She expects to meet her brother Nathaniel (Nate) –her only other living family left– at the docks but is instead greeted by a pair of sisters, Mrs. Dark and Mrs. Black, along with their odd coachman.
The Dark sisters claim to be sent by her brother, and they convince her to come with them as they load her into a carriage marked by a symbol of two snakes eating each other’s tails (an Ouroboros).
Some place else in London, two Teenage Shadowhunters stumble upon the body of a murdered young girl. William (Will) Herondale, and James (Jem) Carstairs, are too late to save her but they discover a strange dagger marked with the same two serpent symbol.
Six weeks later finds Tessa imprisoned in a house with only the sisters, the coachman and a maid.
The unsettling maid named Miranda (an automaton) arrives to take her to the Dark sisters, threatened into obedience by her brother’s safety, Tessa can’t do anything but follow.
During the past weeks the sisters have managed to brutally awaken a hidden ability of Tessa’s, the ability to shapeshift and access the memories and mannerisms of the one she can change into.
In the room in which she always meets the Dark sisters, Tessa is handed a pink bow and demanded to change.
As she changed, she gained the memory of a young girl who had been stabbed but managed to grab hold of the dagger and run to an alley only to bleed to death.
As she finished her change, the sisters –pleased– had complimented her and told her that her training is completed. Informing her that the reason they where training her all this time was because she was to marry a man called The Magister.
Later that night, after a failed escape attempt, Tessa encounters Will Herondale –who had come to investigate the murder of the young girl ‘Emma Bayliss’ whose body they had encountered before.
Mistaking him for the Magister, Tessa becomes startled but soon gets into a rather bizarre conversation before they proceed to break out together and manage to kill Mrs. Black.
Will takes Tessa to the Institute, which is the Shadowhunters’ headquarters in London.
In there, Tessa learns that Shadowhunters are part angel, hidden protectors of the mortal realm from anything supernatural.
In the institute, Tessa meets other Shadowhunters. Charlotte, the compassionate and courageous head of the institute, Henry, her eccentric inventor husband, the stubborn and bitter Jessamine, and last but not least, Jem, the disabled, half-chinese best friend of Will.
Will and Jem help Tessa learn more about Shadowhunters and encourage her to embrace her powers.
While Tessa is adjusting to life with Shadowhunters, Charlotte and Henry meet up with Nate’s old employer, Axel Mortmain.
The pair discover that Mortmain was the one who introduced Nate to the Shadowhunter world via the Pandemonium club, an establishment which is run by The Magister.
Axel Mortmain informs them that the Magister is in fact the head of the vampire clan and a prominent member of the club, Alexei de Quincey.
Camille Belcourt, a vampire informant of the institute, tells them that de Quincey has been hosting human torturing parties hidden from the Clave –The Shadowhunter law enforcement– .
With no other way to get inside, Camille suggests that Tessa infiltrate the party shape-shifted as her, so that the Clave has evidence against de Quincey.
Tessa agreed to go to the party along with Will who had been already been spotted before at Downworld –The place where supernaturals like Vampires, Warlocks, Faeries, etc live– which makes him the perfect person to go with her.
Tessa disguises herself as Camille, with Will accompanying her as her human subordinate. The plan was to gather evidence of de Quincey’s defiance and signal the awaiting Clave as soon as they did.
Together with Magnus Bane’s assistance –A warlock companion of Camille’s– , they explore the premises and uncover the blueprints to animating clockwork automatons using demonic energy.
During their investigation, they witness de Quincey threatening a masked individual, prompting Will to signal for the Clave’s intervention. As the raid unfolds, the mask is removed, revealing Tessa’s brother, Nate.
Tessa and Will narrowly die at de Quincey’s hands, as the Clave intervenes, overpowering the vampires in his influence and saving Nate, who is then brought back to the Institute alongside Tessa.
Despite their efforts, de Quincey eludes capture, leaving hope pinned on Nate’s eventual recovery for crucial intel.
When Nate finally regains consciousness, he reveals that de Quincey coerced him by exploiting his debt to the Pandemonium Club, forcing him to summon Tessa under threat of death.
Nate discloses that both de Quincey and Mortmain were acquainted with their parents through the club and harbored suspicions regarding Tessa’s mysterious abilities.
Furthermore, Nate also informs them the terrible news that de Quincey and the Dark Sisters have perfected the demonic animation spell, intending to amass an army of automatons for the Clave’s destruction.
Acting swiftly on the provided information, Charlotte, Henry, and the Clave hasten to the designated location disclosed by Nate. Leaving Will, and Jem as the only real trained Shadowhunters on campus.
Meanwhile, shortly after their departure, Axel Mortmain arrives at the Institute to alert Will and Jem of a secondary site where the Dark Sisters are conducting their ritual.
After leaving Tessa, Jessamine, and Nate at the Institute with optimism for their timely intervention of the Dark sisters’ plans, Will and Jem depart.
However, upon their exit, Mortmain launches an assault on the Institute using his automatons, ultimately revealing himself as The Magister –the true villain–, with Nate as his accomplice.
Will and Jem head to the Dark sisters’ location only to find it vacant except for Mrs. Dark who had been desperately trying to resurrect her sister.
Finding themselves deceived, Will and Jem find out a little too late that this has all been part of a masterplan, and head back in a hurry to the institute.
Meanwhile, Mortmain threatensto eliminate Jessamine and the Institute staff unless Tessa surrenders herself. He reveals himself as the reason for her existence, alleging manipulation of her mother into a union with a demon.
Everyone puts up a fair fight but in the end, Tessa is left with no choice but to face him head-on, she defiantly rejects Mortmain’s demands and threatens to stab herself, rightly guessing the target of the attack, hoping to catch Mortmain off guard.
As Mortmain calls her threat a bluff, Tessa stabs herself and convinces him of her death.
Mortmain disappears just as Will arrives, Tessa reveals to him that she changed her form to a shot woman’s just she stabbed herself, so that the load of blood that appears on her seems like her own.
Despite everyone eventually figuring out that they had been tricked, they suffer the loss of one of their servants and both Mortmain and Nate escape, taking the institute’s storage of demonic energies.
Tessa and the Shadowhunters are obligated to provide a report of the conflict to the Clave, who will proceed with further inquiry.
Charlotte also extends an invitation for Tessa to stay at the Institute, which Tessa accepts. However, when Tessa seeks out Will to share in their victory, he rejects her. Feeling wounded, Tessa turns to Jem, who reveals his own feelings for her.
