The story, loosely based on actual events, takes place in March 1943, when the Second World War was at its height. The cryptanalysis at Bletchley Park, Buckinghamshire, have a problem: the Nazi Germany U-boats have changed one of their code reference books used for Enigma machine ciphers, leading to a blackout in the flow of vital naval signals intelligence. The British cryptanalysts have cracked the Cryptanalysis of the Enigma#M4 (German Navy 4-rotor Enigma) once before, and they need to do it again in order to keep track of U-boat locations.