During World War II, the English country estate "Bletchley Park" served as a base for the secret service to decrypt German intelligence traffic. After the war, four women and former cryptanalysts meet again. On their own, they investigate a series of murders. Susan, Millie, Jean and Lucy deciphered the codes of the Nazis during World War II. After the war, in 1952, a series of murders occurs. When Susan recognizes a pattern, she rounds up her former colleagues.