The witnesses of a train murder must take the investigation into their own hands if they want to survive.

Six people travel in a railroad sleeping car from Marseilles to Paris. Upon their arrival, a woman is found dead in one of the berths. The police investigate the other five passengers, suspecting that one of them committed the homicide, but the suspects are killed one by one. The last two must solve the case themselves before they become the next victims.—mnfowler