Nathan's father has been arrested. He works for the Ministry of Defence and is accused of leaking top secret information.

But as he is dragged into a police car, he gives Nathan a message.

It leads to a riddle, but it's not from Dad. It's from an ex-Bletchley Park employee, Lily Kenley, and was written in 1940. Nathan begins to follow the clues left behind by Lily.

But how can this war-time story link to his father's fate?

Hope for Dad's appeal is fading fast. He must solve the puzzle. Time is running out.