Six people travel by overnight train from Marseilles to Paris. When they arrive, one of them, a young woman, is found dead in a sleeping berth. The police, led by Inspector Grazzi, investigate the other five passengers, suspecting that one among them was responsible. However, as the investigation is stepped up, the others start turning up dead. It's up to the remaining two to solve the case, lest they become the next victims.

Yves Montand
as L'inspecteur Grazzi

Pierre Mondy
as Le commissaire Tarquin

Jean-Louis Trintignant
as Éric Grandin, étudiant vétérinaire

Jacques Perrin
as Daniel

Catherine Allégret
as Bambi

Simone Signoret
as Eliane Darès, l'actrice éventée (Victime #4)

Claude Mann
as Jean-Loup Gabert, inspecteur stagière de Grazzi

Michel Piccoli
as René Cabourg, l'employé besogneux (Victime #2)

Pascale Roberts
as Georgette Thomas, assassinée (Victime #1)

Charles Denner
as Bob, l'amant sincère de Georgette Thomas