Genvieve Stonefield's husband Angus is missing when she seeks William Monk's help to find him. She is convinced that he has been murdered by his twin brother Caleb, a shadowy figure who lives in the slums bordering the Thames and has always hated his respectable businessman brother. Although worried about Hester Latterly's health as she nurses victims of a typhoid outbreak in Limehouse, and threatened by a personal scandal, Monk is determined to bring one of the most bizarre cases he has ever encountered to its conclusion.