The Germans have hidden a new radar site at Quistreham, France, a threat to the Allied invasion on June 1944. The government decides to send in an agent to locate it. A nurse disguised as a Sister manages to travel the countryside by bicycle searching for the radar site. Along the way, she falls in love with a German expatriate who helps hide her from the Nazis.

With time counting down to D-Day, escaping the daily destruction of RAF air raids, the nurse survives a brutal interrogation by the Gestapo, finally locating the radar site and saving many Allied lives. She becomes a witness to the invasion by British troops at Sword Beach, Normandy.