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?

Cambridge, 1940. It is the first winter of the war and the snow is falling thick and fast. A college porter, crossing the ancient Mathematical Bridge on his nightly rounds, is startled to hear a child’s cries for help coming from the icy river below. Detective Inspector Eden Brooke is summoned by police whistle and commandeers a punt in a desperate attempt to save the child, but the flood carries the boy away into the night. By dawn there is no trace of the victim. The boy was Sean Flynn, part of a group of Irish Catholic children evacuated from a poor London parish. When an explosion causes damage at a factory engaged in war work and the bombers leave an Irish Republican slogan at the scene, Brooke questions whether there could be a connection between the two events. As more riddles come to light, he begins to close in on a killer, but there is one last it seems that Sean Flynn had his own startling secret.

A lone German bomber crosses the East coast of Britain on a moonless night in the long hot summer of 1940. The pilot picks up the silver thread of a river and following it to his target, drops his bomb over Cambridge’s rail yards. The shell falls short of its mark, and lands in a maze-like neighbourhood of terraced streets on the edge of the city’s medieval centre. D I Eden Brooke is first on the scene and discovers the body of an elderly woman, Nora Wylde, beside her shattered bed in a terrace house on Elm Street, two fingers on her left hand severed, in what looks like a brutal attempt by looters to steal her rings. When the next day Nora’s teenage granddaughter, Peggy, a munitions worker at Marshall’s Airfield, is reported missing, Brooke realises there is more to the situation than meets the eye.