Twin sisters Dot and Dash Wilson share many things, and while they are practically inseparable, they are nothing alike. Dot is fascinated by books, puzzles, and Morse code, a language taught to both girls by their father, a WWI veteran. Dash's days are filled with fixing engines, dancing with friends, and dreaming of flying airplanes. Almost always at their side is their best friend Gus--until war breaks out and he enlists in the army, deploying to an unknown front. Determined to do their duty, both girls join the WRENS, Dash as a mechanic and Dot as a typist. Before long, Dot's fixation on patterns and numbers takes her from HMCS Coverdale, a covert listening and codebreaking station working with Bletchley Park in England, to Camp X, a top-secret spy school. But when personal tragedy strikes the family, Dot's oath of secrecy causes a rift between the sisters. Eager to leave her pain behind, Dash jumps at the opportunity to train as a pilot with the Air Transport Auxiliary, where she risks her life to ferry aircraft and troops across the battlefields of Europe. Meanwhile Dot is drawn into the Allies' preparations for D-Day. But Dot's loyalties are put to the test once more when someone close to her goes missing in Nazi-occupied territory. With everyone's eyes on Operation Overlord, Dot must use every skill at her disposal to save those she loves before it's too late.