In post-war Paris, revered German peacemaker Dr. Heinrich Bernhardt addresses a secret conference of United Nations representatives on the controversial topic of German reunification. In another part of the city, a coded message consisting of the number "9850," the European time "21:45," the letter "D" and the place "Sulzbach" is found tied to a dead pigeon. The French police are unable to decipher the message completely, but notify the Allied authorities to be on the alert for spy activities. That night, seven passengers board the same car of a Berlin-bound train: Robert F. Lindley, an American agricultural expert, Lucienne Mirbeau, a French secretary, German businessman Otto Franzen, James Sterling, an English schoolteacher assigned to "re-educate" the Germans, Lt. Maxim Kiroshilou of the Russian army, Henri Perrot, a former member of the French underground, and Hans Schmidt, a mysterious German. One Compartment "D" is occupied by an unseen dignitary, who Robert and James soon learn is Dr. Bernhardt. Robert then discovers that he has been moved to another compartment and that "D" is now empty. At 21:45, in the border town of Sulzbach, the train is stopped momentarily by an overturned horsecart. A few moments later, a man calling himself Dr. Heinrich Bernhardt enters his reassigned compartment and triggers a deadly grenade blast. Later, in war-devastated Frankfurt, the six male passengers are brought in for questioning at the American occupation headquarters. After they interrogate Robert, Colonel Johns and a major talk with Otto Franzen, who turns out to be the real Dr. Bernhardt and Lucienne's boss. Still concerned for Dr. Bernhardt's safety, the Americans insist that he continue his impersonation, but while standing alone at the Frankfurt train station, the German is approached by an old friend, Johann Walther. Seconds later, a woman screams, creating a distraction, and Dr. Bernhardt disappears. A desperate Lucienne reveals Dr. Bernhardt's impersonation to Robert, James, Perrot and Maxim and finally convinces the reluctant group to help her find her kidnapped boss. Dr. Bernhardt, meanwhile, is being kept by his German enemies at Walther's, who confesses that he betrayed his friend to obtain the address of his missing wife. That night, after a fruitless day of searching, the weary group stops at a public bulletin board, where Lucienne sees a notice posted by Walther. Believing that Walther may know something about Dr. Bernhardt, the team goes to his address, but Walther, who was informed by the Germans that his wife is dead, has hanged himself. With no leads, Lucienne is about to give up the search when Perrot suggests that they scour the underground German nightclubs. At one club, Robert and Lucienne notice a German woman smoking a cigarette identical to the type smoked by Dr. Bernhardt, and during a clown and mind reading act, Robert asks the performers if they know Dr. Bernhardt's location. The question leads to a fight, and in the mêlée, the clown, Ludwig, is knocked out and his costume is stolen. Sgt. Barnes, an American soldier who was flirting with the German woman, then leads Robert and Lucienne to her home in a bombed-out brewery. There the double-crossing Barnes, whose German name is Heinz, turns Robert and Lucienne over to Dr. Bernhardt's Nazi kidnappers, who include the clown and leader Kessler. Kessler confesses that he led Robert and Lucienne to the brewery because he believes that Lucienne will tell him the details of his unification plan, which Dr. Bernhardt has refused to reveal, in exchange for her boss's life. During the discussion, Ludwig bursts in and a gunfight ensues in which the fleeing impostor clown is wounded. When Lucienne then refuses to talk, Kessler prepares to execute both her and Dr. Bernhardt, but is interrupted by the arrival of Perrot and some American soldiers, who were tipped off by the dying impostor clown, who turns out to be the mysterious Hans Schmidt. In a corner of the brewery, Perrot gets Kessler alone and reveals himself to be the head spy. Mistrustful of Kessler, Perrot shoots him, then re-insinuates himself into the group. Later, as a grateful Dr. Bernhardt and the others continue on to Berlin, Perrot attempts to strangle the doctor in his train compartment. Robert stops the assassination, however, and Perrot is killed while trying to escape. In Berlin, after Lucienne promises Robert she will see him again some day, the American, English and Russian allies say hopeful goodbyes to one another, then drive off to their respective, separate sectors.