In an aero-naval base, a marine engineer who was about to deliver the name of a redoubtable spy is murdered. Shortly afterwards three different men appear on the crime scene, each of them claiming to be Monsieur Sylvain, the expected sleuth. But which of the three investigators is actually Sylvain? And what about this delightful young lady who arouses one of the police inspectors? Couldn't she be the murderous spy?

The Marquis de Montauran was appointed to command the Chouans whose first two revolts were crushed. An aristocrat, Marie-Nathalie de Verneuil, is sent by Joseph Fouché to seduce and capture him.

Marie Leroux, who is married to Charles, an honest, understanding country doctor, lives an uneventful, rather monotonous life.Her husband is a kind man but he does not give her any thrill or excitement. One day, Marie meets Olivier Dumas-Beaulieu, a handsome industrialist, who is in the process of leaving his fiancée Corinne, despite her being pregnant by him. It is easy for the confirmed womanizer he is, to seduce Marie, who very foolishly thinks she has found true love. Shortly afterward Charles is shot dead by Olivier while the two men were having a quarrel about Marie. The latter, who finds the corpse, believes her husband has committed suicide. Which is not the police's opinion and Marie is arrested and condemned to ten years in prison. Annihilated by such unfair treatment and, worse, by the separation from her beloved eight-year-old daughter, she still manages to survive and to serve her sentence.