Sixteen-year-old Tessa Grey travels to London to find her brother during the reign of Queen Victoria. She journeys to the world below London, a world full of vampires, warlocks and other supernatural beings. This is Downworld, where the Shadowhunters fight against the demons and maintain law and order. Tessa is soon captured by the mysterious Dark Sisters who tell her that she is also one of them, a Downworlder. She has the power to transform at will into another person. Before she has a chance to control her powers, she discovers that there is someone who will do anything to claim it for himself by marrying her. Tessa will need to work with the Shadowhunters if she wants any chance of saving her brother, but she is soon caught between two close friends. The fragile Jem and the blue-eyed Will, both of whom begin to harbour feelings for her.