Carla Lehmann is "Susan" who finds herself in an hospital bed in Algeria just as the Allies celebrate a resounding victory over the Nazis. She recounts a tale to one of her nurses of her role in enabling that victory... She encounters an escaping British prisoner "Thurston" (James Mason) whom she shelters, and who manages to rope her into his plan to obtain a camera that has photographs of the venue of a soon-to-happen top secret meeting essential to the planning of the invasion. Soon she is embroiled in his operation and with it's perils, and they must retrieve the photographs and smuggle them...