Six people travel by train overnight from Marseilles to Paris. When the train arrives at its destination, one of the passengers, a girl, is found dead in a sleeping berth. The police led by Inspector Grazzi investigate the other five passengers, suspecting that one of them was responsible. However, as the investigation is stepped up, the other passengers start turning up dead. It is then up to the last remaining two to solve the case, before they become the next victims.