Cordelia Carstairs is a Shadowhunter, a warrior trained since childhood to battle demons. When her father is accused of a terrible crime, she and her brother travel to London in hopes of preventing the family's ruin and convincing the Shadowhunters of their father's innocence.

Her mother is desperate to see Cordelia married, but Cordelia is determined to be a hero rather than a bride. She is reunited with her childhood friends James and Lucie Herondale and drawn into their glittering world of midnight dances and supernatural salons, where vampires, warlocks, merfolk, and humans mingle. Yet Cordelia conceals a secret love for James, who is pledged to another.

Then a chain of grisly demonic murders strikes London. The creatures are unlike any the Shadowhunters have faced: they walk in daylight, inflict an incurable poison, and seem impossible to kill. As the city falls under quarantine, Cordelia and her friends discover that a link to a dark legacy has granted them incredible powers - and they learn the true, cruel price of being a hero.