Twelve-year-old Robyn has grown up in Bletchley Park, where her father works as a driver. When she's not at school, there's nothing she likes more than helping her dad in the garages.

Then the war begins and everything at Bletchley changes.

Robyn is assigned to help with the carrier pigeons that take messages to the Allies. But first, she must sign the Official Secrets Act and is ordered not to leave the grounds of the park. While Bletchley is buzzing with people recruited for the war effort and all eyes are on the skies, Robyn becomes convinced that there's something sinister going on within Bletchley Park itself.

Together with her friends Mary and Ned, Robyn resolves to uncover the enemy in their midst . . .