Seventeen-year-old Will is the sort of young man nice Victorian girls were warned against. He drinks, gambles, and enjoys the company of ladies of questionable virtue—at least, as much as he enjoys anything. Will hates everyone and everything, with the possible exception of Jem, and even that's in some doubt. Charlotte despairs of keeping him alive past the age of nineteen. When he finds himself oddly drawn to Tessa, his friends begin to hope that the Downworlder girl will prove Will's unlikely salvation—but as Tessa grows closer to the bleak secret that makes his life a prison, danger threatens to destroy them both if she ever finds out the truth.

Tessa has spent the first sixteen years of her life being overprotected by her aunt after the death of her parents in a carriage accident. She lives almost all her life in books, dreaming of having the kind of adventures the heroines she reads about do—until she is forced to travel from New York to London when her brother Nathaniel goes missing and only a pair of mysterious—and sinister—women called the Darke Sisters seem to know where he might have gone. The Shadowhunters of the London Institute may be Tessa's only hope of finding Nate, but their mysterious world of demons and Downworlders holds many dangers for her—especially when she realizes she might be more a part of it than she'd thought.

Brought up in the Institute in Shanghai, Jem's parents were killed by a demon who allowed Jem to live—but with a terrible price to be paid. Frail and silver-haired, Jem seems an unlikely demon-killer, but his skills and swift intelligence make him a formidable foe. He finds a kindred spirit in Tessa as both of them feel they are torn between two worlds, neither belonging completely to one or to the other. As Jem's affection for her grows, will Will have to hurt the only person in his life he's ever seemed to care about?

Jessamine's parents, like Will's were Shadowhunters who left the Clave. Their deaths in a fire orphaned her, leaving her to be brought up in the Institute by Charlotte and Henry. Jessamine hates everything about being a Shadowhunter, from the training to the danger to the existence of demons, which she seems "smelly and disgusting." Armed with a razor-edged parasol and an equally sharp tongue, Jessamine can be dangerous if she wants to be—to herself as much as to the supernatural creatures she's sometimes forced to encounter.

Absent-minded and brilliant, Henry spends his waking hours in the crypt of the Institute, inventing fabulous machines and weapons out of cogs cams and gears. Unfortunately just as many of Henry's inventions don't work as the ones that do, and Henry is as likely to set himself on fire as he is to invent something new and amazing. When Tessa finds herself followed by a silent band of assasins who turn out to be automata—human beings made from clockwork parts—it is Henry who must decipher how they work in order to find the demonic mechanical genius at the heart of the most evil plan London's supernatural world has ever seen.

Twenty-two year-old Charlotte runs the entire Institute with a capable hand. Kind and loving, she does her best to care for the orphaned Shadowhunters who live under her roof while hiding her loneliness from the absent-minded husband she loves. It seems as if there's nothing Charlotte can't manage—until Tessa's search for her missing brother begins to uncover a web of corruption and deceit in London's Downworld that threatens to tear London's Conclave of Shadowhunters apart.