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.