Mary Dumont is a third officer in the Women's Royal Naval Service working at the Admiralty Communications Centre in London in 1942 where she is recruited by the Special Operations Executive (SOE) to become a wireless operator-saboteur. Parachuted into France, Mary joins a small band of resistance fighters where she leads a double life, a schoolteacher by day, a wireless operator by night; all goes horribly wrong, Mary is captured and tortured by an SS major who is determined to obtain both her codes and the names of her companions. Mary makes a daring escape during an air raid, unknown to her, German Intelligence has infiltrated SOE, a double agent known to Mary has also been parachuted into France to capture a Dutch scientist who is escaping to England with plans for the German V1 flying bomb. Evading capture, Mary's resistance cell must get the scientist to England before he can be handed over to the Gestapo, but first Mary must eliminate the German double agent. Just when she thinks it is safe on returning to England, Mary and her companions discover the identity of the traitor within SOE, they must try to eliminate him before he can return the scientist to the Germans.