Spring 1941. It is the third year of war, and when the siren sounds the people of Cambridge trudge to the city's public bomb shelters. Crowded, smoky, often raucous, the shelters have become a way of life for the poor. At dawn, the body of a young man is found in a shadowy corner of the Trinity Shelter, one of three on the city's great open space - Parker's Piece.

Detective Inspector Eden Brooke searches the body and finds no wallet or papers, save for a cinema ticket dated six months earlier. PC Vanessa Hill - a recruit to The Borough police from Girton College - uses her skills in fine art to sketch the dead man's face for a poster. An autopsy reveals the only clue to his death is the wound left by a hypodermic needle in the back of his neck. Brooke has a very puzzling case on his hands .