It's December 1941, and almost Christmas, when a bomb falls on Deepdean School for Girls. May and Nuala are whisked away from the chaos and sent to stay with May's big sister, Hazel Wong, who is working at Bletchley Park.

Within seconds, they're thrown straight into their most dangerous case yet. A codebreaker has been shot in what appears to be a tragic accident - until a suspicious and top-secret message is discovered in his pocket. Was he a spy, working for the other side? Was the message planted on him? Most importantly - was this murder?

Worst of all, Daisy's big brother, Bertie Wells, is being blamed. It’s up to May, Nuala and their friend Eric to prove Bertie's innocence and solve the mystery. But Bletchley is a strange and secretive place, full of Britain's most ingenious minds. They'll need to watch, listen, and puzzle out the incredible, unthinkable truth…