On a cold fall morning, Nathalie Castellane is found murdered near a swamp. The night before she was at the restaurant with Louis and Hélène, her rich parents, and Eric, her brother. There she had told them about her intention to marry Philippe Laclos, the manager of the Castellane group. When leaving the restaurant, Nathalie had looked very worried... After being notified of Nathalie's death, all the protagonists sink into silence. Which will not help police inspector Tourneur to establish the truth...