France, 1939: A talented singer, Ruth Lacroix has left Maine to live with her aunt and uncle, dreaming of performing at the Casino de Paris. But with the outbreak of war, and the heartbreaking news that her cousin has been killed by German forces, that goal is supplanted by another—to support France in any way she can.

Though Ruth has never driven a vehicle larger than the tractor on her parents' farm, she joins a friend in enlisting as a driver for the French ambulance corps. On their way to transfer injured soldiers to Dunkirk for evacuation, they encounter Jimmie, a British Royal Air Force pilot with No. 73 Squadron RAF, who has bailed out of his burning plane. As Dunkirk falls, blocking off the route to the northern coast, word spreads of a daring Allied plan to rescue the remaining troops and civilians from ports in western France: code name Operation Aerial.

Over two hazardous weeks, Ruth and Jimmie will journey hundreds of miles together, helping other refugees as they rush to reach the sea before they are overtaken by the Germany army. But all their courage and resilience offer no certainty in wartime, when a single stroke of luck, or a split-second decision, can mean the difference between life and death . . .