Billy Harrow works at a London museum. During the excursion, the main exhibit, an aquarium with a giant squid, is missing, followed by the disappearance of a security guard. During interrogation by the police, Billy learns about the existence of a special department dealing with numerous sects in London. For an unknown reason, Billy becomes the object of a hunt for immortal assassins, and then a key player in the party of an occult London panicking before the apocalypse, populated by magicians, monsters and gods.