Jeff Warren, a returning Korean war veteran, resumes his job as railroad engineer with longtime friend Alec Simmons. Uncertain of his future, Jeff gratefully accepts Alec's invitation to reoccupy his room at the Simmons home, where he is cheerfully greeted by Alec's wife Vera and their daughter and Jeff's childhood friend Ellen. At the railroad yard, Alec introduces Jeff to Carl Buckley, the assistant yard master. A few days later Carl informs his wife Vicki that due to a fit of temper with his boss, he was fired. Vicki is supportive and assures Carl they can move or she can return to work. Carl refuses to consider either possibility, then pleads with Vicki to contact the powerful John Owens, her mother's former boss, to use his influence with the railroad line to have Carl reinstated. Vicki resists the suggestion, but under Carl's continued pressure, at last agrees. The following day Carl and Vicki take the train to the next town, where Vicki insists that she meet Owens alone. Vicki returns late and tells Carl that Owens has gotten him rehired, but Carl grows suspicious of her long visit and questions her harshly. When Vicki protests, Carl beats her, then forces her to write an intimate letter to Owens, asking to meet him the next day on the train. The following day, Carl and Vicki board Owens' train, as does Jeff. Once the train is underway, Carl drags Vicki to Owens' compartment, then knifes the older man to death. Afterward, Carl takes Vicki's letter from Owens' body and his wallet to make the attack appear to be motivated by robbery. As they are about to leave Owens' compartment, Carl notices Jeff smoking between rail cars and sends Vicki to distract him. Vicki lures Jeff to another compartment, where he tries to kiss her, before she flees back to Carl. Upon arriving at the station, Jeff is dismayed to run into Carl, who formally introduces him to his wife. During testimony at the inquest of Owens' murder, Jeff avoids identifying Vicki as the passenger he witnessed near Owens' compartment. Later, Jeff runs into a morose Vicki and drunken Carl at a bar and helps Vicki get her husband home. Vicki thanks Jeff for his testimony and explains that she went to Owens' compartment to ask him for help with Carl's job, but found him dead. Vicki then confides that Carl abuses her and shows him several welts on her shoulders. The next day Vicki and Carl quarrel over Owens' murder, and Carl refuses to return Vicki's letter. Several days later, Vicki calls Jeff and meets with him to bemoan her crumbling marriage. Jeff is sympathetic and the two begin an affair. A few days later Ellen come to the train yard to visit Jeff. She asks Jeff about his dating Vicki but he remains evasive and, hurt, Ellen leaves. On the train, Alec cautions Jeff about seeing a married woman. Several days later in a borrowed apartment, Jeff complains to Vicki about their covert meetings and suggests they go to the police about Carl. Vicki admits that she lied about Owens' murder and reveals the truth, ending with Carl blackmailing her with the letter. Jeff grows angry, declaring that Vicki has now made him an accomplice by confiding in him and asks her if Carl had reason to be jealous of Owens. Vicki describes how she and her mother lived with Owens when Vicki was a teenager and Carl became obsessed over it. Jeff remains dubious, but promises to stay with Vicki. One evening Vicki calls Jeff and upon meeting him, declares that Carl has been fired again and intends to sell the house and leave town. Startled, Jeff nevertheless demands Vicki stay and vows to get the letter from Carl. Vicki fears that Carl will kill them both should he discover their affair and makes a veiled suggestion that Jeff kill him. That night, Jeff finds Carl drunk in a bar and follows him across the yard, intending to murder him. Later he returns to Vicki and confesses he was unable to murder Carl and helped him to the hospital instead. Jeff asks Vicki again about Owens, and she angrily declares that the older man assaulted her when she was sixteen and once Carl learned this, it ruined their marriage. She accuses Jeff of harboring the same jealousies as Carl. Feeling that he has been used, Jeff breaks with Vicki but leaves her the letter, which he found in Carl's pocket. The next day before his regular run, Jeff contemplates Ellen's invitation to a dance as Vicki boards the train. Just as the train departs, Carl drunkenly stumbles into Vicki's compartment and demands to know where she is going. She informs him that she is leaving him and laughs when he pleads with her to stay and offers to destroy the letter. When Carl accuses her of having an affair with Jeff, she agrees, then admits she did allow Owens to seduce her to win Carl's job back. Hysterical, Carl strangles Vicki to death. Oblivious in the engine car, Jeff and Alec mend their friendship.