In the Rocky Mountain division of a western railroad, the Barton brothers, Blake, Leroy and Gabby, a trio of thieving assassins, murder a guard on the Cheyenne express train. Railroad detective Luke Smith, called "Whispering" for his secretive way of investigating, is assigned to the case. In a night manhunt, Luke jumps a train and is reunited with an old friend, Murray Sinclair, who is in charge of the railroad's wrecking crew. A fire up ahead causes the train to stop as the Bartons shoot a depot operator. Arriving on the scene, Luke shoots Gabby and Leroy dead, but Blake escapes. In the mêlée, Luke is shot, but is saved by a harmonica in his pocket that was given to him by Murray's wife Marian, with whom he was once in love. Because his arm was nicked, Luke recuperates at Marian and Murray's ranch and discovers that Murray is involved with nefarious rancher Barney Rebstock and his gang. One day, Marian, who still loves Luke, tells him he should have married her five years earlier, even though he did not think he had enough to offer a wife. Believing Rebstock is hiding Barton in nearby Williams Canyon, Luke goes to town and gives him two days to run Barton out of the canyon. A gunfight follows, in which Luke kills Barton and is nearly killed by Rebstock's partner, Whitey Du Sang, whose involvement proves Rebstock's alliance with Barton. Marian, who has convinced Murray to stop the fight, arrives too late, and runs to Luke in relief, proving to Murray that his best friend and his wife are still in love. Later, Luke visits his old friends, Bill and Emmy Dansing, who run the town boardinghouse, and discusses with Bill the possibility that Murray has been funding his ranch by illegal means. Luke's suspicions are confirmed when, at the scene of a railroad wreckage, Murray orders his wrecking crew to fill his wagon with the spoils--much of which is salvageable. After George McCloud, the new division superintendent, orders Murray to unload the "loot" and is backed up by Luke, Murray and his crew defiantly smash the goods, and are fired. Luke admits to McCloud that Murray is stubborn, but agrees to appeal to the railroad president on his behalf to have him reinstated. Luke's request is denied, however, and Murray childishly blames Luke, who leaves town that night without even a good-bye from Murray. Rebstock then makes Murray his partner, and together they sabotage the railroad, derailing trains and stealing transported goods. Murray also begins frequenting the saloon in the evenings and consorting with friendly saloongirls. One night, Marian frankly tells Murray that his friends are wicked men, but he ignores her pleas. The railroad has, meanwhile, hired Luke to investigate the recent thefts, and he returns that night and comforts Marian. In the saloon, Luke tells Murray that everyone knows he was responsible for a recent theft of cattle, and offers to cover for him if he will break with Rebstock. Murray jealously accuses Luke of trying to steal Marian, and slugs him. Marian later tells Luke that she is leaving Murray, but Luke advises her to take Murray away instead so that he has a chance to reform. Murray agrees to leave after one last deal with Rebstock, and that night, during a train hold-up, Whitey kills a guard as Murray looks on. The next day, Luke, Bill and the sheriff lead a posse in search of the killer, and Whitey is identified. Marian tells Luke that Murray never came home after riding out with Rebstock the previous night. Meanwhile, Rebstock distributes the robbery's profits and pays Murray for his ranch. After Murray leaves, Whitey shoots Rebstock and absconds with the money. The posse locates the gang, and during a shootout, Murray is hit in the shoulder. Later, Luke kills Whitey, but allows Murray to escape. Murray returns to the ranch and, finding Marian packing, accuses her of running away with Luke. He then knocks her to the floor and orders her to prepare the buckboard. She finds Luke and Bill waiting in the barn, and Luke sends her with Bill to get a doctor. When Murray tries to shoot Bill and Marian, Luke shoots him in his side. Back in the house, Murray apologizes, while hiding a gun in his shirt. When Luke goes to fix him a drink, Murray pulls his gun on him, but is beaten to the draw by Luke. As his friend lays dying, Luke tells him he would have played it differently, but the only cards he had were the ones Murray dealt him. Murray dies, and Luke slowly rides away from the ranch.