Siblings Lizzie, 14, and Jakob, 19, who have grown up in England, are the children of an American mother and Polish father. Their father died years before. Now, with England fighting World War II, their mother, who worked for the American embassy in London, is presumed dead after being caught in Poland on embassy work as the German invasion there began. Irrepressible Lizzie refuses to believe their mother is dead. She evades the chaperone accompanying her to their grandmother in Cleveland and makes her way to their London apartment, where she finds documents her mother secreted under a floorboard. Cautious Jakob, annoyed by his sister’s rogue behavior and stressed by his responsibilities, reluctantly brings Lizzie back to his lodgings near Bletchley Park, where he, a math student at Cambridge, is now working on the secret Enigma codebreaking efforts. Alternating chapters focus on Lizzie’s efforts to prove their mother is alive, helped by two new friends, and Jakob’s work in Hut 6 on breaking the hold of Enigma on German messages. Slowly, their efforts converge, bringing them closer as they realize their mother was involved in helping Polish codebreakers—who contributed greatly to the work the British are doing at Bletchley—flee the Germans during the Polish invasion. This swiftly paced story is steeped in real as well as fictional intrigue.