In the Soviet Union, American reporter McKinley "Mac" Thompson (Clark Gable) secretly writes unflattering stories about the Soviet Union, attributed to "Comrade X", for his newspaper. His identity is discovered by his valet, Vanya (Felix Bressart), who blackmails Mac into promising to get his daughter, a streetcar conductor named Theodore (Hedy Lamarr), out of the country. Theodore agrees to a sham marriage so she can spread the message of the benefits of Communism to the rest of the world. However, Commissar Vasiliev (Oscar Homolka) is determined to unmask and arrest Comrade X.