes out of the room, and returns with a piece of paper. He explains that it is Barbara Lightwood's death report, which he got from the Silent Brothers when he and Tessa visited Jessamine. He reminds everyone of Benedict's story to his children; that their mother committed suicide due to grief at her brother's death. Jem reads the report again and concludes this was partially a lie. While she did slit her wrist, according to the report the Silent Brothers recorded a shield-shaped rash on her shoulder, indicative of astriola, another name for demon pox. They realize that she killed herself because she knew her husband had given her the disease, and that Benedict lied to his children about it, and covered it up even after the Silent Brothers, obligated, told him what they had found.
The Shadowhunters come up with a plan to blackmail Benedict, using their newfound knowledge to threaten him, as the Clave would see Barbara's death as the deliberate murder of a fellow Shadowhunter, accomplished through demonic means, which would call down the ultimate punishment. Everyone, besides Henry and Jem who are staying behind, begins preparing for a trip. Tessa is stopped by Will who says he needs to talk with her, and she convinces him to wait until after they return. Even though Tessa has accepted Jem's proposal, she has conflicting emotions with regards to Will.
Tessa continues on to her room and is surprised to see new clothes laid out for her, which Sophie explains are from Charlotte. She apologizes to Sophie for questioning her about Gideon, and the latter forgives her.
Charlotte, Tessa and Will all ride to Benedict's house in Pimlico, and meet him in his drawing room with Gabriel and Gideon behind him. Charlotte reveals what they know, and demands that he take back his claim on the Institute and speak out on her behalf lest they tell the Clave the truth. During the conversation, his sons learn of the demon pox, though Gideon appears to already have known. When blamed by his father of betraying the Lightwood name, he removes his family ring and leaves with Charlotte, stating that he was not a criminal. Gabriel chooses to support his father, despite his brother's attempts at convincing him to withdraw his support.
In the carriage ride back, Will asks Gideon how he knew about the demon pox, and Charlotte reveals that she had Cyril deliver two notes when he delivered the calling card to Benedict. Gideon says Cyril slipped it into his hand when Benedict wasn't looking. Tessa asks him why he chose to believe it so quickly, and Gideon simply says that the story his father told him about his mother's death never made sense, but this did.
Back at the Institute Will drags Tessa to the drawing room to talk and locks the doors. He kisses her, and she pushes him away. They have a conversation, where Will reveals the truth about the curse and professes his love for her, and Tessa tells him about her engagement to Jem. He is devastated, but accepts it and leaves after she tells him she loves Jem and not him. She maintains this is the right thing as she could never be responsible for hurting either of the boys. Wanting to feel something she takes a hot poker from the fireplace and grips it in her hand, screaming, and bringing Sophie running to take her to her room. While Sophie dresses the wound she admits to Tessa that she's insecure about Gideon living in the Institute and seeing her do her duties as a maid.
Will visits Jem, intending to tell him of his feelings for Tessa, but before he can do so, Jem tells him he loves Tessa, having mistaken Will's saving her life for a favor Will did for him. Jem apologizes to Will for hitting him, and Will decides to keep his feelings for Tessa to himself and pretends to be glad for Jem.
The Shadowhunters attend another Council meeting to hear the decision regarding the Institute. Charlotte recounts the preceding two weeks before the Council, impressing almost everyone with her handling of the situation with Jessamine. Benedict stands up and speaks out for Charlotte, retracting his claim on the Institute. The Council votes and Charlotte is allowed to keep the Institute. Shortly afterward the Council Chamber is attacked by large, flying, beetle-like automatons sent by Mortmain to spy on the proceedings, as he had lost all his sources of information. They disappear, but not before Tessa captures one for Henry to study.
Back at the Institute, the Shadowhunters have a celebratory dinner. Jem, in a fit of happiness, announces to them all that he has proposed to Tessa and she has accepted him. Charlotte takes the chance to announce that she is pregnant. While everyone is congratulating each other Cyril announces that a guest has come to see them. Charlotte tells him to let them in, and Will's sister, Cecily Herondale, enters the room and introduces herself, saying she has come to be trained as a Shadowhunter.
Prologue: The Outcast Dead
- Tessa Gray - the main protagonist of the series, Tessa is a warlock who grew up in the mundane world.
- Magnus Bane - a warlock who helps Will with a predicament.
- Ragnor Fell - the High Warlock of London who has kept a close eye on Will's family for years.
- Woolsey Scott - Magnus's influential werewolf friend and leader of the London werewolf pack.
- Will Herondale - a Shadowhunter and ward of the Institute who struggles with his feelings for Tessa and protecting her, Jem, and those he loves from his "curse".
- Jem Carstairs - Will's parabatai, Jem is a ward of the Institute whose life depends on a drug that is slowly killing him and who is growing closer to their guest Tessa.
- Charlotte & Henry Branwell - the heads of the London Institute.
- Jessamine Lovelace - a ward living in the London Institute who is unhappy with the life of the Shadowhunters and wishes only to become a mundane.
- Aloysius Starkweather - an old man who heads up the York Institute that our heroes are tasked to meet.
- Benedict Lightwood - the patriarch of Lightwood family who wishes to take Charlotte's place as head of the London Institute.
- Gabriel Lightwood - Benedict's son and Gideon's younger brother who despises Will.
- Gideon Lightwood - Benedict's eldest son who becomes Tessa and Sophie's trainer; unlike Gabriel, Gideon, due to his time abroad, is not blindly loyal to their father.
- Axel Mortmain - a wealthy and very influential mundane with a special connection to Tessa's past.
- Nate Gray - Tessa's trouble-prone brother who is working for the Magister.
- Sophie Collins - the maidservant of the ladies in the London Institute who becomes a good friend to Tessa and the subject of Gideon Lightwood's attention.
- Cyril Tanner - Thomas Tanner's brother who comes to work for the London Institute after his brother's passing.
- Bridget Daly - the cook of the London Institute after Agatha's passing who frequently perturbs the other residents with her morbid ballads.
- Collector's edition: The collector's edition includes a copy of the letter Will wrote to Tessa—written on the inside cover of their copy of A Tale of Two Cities—as well as one of the many unsent letters Will wrote to his parents, specifically one he wrote on his seventeenth birthday. Both of these were mentioned in Clockwork Princess.
- The title refers to Axel Mortmain, whom his adoptive father John Thadeus Shade called his "clockwork prince".
- The original (now hardbound) cover features Jem Carstairs holding his jade cane in front of Temple Bar in the background. The repackaged (paperback) cover also features Jem, this time on the ground and playing his violin while an illustration of the clockwork angel stood behind him.
- The cover model for Jem on the original US cover is Forrest Hoffman; the model's brother is on the cover of The Bane Chronicles as Magnus Bane. The model for the repackaged edition is Aaron Bernards; he went on to reprise the character on the Ghosts of the Shadow Market cover.
- In the trailers and video for the book, Will was portrayed by Carson Nicely, and Tessa by Mandi Lee. The teaser trailer and the DSBS scene was produced by VLC Productions, and the full trailer was produced by Dirty Robbers.
- "The model for the Clockwork Prince cover is the enigmatic, Forrest Hoffman..." Cliff Nielsen on Facebook
- "He is, yes, the same guy, Aaron, who portrayed Jem on the cover of Clockwork Prince!" Cassandra Clare on Twitter
- "The new one, sorry!" Cassandra Clare on Twitter
- "Dirty Sexy Balcony Scene now available on YouTube" — TMI Source
- "First still from full-length ‘Clockwork Prince’ book trailer by Dirty Robber" — TMI Source