End of WW2 : the German officers lock all the men between 18 and 65 into the "Fort National" on an islet in Saint-Malo.

Cora, the wife of a wealthy Parisian businessman, Daniel, is involved in an affair with Henri, an unscrupulous garage mechanic who has set his sights on Cora's fortune. Daniel becomes suspicious and hires a detective to investigate, but the detective is unsuccessful in pinpointing Cora's extramarital activities.

A young woman is forced into espionage so that her father can escape blackmail. The love of an aviator helps her regain her freedom and secure the arrest of the members of the network, whose leader commits suicide before the police arrive.

Married to a businessman who neglects her, Laurence cheats on him with Mauliaire, a young actor. Letters and photos of the young woman addressed to her lover are stolen by a blackmailer who demands a large sum in exchange. Distraught, Laurence goes to see Maître Tom Savier and displays such persuasive charm that the lawyer gives in to her demands and decides to get her out of this predicament. But he is soon spotted by the blackmailer. Meanwhile, Laurence disappears. Following a visit to Mauliaire, Tom discovers that the woman he has met is not Laurance, but Lydie Audry, the industrialist's mistress. As for Laurence, her mutilated corpse is discovered at a level crossing. Was it a crime? Accident? Suicide? Inspector Muller is put in charge of the investigation, and his suspicions turn to the blackmailer. After many twists and turns, the real culprit is unmasked.