Billy Harrow works at a London museum. During a guided tour, the main exhibit vanishes—a tank with a giant squid—and then a security guard disappears as well. During questioning by the police, Billy learns about the existence of a special department dealing with the many sects of London.
For unknown reasons, Billy becomes the target of hunting by immortal assassins—and then, he turns into a key player in a game unfolding in occult London, gripped by panic about the approaching apocalypse. A city inhabited by wizard-fishers, monsters, and gods.