1940. As England prepares to fight the Nazis, three very different women answer the call to mysterious country estate Bletchley Park, where the best minds in Britain train to break German military codes. Vivacious debutante Osla is the girl who has everything, but she burns to prove herself as more than a society girl. Imperious self-made Mab, works the legendary codebreaking machines as she conceals old wounds. Both Osla and Mab are quick to see the potential in local village spinster Beth, one of the Park's few female cryptanalysts. But war, loss, and the impossible pressure of secrecy will tear the three apart. 1947. As the royal wedding of Princess Elizabeth and Prince Philip whips post-war Britain into a fever, three friends-turned-enemies are reunited by a mysterious encrypted letter, the key to which lies buried in the long-ago betrayal that destroyed their friendship and left one of them confined to an asylum.

Joining the elite Bletchley Park codebreaking team during World War II, three women from very different walks of life uncover a spy's dangerous agenda years later against the backdrop of the royal wedding of Elizabeth and Philip.