1940, Bletchley Park, Buckinghamshire.

Three very different women are recruited to the mysterious Bletchley Park, where the best minds in Britain train to break German military codes.

Vivacious debutante Osla has the dashing Prince Philip of Greece sending her roses - but she burns to prove herself as more than a society girl, working to translate decoded enemy secrets. Self-made Mab masters the legendary codebreaking machines as she conceals old wounds and the poverty of her East-End London upbringing. And shy local girl Beth is the outsider who trains as one of the Park's few female cryptanalysts.

1947, London.

Seven years after they first meet, on the eve of the royal wedding between Princess Elizabeth and Prince Philip, disaster threatens. Osla, Mab and Beth are estranged, their friendship torn apart by secrets and betrayal. Yet now they must race against the clock to crack one final code together, before it's too late, for them and for their country.