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 - beauty, wealth, and the dashing Prince Philip of Greece sending her roses - but she burns to prove herself as more than a society girl, and puts her fluent German to use as a translator of decoded enemy secrets. Imperious self-made Mab, product of east-end London poverty, works the legendary codebreaking machines as she conceals old wounds and looks for a socially advantageous husband. Both Osla and Mab are quick to see the potential in local village spinster Beth, whose shyness conceals a brilliant facility with puzzles, and soon Beth spreads her wings as 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. A mysterious traitor has emerged from the shadows of their Bletchley Park past, and now Osla, Mab, and Beth must resurrect their old alliance and crack one last code together. But each petal they remove from the rose code brings danger - and their true enemy - closer...