Charlotte loves puzzles. On her ninth birthday, she receives a card from her great-grandmother, on which she finds a difficult code. Soon she embarks on a quest full of codes and riddles. While she tries to solve them, Charlotte discovers important details about the role her great-grandmother played in the Second World War. She also learns about three historical sites in England: Parker's Piece, the River Cam, and Blechley Park. You'll learn about these places, too!