1940. As England prepares to fight the Nazis, three very different women answer the call to Bletchley Park, where the best minds in Britain train to break German military codes. Osla puts her fluent German to use as a translator of decoded enemy secrets. Mab works the legendary codebreaking machines and looks for a socially advantageous husband. Beth's shyness conceals a brilliant facility with puzzles, and she becomes one of the Park's few female cryptanalysts. 1947. As the royal wedding of Princess Elizabeth and Prince Philip whips post-war Britain into a fever, the three women 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.