Germany, 1945: The 25-year-old Ellen is active in the resistance movement against the Nazis. When she meets Robert, a charismatic English secret agent, the two begin to have an affair.
Ellen is ordered to spy on the German naval officer Hans. He is scheduled to serve as Chief Officer aboard the submarine U 864, which has been given a secret mission. Against her will, Ellen falls for Hans. Although he is engaged to another woman, Hans is strongly attracted to her as well.
Eventually, Ellen succeeds in delivering critical information the submarines precise route to Japan. The English had suspected that Hitler planned to deliver his ominous wonder weapon to the weakened Japanese. The U 864 leaves its port. Because the war has been going badly for the Germans, the mood onboard is grim.