Inge Karsten receives an unexpected visit from her former student friend Margit from the West. She is even more surprised to hear that she is married to her stepbrother Herbert, who was long thought to be missing and whom she had almost forgotten. He last lived in Canada. This presents Inge's husband Horst, who has an important job as a military pilot, with a conflict of conscience. The situation is even more serious than it seems at first glance. In truth, Margit is an agent and the whole story is a staged game by the West German secret service...