Because of his German parentage, government agent Carl Steelman is assigned to by his chief, Craig, to join the Nazi Bund, to obtain information on the underground activities of the group. Steelman is welcomed and becomes a trusted member of the group. His father, Julius Steelman, unaware of this son's real mission, drives him from their home. The government agents raid a Bund meeting and Steelman assumes the identity of one of the members who was about to depart to Germany for training. In Berlin, Steelman meets and falls in love with Helga Lorenz, a member of the German underground. In order to hide his true identity and complete his mission, Steelman has to betray the girl to the Gestapo, but later manages to rescue her and see her safely on her way to England. Frau Reiter, wife of the man whose identity Steelman has assumed, threatens to turn him over to the police. He convinces the Gestapo that she is crazy and she is confined to an insane asylum. His training completed, Steelman is sent to America, via submarine, in charge of a group of eight saboteurs. Before they depart the sub, off the Long Island coast, Steelman plants a time-bomb in the sub.