In 1939, cruel and tyrannical businessman Simon Le Tescou, tries to reunite his family for his 70th birthday celebration only to be found dead inside a locked room on the night of his birthday feast.

While investigating Simon Le Tescou's death, Commissioner Larosière and Inspector Emile Lampion soon discover that nobody has an alibi and everybody has a motive.

Commissioner Larosière and Inspector Lampion decide to stay overnight in the castle in order to set a trap for the murderer.

As the killer's tally rises, Commissioner Larosière is recovering from his injuries in a hospital, but he remains determined to extract a confession.