Robert Crain (Marlon Brando) is given an assignment to work undercover aboard a German ship that is carrying rubber into the war area. He has to pretend to be a Nazi. He also has to look cruel in order to appear real. Muller (Yul Brynner) is in charge of the ship and is trying to hold on to his job in the merchant marines for Germany. He is kind, just, and has to control his first officer, who has been assigned to work under him but is a true Nazi. The ship also rescues U-boat survivors such as Esther (Janet Margolin), who used to live in a prison camp and is filled with anger and sorrow over her experiences.