Wicked Frenchwoman Marie Chaumontel is the mistress of Captain Henry Ravignac. She steals some papers from him, which she gives to the Germans. Then she escapes to Berlin while her lover is tried for her wrongdoing. He is found guilty of neglect, and he commits suicide. His brother, Lieutenant Charles Ravignac swears revenge, and he becomes a counterspy by going to work for the Germans.

Based on the true story of Melbourne Johns, an aircraft factory foreman sent to France to prevent the Nazis getting hold of some vital equipment.