Officially, The Bletchley Circle's Millie (Rachael Stirling), Susan (Anna Maxwell Martin), Lucy (Sophie Rundle), and Jean (Julie Graham) did clerical work during World War II. In reality, the women were part of a group of military codebreakers who helped the Allies win the war. Nine years later, they've all moved on with their lives. That is, until Susan starts seeing a pattern in reports of a British serial killer preying on young women. She thinks with the help of her three ex-coworkers, she can find the killer. The only problem? He might find them first.