American agent John Phillips has been given a dangerous assignment. He boards the Paris to Zurich Express carrying important papers related to his mission. He retires to his sleeping berth but in the morning, when he cannot be awakened, the coach attendant and another passenger break into his compartment to find him dead of a gunshot wound. Inspector Girard of the Swiss Federal Office of Police, investigates the matter which at first appears to be a case of suicide. But subsequent forensics testing shows that the agent has been murdered, perhaps for the papers he carried. How could this have happened? The door of the compartment was locked and a safety chain engaged from within. The inspector and his team of detectives are called upon to pit their wits against the cunning of a ruthless killer.