Clockwork angel is the first book in a Prequel Series, The Infernal Devices, to the Mortal Instument Series. The Infernal devices tells the story of a Shadowhunter Institute in London in the late 1800’s. It is 10 years after the Accords have been created between the shadowhunters and the downworlders.
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London, April 1878. Shadowhunter William Herondale kills a Shax demon and searches for Jem, his “hunting buddy” (parabatai). Jem is in an alley and has found the body of fourteen-year-old Emma Bayliss. They find a dagger imprinted with two snakes each biting the other’s tail, an ouroboros.
Tessa Gray arrives in the port of Southampton from New York to live with her brother Nate after their Aunt Harriet died. However, she is approached by two strange women calling themselves the Dark Sisters that give her a letter apparently from her brother. In it, he tells Tessa that he can’t meet her off the ship, but the Dark Sisters will take her to him, and warily, she enters the Dark Sister’s carriage.
Six Weeks Later, Tessa has been held prisoner by the Dark Sisters. They have been training her to Change (a process by which she takes on the appearance of someone else, living or dead, by holding something which belongs to them). This time Tessa becomes Emma Bayliss and relives her last few moments alive. The sisters are impressed and tell Tessa that they have been preparing her for an arranged marriage to a mysterious individual known as “the Magister.” They expect the marriage to take place that day or the next. The sisters leave and Tessa tries to run away but is caught by the Sister’s coachman. Tessa is tied to her bed, she attempts to escape again by using the Change. Just as she escapes her bindings the door opens. It is Will, who followed the trail from Emma’s body to an organization called the Pandemonium Club and the Sister’s house. Will and his friends proceed to help Tessa escape and when the Dark Sisters catch up a battle begins in which Mrs. Black is killed, and Tessa is knocked unconscious.
Tessa awakens in the London Institute to find herself being examined by Brother Enoch, one of the Silent Brothers, who informs her that she is a shape-changer and a Downworlder. Tessa meets the inhabitants of the Institute. The next morning Jessamine takes Tessa shopping; Charlotte and Henry look for Tessa’s brother; and Will and Jem inspect the Sister’s house. Will and Jem find a clockwork automaton wrapped in human flesh. A vampire named de Quincey is identified as the Magister and is said to have Tessa’s brother. Tessa and Will infiltrate one of de Quincey’s parties. At his parties de Quincey slowly drains a human victim; this is against the Accord, which allows the Shadowhunters to attack de Quincey. Things go terribly wrong at the party when de Quincey’s victim is revealed to be Tessa’s brother Nate.
Tessa and the Shadowhunters kill the vampires at the party, save Nate and bring him to the institute, but de Quincy escapes. Will bites a vampire, while fighting, and accidentally swallows some of the vampires blood. To get the blood out of his system, so he won’t turn into a vampire, he has to drink holy water. When Tessa brings him a bucket of holy water they kiss. Tessa also noticed he tasted like holy water and blood. Tessa and Jem go for a walk and are attacked by the Magister’s clockwork army. They make it to the institute where the Shadowhunters barely fight the army off. Jem loses consciousness and reveals that he is addicted to demon blood and is slowly dying from it. Most of the Shadowhunters leave to kill de Quincy who is hiding out. Mortmain, the mundane who informed the Shadowhunters of de Quincey comes back to the institute and tells Will and Jem the location of the Dark sister who is trying to help the Magister with his army. They rush off to kill her leaving Nate, Jessamine and Tessa alone. Mortmain comes back with the clockwork army and reveals himself as the real Magister. Nate reveals himself as Mortmain’s accomplice. The girls lock themselves in a room, but Mortmain tricks them to open the door. Tessa agrees to marry Mortmain if he will leave everyone alone and he agrees. Once the army has left the room Tessa stabs herself in the heart and Mortmain leaves.
Will rushes back when he realizes his mistake. At the Institute he runs into the army and Nate. Nate orders the army to kill Will. Jem makes it back and helps Will and then the rest of the Shadowhunters arrive. The army leaves. Will finds Tessa who is alive; she Changed quickly just before stabbing herself and the blood was from a gunshot victim she had Changed to before with the Sisters. Tessa is asked to stay at the Institute and she happily accepts. Tessa wants to have a serious relationship with Will, but he suggests a strictly sexual relation and tells Tessa that she cannot have children (she is assumed to be a warlock) so it is impossible. Jem comforts her though she will not tell him exactly what Will did to annoy her. Will seeks Magnus Bane’s help, but it is not revealed for what reason.