Set in 1952, `The Bletchley Circle' tells the story of four ordinary women who have returned to their normal lives after working as code-breakers at Bletchley Park during World War II, work that produced crucial intelligence that helped the Allies to victory while shortening the war. One of the women is Susan, who seems to be a regular 1950s housewife and mother. But underneath that conventional exterior lies a steely determination that is not to be underestimated. After not doing anything adventurous for nine years, she decides to try to solve the unresolved murders of Jane Hart and Patricia Oakes, having spotted a pattern of behaviour that no one else has discovered. But the police refuse to take her theory seriously when she approaches them. So Susan realises she must take matters into her own hands and bring the culprit to justice with the help of her former friends and fellow code-breakers.