1939, Cambridge: The opening weeks of the Second World War, and 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. 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, to a life lived away from the light. Within hours The Great Darkness has claimed a second victim. War, it seems, has many victims, but what links these crimes of the night?