After having to bury her Aunt Harriet and sell everything including most of her wardrobe in order to do so, Tessa Grey leaves New York and sails to London. She has arranged to meet her brother Nate at the docks, but met the Dark sisters instead. The Dark sisters tricked her into going back to the house with them where she is held captive, and made to do things she didn’t want to do.
When the Dark sisters were satisfied with Tessa’s transformations into other people they arranged for the magister to come and marry her. While waiting on the arrival of the mysterious magister, Tessa had escaped the straps that bound her to the bed however, she could not escape the bedroom let alone the house itself. She was searching for a way out when the handsome Will busted in the door and made her escape possible. Will had been searching for clues of a murder that had taken place and was directed to the Dark sister’s house by some of the downworlders. The escape ended with a fight, Will had managed to kill one of the sisters, but Tessa was thrown across the room and knocked unconscious.
Tessa woke in the institute to the shocking appearance of Brother Enoch, and was very frightened until Charlotte—who runs the institution—calmed her down. Tessa had learned from her stay in the Dark house that their where things in the world that was not normal, and in fact she was not normal. However, she thought it to be a delusion, that she was trapped in a nightmare until now at the institution when Charlotte cleared things up a bit more, and made Tessa more acceptable to the odd things that were hidden from her.
Tessa was told about the difference between downworlders—faeries, werewolves, vampires, and warlocks—and shadowhunters—“Warriors dedicated to ridding the world of demons”—and learned that she herself was in fact a downworlder, though no one knew just what kind.
The accords that help keep peace with the downworlders bound the shadowhunters to help Tessa. Charlotte gave her word she would help find Tessa’s Brother Nate, and demand nothing but help in the ongoing investigation of the mundane murders and the Pandemonium club members in return. Tessa might be able to help with the Dark sisters and the house that imprisoned her.
Tessa met most of the members of the institute over dinner. Some she had met during her escape from the Dark house like Henry—Charlotte’s husband—and Will—the arrogant and handsome teenage shadowhunter. Then there was Jessamine and her servant Sophie. Jessamine was hateful and questioned Tessa’s innocence. Sophie on the other hand was kind and not like other servants, she could speak freely in the institute without punishment or disapproval. Tessa didn’t meet Jem—orphan from Shanghai China—until later when his violin playing woke her up out of a deep sleep. He was kind enough to her, more so than Will had been earlier that day.
The next morning discussions of investigation took place over breakfast. Will and Jem went to search the Dark sister’s house and found a human-looking machine screaming warnings at the two nephilium. Charlotte and Hennery went to talk with Nate’s employer about his disappearance and to their surprise Mr. Mortmain—once a traveler and trader of spices, now in the machinery business—knew who they were. Charlotte was able to get some helpful information out of Mr. Mortmain, including the name of the magister—de Quincey.
Later on that day a trusted vampire by the name Camille came to visit and observe Tessa and her abilities. She had a plan of getting the shadowhunters into one of de Quincey’s very private parties where killing of mundane take place. If the plan went well, Tessa would transform into Camille and be escorted by warlock Magnus Bane, and by Will as her subjugate. They would witness the murders for themselves and bring the enclave down on them.
At the party the trap took place as planned. Tessa and Will met Magnus Bane and he led them into a library that also served as an office. Will searched until he came up with blueprints for the automatons, like the one Jem and he found at the Dark sisters house. When the ceremony started the three of them took seats among the other vampires and observed de Quincey slash a cut into the throat of a hooded mundane. That was Tessa’s clue to leave with Magnus so Will could hit the button on Hennery’s device and signal the enclave.
Just when Tessa was leaving the room, de Quincey unmasked the mundane and Tessa saw it was her brother Nate. She screamed at Will, Will hit the button, but instead of a white light shooting out like appose to, the curtains burst into flames. Ciaos broke out in the room full of vampires and the enclave joined in to help slaughter the evil downworlders. In the end the only vampire to escape was de Quincey, and Tessa had her brother back. Back at the institute Brother Enoch helped to cure him, but Nate stayed asleep burning with fever.
Tessa went to see Will that night after getting kicked out of Nate’s bedroom. He too was burning with fever from biting de Quincey and consuming vampire blood. The two of them shared a hot kiss than Will demanded Tessa to leave his presence. The next night Tessa woke after sleeping the day away. She overheard Will’s raging fit in the dining room and stormed in to give the shadowhunters a piece of her mind.
When Jem took Tessa on a walk to cool off, they were attacked by an army of automatons. Jem had collapsed when they reached the steps of the institution and the other shadowhunters came out to battle the machines. The few clockwork machines that survived ran off. It was then that Tessa learned about Jem’s “illness.” He told her how he was tortured with demon poison when a demon had entered the institution in Shanghai where he lived with his parents. His parents were both killed but somehow he managed to survive. However, his body had been changed. He could only survive on the demon poison. It has been like living with a drug addiction. If he stopped taking the drug he would die, if he uses the drug he would prolong his life, but would eventually die anyway because of the drug.
When Nate woke up he was very startled by the shadowhunters and the institution that he hid and would only allow Tessa to calm him down. Nate told the shadowhunters that de Quincey is the magister and he is planning to bind demon energies into the clockwork machines in order to make a grand army that is not born of God nor the devil. His purpose is to wipe out the shadowhunter clan that he despised so much. Nate told them that this binding would take place under the full moon. When the shadowhunters heard this they realized the full moon was that night and they rushed to gather the enclave to stop de Quincey in his plans.
After Charlotte and Hennery took off to meet with the other clan members, Mr. Mortmain showed up at the door and told Will and Jem where the Dark sisters were. The shadowhunters bad been told that the ceremony was being carried out by the Dark sisters so Will and Jem rushed off to stop them. As soon as they left the institute was attacked. Mortmain and an army of automons discovered a way of getting into the institute. Since the doors would not open to anyone without shadowhunter blood, the automon who attacked Jem still wore his blood on its hands. It was enough to open the institute’s doors.
The others realized that they had been tricked. That Mr. Mortmain and even Nate told a believable story that had all the fighters off chasing lies and now they were free to grab Tessa and the trapped demon energies in the weapons room. Mr. Mortmain turned out to be the magister who wanted Tessa. He even told her he had made her, though didn’t clarify how or why. Nate explained that he felt he didn’t have a sister. That when he found out what she was, she was dead to him. In the end the other shadowhunters made their way back to the institution and destroyed what was left of the machines. Nate on the other hand made off with the box of demon energies and when Tessa faked her death, Mr. Mortmain made his escape as well.