In 1929, Bill White, Grant Withers, is a steam locomotive fireman and boozing womanizer who is evicted from his boarding house for excessive drinking and late rental payments. Needing a new place to live, he accepts the invitation from his longtime friend and fellow engineer, Jack Kulper, Regis Toomey, to move into his home, where he resides happily with his wife Lily, Mary Astor. This new living arrangement brings changes to relationships as the months pass.
Bill and Lily's own friendship, which at first is playful and innocent, evolves into a passionate love between them. Hesitant to hurt Jack, they try to keep their feelings secret, at least for a while; but Jack begins to notice differences in his wife's demeanor and becomes suspicious when he finds that Bill has suddenly moved out of their house.
Inside the cab of the oil-fired steam locomotive that the two men operate together at the nearby rail yard, Jack confronts Bill and, in a moment of brutal honesty, Bill admits that Lily and he have fallen in love. In the fistfight that ensues, Jack falls during the struggle, strikes his head, and is permanently blinded by the injury.
During his convalescence at home, Lily tries to rededicate herself to her marriage; however, Jack resents his dependency on his wife. Increasingly frustrated by his situation, he insists that Lily leave town for a few weeks to visit her parents, explaining that he needs emotional space and that he also wants her away from the dangers of expected floods due to rainstorms in the area. Shortly after Lily's departure, Jack learns from rail workers that Bill plans to drive a train of flatcars stacked with bags of cement onto a vital river bridge, the desperate hope being that the combined weight of the train and its load will bolster the bridge and prevent it from being swept away by the rising floodwaters. Stumbling that night through a heavy downpour and literally feeling his way to the rail line, sightless Jack manages to locate Bill and knock him unconscious before he begins what everyone deems a suicidal mission. Jack then takes charge of the engine's controls, but before moving onto the wavering bridge, he pushes Bill off the locomotive to safety. Once on the bridge, the entire train plummets into the river as the structure collapses, and Jack drowns in the raging river.
Months after the tragedy, Bill, still employed as a locomotive fireman, goes into the depot's diner for some quick food before returning to his train. Nearby, Lily arrives on another train and enters the same restaurant carrying her luggage. The two see one another and engage in some awkward small talk before Lily remarks that she intends to remain in the community, fix up her house and yard, and plant a new spring garden. Then, with a warm smile, she invites Bill to drop by to help her with the work. Bill runs out of the diner to re-board his moving train. Lily stands in the restaurant's doorway watching Bill climb to the top of a long line of freight cars and then running forward toward the engine. As he jumps from one car's roof to the next he raises his arms skyward.
