While in Hamburg in the 1930s, Jack Clarke has run into some distant relatives. There is Johannes, an enthusiastic Nazi - and his elder brother, Kurt, who is in love with Magda, a wealthy Jewish girl. Jack and Kurt become friends and when Jack is advised to leave Germany as the political situation worsens, Kurt asks him to help Magda escape. Kurt, whose loyalty to his country has been strained, also reveals to Jack that the Germany navy has built a super-U-cruiser for commerce raiding in the event of war. On his return to England, Jack passes on this information. But the moral confusion of Jack and his relatives has begun to extend beyond politics, now that Jack's feelings for Magda have begun to take a treacherous turn.