At Dunkirk during World War Two, the unlikely paths of Eve Lacy and Isobel Bloomfield cross. Eve has been nursing injured soldiers on the front line and Isobel, a London debutante, is biding time until she is accepted for the Air Transport Auxillary. A shared birthday provides the basis of a friendship which continues when they arrive in back in England. The friends go their separate ways but only after they encounter Jack Wallace, an ace fighter pilot, who connects them by a fragile but invisible thread. Will this be a test of their loyalty?