A violent murder is discovered in a locked sleeping-car on an international express, prompting the conductor to detain the passengers and involve local authorities. The narrative confines attention to the small group aboard and the officials who question them, as suspicion moves among occupants and procedural difficulties and language differences complicate inquiries. Tightly observed scenes aboard the train and at subsequent examinations reveal contrasting personalities, social manners, and the mechanics of police procedure, while the plot advances through clues, alibis, and gradual revelations that lead toward identifying motive and perpetrator.