“Oh, joy, a new Peculiar Crimes Unit case by Christopher Fowler . . . the best fun is running all over the city with these amiable partners.”—The New York Times Book Review 

The brilliant Arthur Bryant and John May take the late, late shift in a cat-and-mouse hunt with a killer who preys on his victims at the same time every night—the lonely hour of 4 A.M.

When a man is found hanging upside down inside a willow tree on Hampstead Heath, surrounded by a baffling assortment of occult objects, the Peculiar Crimes Unit is called in to investigate. Was this a botched satanic ritual pulled off by bored teenagers, a gang initiation, or the work of a mastermind with grander intentions? Bryant and May set off in search of answers and are soon reminded that London is a city steeped in blood and magic. 

When another body is pulled from the river at dawn, it becomes clear that a killer lurks in the night. To catch him, the PCU switches to graveyard shifts, but the team still comes up short. As they explore a night city where the normal rules do not apply, they’re drawn deeper into a case that involves murder, arson, kidnapping, blackmail, loneliness, and bats. 

May takes a technological approach, while Bryant goes in search of his usual academics and misfits for help, for this investigation reveals impossibilities at every turn. How do you stop a killer who appears not to exist? Luckily, impossibilities are what the Peculiar Crimes Unit does best.