Plot
The book takes place during the 1900s. However, several flashbacks (known as "Days Past") are scattered between the official chapters.
Part one
In 1897, in Idris, ten-year-old Lucie Herondale, daughter of Shadowhunter William "Will" Herondale, and half warlock/half Shadowhunter Theresa "Tessa" Gray, gets lost in the forest. She finds her way back with the help of a mysterious young boy.
Six years later, in 1903, James Herondale and his friends, his parabatai Matthew Fairchild (son of Charlotte Fairchild), Thomas Lightwood, and Christopher Lightwood, defeat the first demon they have seen in a year. The demon recognizes James, who falls into the shadow realm (a unique power as a result of his warlock heritage) for the first time in 3 years.
Meanwhile, Cordelia Carstairs enters the London society along with her brother Alastair and mother Sona. Her father has been arrested for supposedly mishandling a vampire raid, and the Carstairs wish to make powerful allies in London to save their reputation. The Carstairs are greeted by Tessa, James (Cordelia's childhood friend who she has a crush on), and Lucie (James' sister). Cordelia believes that her feelings for James might be requited.
At the next day's ball, James requests Cordelia dance with him, but disappoints her when he suddenly leaves to greet Grace Blackthorn, the adopted ward of Tatiana Blackthorn. James met Grace in 1899, and has been in love with her since he was 13 years old. Lucie sees the mysterious boy from her childhood, who reveals himself to be the ghost of Jesse Blackthorn, the late son of Tatiana. His mother asked Downworlders to preserve his body, so his spirit has remained trapped in the in-between. James is swept into the Shadow Realm and sees a partygoer being attacked seconds before she collapses in the real world.
The day after, Cordelia takes James aside and he reveals his relationship with Grace to her. Cordelia is heartbroken but happy for him. A demon attack wounds Ariadne Bridgestock and Barbara Lightwood, Thomas's oldest sister.
In a flashback, James meets Grace once again. She points out that he was in love with Cordelia, which he half-heartedly denied. Later on, Grace requested him to use his shadow abilities to retrieve her Cartwright familial bracelet which Tatiana seized.
A demon kept in Tatiana's greenhouse destroys it, and she blames James. Later, James reads a missive addressed to Matthew, signed by Grace. James immediately goes out alone.
Jesse visits Lucie in her bedroom and informs her that James is in danger, as Grace was being followed by the Blackthorn family demon and would probably murder anyone who wasn't a Blackthorn. She goes to save James. Cordelia, Matthew and Anna Lightwood (Christopher's older sister) visit Malcolm Fade and Hypatia Vex, where they inquire about the attacks and if the downworlders might have heard of it. Cordelia saves the warlocks' lives, and in return they send for Ragnor Fell who might be able to help Cordelia.
In a flashback to 1902, Thomas Lightwood had been gone for his travel year and stopped at Paris. He had met Alastair Carstairs there, and they had toured the whole city amiably. Around that time, Thomas had felt himself warming up to Alastair.
Back in the night, James meets with Grace as per her instructions and she asks James to forsake his Shadowhunter blood, run away, and marry her. It’s the only way she can escape her cruel mother. James silently considers it but decides there’s no way he can forsake his family, friends, or parabatai. Grace is angry at his declaration, but she’s cut short when demons attack them. Lucie, Christopher, Matthew, Thomas, Cordelia and Alastair come to their aid. The strongest demon recognizes James and asks why James is fighting against the creatures who worship his own grandfather. The demon calls Tessa’s demon father a prince of hell, which is a fallen angel as powerful as Raziel. James is shocked at the revelation. He lets the remaining demons go in the name of his grandfather as long as they go back to their own dimension. They will do so as long as no one present tells any other Shadowhunters about what transpired at the waterfront. After they’re gone, Tatiana pulls up in her carriage searching for Grace. James begs Grace to come to the Institute with him. His father will grant her safe haven. Tatiana promises to produce evidence to ruin Grace's reputation if she doesn’t come home with her.
Ragnor visits the teenagers-sans Cordelia and Alastair, and gives them a lead - Emmanuel Gast, who he thought might be able to dredge up information. Shocking news arrives when Barbara Lightwood is pronounced dead. At the Carstairs manor in Cornwall Gardens, Charles visits late at night. Cordelia, afraid that her brother might tell him of James' grandfather, eavesdrops on her brother and Charles' conversation and finds out that Alastair and Charles are gay and Charles was marrying Ariadne for power only. He was scared of rejection by the society and hence couldn't publicly display his sexual orientation. The next morning, James visits Cordelia and they, along with Matthew and Lucie pay a visit to Emmanuel Gast. However, he is already dead. Lucie summons his ghost and learns that he had raised the demon who was wreaking havoc over London. He claimed that he knew her grandfather, but she banished him before he could reveal much. Later they find splinters of wood with strange inscriptions. The four head back to the institute. Charles arrives in a carriage, bringing Grace and a bloodied Tatiana to the Institute. Once Tatiana is being treated in their infirmary, Grace pulls James aside to talk to him.
James takes Grace aside where she informs him that she is now engaged to Charles - who had broken off his engagement with Ariadne whilst she was unconscious. Grace demands her bracelet back from him and when he returns it, he feels physical pain from the separation. He goes to the Blackfriars Bridge to relieve his heartache. Grace finds Matthew and tells him what has transpired. Before she leaves, she seemingly holds Matthew in place with words alone and kisses him. She threatens to tell James about this kiss if Matthew ever breathes a word of their conversation to anyone. Matthew later finds James sitting on the bridge and in a moment of recklessness James drinks from his flask, and loses consciousness due to exhaustion. The next morning when he wakes up, he finds the hurt gone, and feels oddly light. Cordelia meanwhile had accompanied Anna to the infirmary and seen her put her guard down and sob for Ariadne. Will and Tessa have gone with the Silent Brothers to transport Tatiana and the other remaining wounded Shadowhunters to the Silent City. Grace will be staying with the Bridgestocks near the City to be close to her mother. Cordelia receives a letter from Charlotte, answering her inquiries about her father. He’s actually already been questioned with the Mortal Sword, which Cordelia was sure would clear his name as it requires the holder to speak only truth. Unfortunately, the demon attack has been erased from his memory, so he cannot clear his name with the sword.
Christopher arrives with the results and the group discovers that the shards of wood were pieces of a Pyxis. James informs them that the inscription were written in Old Persian and they realize that the demon was in fact a Mandikhor demon, which can produce smaller demons. They form a plan to capture the demon but for that they would need a Pyxis. Cordelia recalls seeing one at Hypatia's salon and they decide to approach Anna for aid. James escorts Cordelia home and Sona presumes that they both were getting closer than before and James had marital intentions, to which Cordelia replies in the negative. Cordelia goes to Alastair to gently break the news of Charles’ new engagement to him. Alastair already knew and is not happy when Cordelia tells him she eavesdropped on his recent conversation with Charles.
Part two
The next evening, James, Cordelia and Matthew accompany Anna to Hell Ruelle, where Hypatia is hosting a party. Lucie, Christopher and Thomas spy on the Enclave meeting and find out that Oliver, Barbara's ex-fiancee had passed away as well, seemingly due to the fact that Barbara had scratched him before her death. This shocked the entire Enclave as demon poison had never been contagious before, and London is placed immediately under quarantine. In Hell Ruelle, Anna seduces Hypatia and Cordelia performs in front of the gathering. James and Matthew are impressed, however they notice Charles entering the club. Matthew pushes the couple away and they hide in The Whispering Room, where James and Cordelia kiss passionately.
The next morning the Merry Thieves, Lucie, and Cordelia take advantage of the fact that Will and Tessa are gone for day-patrols, an added precaution by the Clave, and empty the Pyxis by letting out a Palpis demon named Agliarept, so that they may use it to trap the Mandikhor. They kill the demon before he could sway them. Since they performed it in the Sanctuary, Magnus walks in on them inquiring about their actions. They explain to him, their plans of ridding the shadow world of the demon and Cordelia implores Magnus to help them by casting an illusion over Tower Bridge, to protect and distract the mundanes. Magnus agrees. While they are going towards Tower Bridge, Magnus disproves James' theory of his grandfather being Belphegor.
In 1900, at the home of the Carstairs in Idris, there was an argument between the Carstairs siblings regarding the possession of Cortana. Cordelia stated that she wanted to be a merciful hero and Alastair merely said that the sword belonged to him as he was the Carstairs heir. However, Cortana flew back into Cordelia's hand, "choosing its owner."
The group goes to Tower Bridge to try to fight and capture the demon, but it breaks free of the Pyxis. During their fight against the Mandikhor, Cordelia is knocked into the Thames, but Lucie summons all the ghosts of London to her aid and they pull Cordelia out of the waters. When the demon departs, it speaks to James and tries to convince him to go to Belial's realm. Christopher has figured out the cure but cannot make it work without an illegal ingredient. James locates where it may be found but before they could retrieve it, Christopher is attacked and poisoned himself. Cordelia's mother reveals that she's pregnant. While receiving treatment in the Silent City, James, Cordelia, and Matthew sneak in and talked with Christopher. Christopher, under the influence of the infection, grabs James's wrist, infecting him, while trying to warn him that he was in danger and all the attacks were about him.
Meanwhile, Thomas searches the Chiswick grounds for the root and Lucie wanders off to a shed, where she spies on Grace Blackthorn. She notices Jesse Blackthorn's body, in a glass coffin, where he appears to be asleep. She hears Grace talking about her mother hurting James when suddenly a demon attacks her. It scoffs at her for removing James' bracelet and reaches out to destroy Jesse's coffin when Lucie reveals herself and helps Grace fight. Jesse kills the demon, saving them. Lucie and Thomas Lightwood retrieve the plant to finish the antidote for the poison Christopher had developed. Thomas and Alastair create the elixir and go to the Silent City to distribute it, which is a huge success.
After fleeing the city, James realizes that the person behind the attacks had to be his demonic grandfather. He enters the shadow realm to confront and stop Belial once and for all. After some time, Cordelia uses Cortana to cut through the Portal to help him. Belial reveals that the Mandikhor demon was raised partially in the physical world and partially in his realm, making it immune to sunlight. They battle the Mandikhor, but are gravely outmatched until James used his connection to Belial's realm to summon a tornado and kill it. Belial then approaches James to proposition him once again, to which Cordelia ran him through with Cortana. With Belial being severely wounded, the realm collapses, and James and Cordelia return to their own realm. Having been poisoned and depleted his strength when reaching into the realm, James is dying when they returned. Matthew tries healing runes, but they are not strong enough to save him. Unbeknownst to him, the ghost of Jesse Blackthorn gave James his last breath — which had been preserved in his locket to one day resurrect himself — saving his life.
Days later, while still recovering from his ordeal, James is visited by Grace. Grace pleads for his understanding of her actions and then tells him of her mother's involvement in necromancy and her plans against Nephilim. She forces her bracelet back on his wrist and asks that he go to Blackthorn Manor to destroy an automaton there. With the bracelet's control strengthened, James agrees and in the process of destroying it, burns down the manor. At the following council meeting, Tatiana accuses James of having orchestrated the Mandikhor attacks and then burning her house down to destroy her evidence. In an effort to protect him, Cordelia provides him with an alibi, claiming he had been with her the night before, tarnishing her reputation. While Tatiana is being arrested and sent to the Adamant Citadel for the dark magic items found in the rubble of her home, James proposes to Cordelia in order to save her reputation, with the plan to divorce in a year, and Cordelia agrees.
At James and Cordelia's engagement party, Matthew drinks heavily in an attempt to drown his sorrows, realizing he is in love with Cordelia. Charles announces his engagement to Grace and later seeks out Alastair, who breaks up with him in a lone corridor with Cordelia there to support him. Because of his drunken state, Matthew reveals the rumors Alastair had spread about their parents to Thomas, in an effort to prevent their friendship, causing Thomas to threaten Alastair, who runs out of the room crying. Ariadne approaches Anna, telling her that she heard her when she spoke to her at the infirmary, making Ariadne realize that Anna still loved her. When Anna asserts her reputation of being a pessimist about love, Ariadne tells her that she will win her back. Grace emerges from the shadows and she and Lucie talk about resurrecting Jesse.
Magnus Bane finds Matthew feeling desolate, as he notices the younger boy looking towards James and Cordelia dancing merrily. It is then that he reveals his attraction towards Cordelia and hints about the false marriage. Magnus is intrigued and decides that he needed to stay in London for a little while longer. Cordelia reflects on how drastically her life had changed since her arrival in London. James talks to her and tells her that he wishes she could be happy. He also states that the events of the whispering room had been a pretense. Cordelia is saddened but resolves to spend this one year with James happily.
Meanwhile, in Chiswick manor, Tatiana, who is now being banished to the Adamant Citadel, visits her son's coffin. She finds Belial waiting for her, and he says that he will rise much earlier than the Nephilim assumed. He also states that since Tatiana was now in the Adamant Citadel, they will strike at the heart of the Shadowhunters - their Adamas. Saying this he vanishes again, leaving Tatiana satisfied.
