Autumn, 1941. The third year of war and still the siren sounds, driving the people of Cambridge from their beds to trudge to one of the city's crowded bomb shelters. At dawn, the body of a young man is found in a shadowy corner. Everything points to suicide, but Detective Inspector Eden Brooke has his doubts given the unidentified man has a suspiciously tropical suntan and the detective's office telephone number scrawled on the back of his hand. Did the victim know his life was in danger?

Brooke's caseload doubles when a local factory making submarine periscopes discovers it has a saboteur on the workforce. The city's police are at full-stretch. Then two more men are found dead in the city's shelters. A blackout killer is on the loose. The trail leads Brooke to the university, a laboratory which hides a brutal secret, and the key to a startling mystery.