This story of espionage in World War I is based on a true story. Marthe McKenna (Madeleine Carroll) is a nurse from Belgium who uses her beauty and charm to serve her native land during the war. McKenna is able to ferret out secret information from German officers and pass it along to Allied intelligence officers with the help of fellow spy Stephan (Herbert Marshall). When the evil Commandant Oberaertz (Conrad Veidt) discovers what McKenna has been doing, she's sentenced to be executed, and Stephan must step in to save her. I Was a Spy was the first American sound feature for German actor Conrad Veidt, who electrified audiences with his performance in the silent classic Das Kabinett des Doktor Caligari; Veidt left his homeland when the Nazis began their rise to power, though ironically he was to play a number of Nazi villains during his stay in Hollywood.