Agents of the Federal Bureau of Investigation alert the local FBI chief, Ken Crowley, that a top Communist agent, Gerhardt Eisler, is coming to Pittsburgh. Meanwhile, Matt Cvetic, a second-generation Slovenian American, is at a family gathering where everyone except his mother is uncomfortable around him, including his sixteen-year-old son Dick. When Matt must leave in response to a call he receives ordering him to Eisler's hotel suite, his brother Joe accuses him of being a "Red." After he reports by pay phone to Crowley's assistant Mason, Matt arrives at Eisler's luxury hotel room, where an expensive spread, with caviar and champagne, is laid out for the local Communist officials. Jim Blandon, the Pittsburgh party leader, introduces Matt to Eisler, commending Matt for bringing Slovenians into the party and using his position in the personnel office of North American Steel Company to hire party members for key positions. Eisler promotes Matt to the position of chief party organizer for the Pittsburgh district, then orders the group to create discontent in the Pittsburgh workers. The next day, Matt is called to Dick's school, where he learns that Dick has been scrapping with students over the question of Matt's party membership. When Matt privately admits to Dick that he is a member, Dick expresses his shame. That evening at his apartment, Matt writes a letter explaining why he works undercover for the FBI as a Communist party infiltrator, planning to entrust it to the priest, Father Novac, with instructions to give it to Dick should he die. Although weary of the burden of his double life, Matt cannot clear his name without jeopardizing the FBI's work. He hides his displeasure when Eve Merrick, who introduced herself to him that afternoon at the school, shows up to confide that she, too, is a party member. Later, at the Communist headquarters, Blandon tells Matt that Eisler wants them to engineer a strike at the steel mill. As the room has been wired with a hidden microphone by the FBI, Crowley is able to warn Matt later that Eve has orders to shadow him. Matt's mother dies, and after the funeral, Novac returns Matt's letter, as he is being transferred to Rome. Matt, who is accompanied by Eve, puts the letter in his coat, but it falls to the ground when Joe, angered by the presence of Matt's "comrades," slugs him. When Matt later discovers the letter missing, he fears that Eve has it, until he hears a secret FBI recording of her report to Blandon, during which she does not mention it. At the strike, Blandon has thugs physically assault opposing union members, and Eve is disgusted by the violence. Matt realizes that they are being watched by Blandon's informers, so he reprimands her for her disloyalty. Then, after Crowley and Mason promise protection for Eve, Matt reports the incident to Blandon. To keep his cover intact, Matt later confronts Eve about her loyalty in front of the other officials, and she admits her disillusionment and quits. Matt guesses that Blandon has ordered her death and convinces her to leave town. Eve returns his letter, saying how it helped her to repudiate Communism. After Eve leaves with Matt for the train station, Blandon's thugs break into her apartment and stab a mysterious man who is trying to arrest them. Meanwhile, seeing two men follow Eve, Matt boards her train and stops them from killing her. After a fight in which both of the would-be assassins are killed, Eve is sent safely out of town and Matt resumes his cover, as no one alive can incriminate him. However, after the newspapers report the deaths of the two men, Matt is interrogated and beaten by Blandon and Clyde Garson, a visiting Communist leader. Policemen show up fortuitously to arrest Matt for the murder of Jim Broderick, an FBI agent found dead in Eve's apartment. After Matt is taken away, Mason and Crowley, who were behind his rescue, order him to testify against eleven New York Communists and suggest that, until the trial, he is safer behind bars. Later, the district attorney decides that Matt's testimony is unnecessary to convict the defendants, and Blandon springs Matt from jail, after having second thoughts about his suspicions. All of the Pittsburgh Communist leaders are subpoenaed to appear before the House Un-American Activities Committee. On the day of Matt's appearance, Crowley brings Joe and Dick to hear Matt's testimony, in which Matt states that he has been an undercover FBI agent for nine years. Afterward, Matt is free to resume a normal life with his proud family.