In post-World War II Paris, a diverse group of passengers boards a secure U.S. Army train bound for Frankfurt. The travelers include American, Russian, British, French, and German diplomats, scientists, and military personnel, all cleared to travel. A German passenger, Walther Reinhold Schünzel, tries to mingle with the group, but his overtures are rebuffed because of his nationality. A British man on board, Sterling Robert Coote, comes to believe he is Dr. Heinrich Bernhardt Paul Lukas, a prominent diplomat working to restore a peaceful Germany, and the mood on the car shifts dramatically. After retiring to his compartment, the man is killed by a bomb, and the others learn that the dead man was actually one of the doctor’s bodyguards. The real Bernhardt had been posing as an anonymous passenger, traveling with his secretary, Lucienne Merle Oberon.
At the bustling Frankfurt railway station, Bernhardt greets Walther, an old trusted friend, but before anyone realizes what has happened, Bernhardt is kidnapped. The U.S. Army launches a city-wide search, but when Lucienne begs her fellow travelers to help look for him, they initially refuse. One by one, however, they reconsider. Lucienne suggests they find Walther, unaware that he has betrayed Bernhardt in order to secure his detained wife’s return. When they reach his home, they discover Walther has hanged himself in the trauma of learning his wife had been dead all along.
The group splits up to cover the city. Lindley [Robert Ryan] accompanies Lucienne to various German-only nightclubs. At one, Lindley notices a woman smoking an unusually long cigarette, just like the ones Bernhardt has made specially for him. He pockets a discarded butt and shows Lucienne that it bears Bernhardt’s monogram. When the woman turns out to be an entertainer, a “mind-reader,” Lindley asks her where Bernhardt is. She flees, and her assistant prevents Lindley from following her. When Lindley and Lucienne question the American soldier who had been with the woman beforehand, Sergeant Barnes [Michael Harvey] reluctantly agrees to lead them to her home.
When they arrive at an abandoned brewery, Barnes turns out to be working with the kidnappers. Bernhardt is being held prisoner, and Lucienne and Bernhardt are taken captive. An undercover agent assigned to protecting Bernhardt had trailed the others to the hideout. He is shot when discovered but manages to escape and inform authorities of the location. American soldiers break in just as Bernhardt and Lucienne are about to be shot and free the three unharmed. Kessler [Otto Waldis], the ringleader, is chased by Perrot [Charles Korvin], a French train passenger and part of the search party. In a room alone together, Perrot shoots him dead after revealing that he is actually a German collaborator usurping Kessler’s position.
The passengers board the connecting train for Berlin. Perrot suggests that each of them take turns guarding Bernhardt in his compartment. He volunteers to be first. Uneasy, Lindley pieces together various clues suspecting Perrot, but they are dismissed by the others. At a stop Lindley sees a reflection of Perrot strangling Bernhardt in the window of an adjacent train and saves his life. Perrot is gunned down as he tries to escape.
The rest of the group is driven to the Brandenburg Gate for transit to their destinations in Berlin. Seeing them depart, Bernhardt wonders if there can be cooperation between nations, and after a moment of despair, he observes camaraderie between the once fractious British, American, and Russian men as they say their goodbyes. His hope is restored.
