Cambridge, 1939: In the opening weeks of the Second World War, the first blackout - the Great Darkness - covers southern England, enveloping the city. Detective Inspector Eden Brooke, a wounded hero of the Great War, takes his nightly dip in the cool waters of the Cam. Though the night is full of alarms, in this Phoney War the enemy never comes. But daylight reveals a corpse on the riverside, the body torn apart by some unspeakable force. Brooke investigates, calling on the expertise and inspiration of a faithful group of fellow 'nighthawks' across the city, all condemned, like the detective himself, to a life lived away from the light. Within hours, the Great Darkness has claimed a second victim. War, it seems, has many casualties. But what links these crimes of the night?