Summer, 1940. Nineteen-year-old Jakob Novis and his younger sister Lizzie share a love of riddles and puzzles. And now they're living inside of one. The siblings find themselves amid one of the greatest secrets of World War II - Britain's eccentric codebreaking factory at Bletchley Park. As Jakob joins Bletchley's top minds to crack the Nazis' Enigma cipher, fourteen-year-old Lizzie embarks on a mission to solve the mysterious disappearance of their mother. Could the riddles of Enigma and their mother's disappearance be somehow connected? Jakob and Lizzie must work together as they race to decipher clues which unravel a shocking puzzle that presents the ultimate challenge: How long must a secret be kept?