Tom Andrews (Randolph Scott) is a surveyor involved in the building of the Canadian Pacific Railway, which is stalled by the Rocky Mountains. While mapping a route through the mountains, Andrews is shot at by Dirk Rourke (Victor Jory), a fur trader, and his accomplice, Cagle (Don Haggerty). When Andrews returns to the construction camp, he sees Cagle working there and attacks him. Dr Edith Cabot (Jane Wyatt) intervenes - she is a pacifist and disapproves of violence.
Andrews heads for Calgary to his girlfriend, Cecille Gautier (Nancy Olson). Andrews and Cecille's father (John Parrish) attend a meeting at which Rourke campaigns against the railway, claiming it will mean the end of trade in the area. Andrews tries to convince the crowd that the railway will benefit them, and that Rourke objects only because it will end his business monopoly. He and Rourke get into a fist fight, which is broken up by Père Lacombe (John Hamilton). To keep the peace (and because Cecille's father sides with Rourke), Andrews decides to return to work on the railway; Cecille, not understanding and upset at his intention to spend another year away, breaks their engagement.
At the camp, Dynamite Dawson (J. Carrol Naish) tells Andrews of his suspicion that Indians have been stealing dynamite; Andrews later finds several cases of dynamite buried at an Indian village. The chief says that young braves had been paid by a white man to steal them. Back at the camp, Cagle and Rourke shoot at one of the cases as Andrews unloads them. Andrews is seriously injured in the resulting explosion. On board a train back to the base hospital, Dr Cabot transfuses her own blood to save Andrews's life.
Meanwhile, back at the ranch, Cecille overhears Rourke, her father and others plotting to stop the railway by inciting the Indians to cause trouble. When she objects, her father threatens to disown her. Suspecting Rourke of having caused the explosion that injured Andrews, she decides to join him at the hospital, but is dissuaded by Dynamite, who persuades her to return home to gather evidence and to pass it on to him.
Saboteurs cause a series of explosions, closing down railway construction for the winter. By the spring, Andrews has recovered, and has wooed Edith. Hearing of trouble at the camp, Andrews prepares to return, but Edith persuades him not to wear a gun. Rourke has stirred up the Indians and has established a saloon and gambling hall at the camp. When a man is shot at the saloon, Edith goes to help but is too late. Andrews straps on his guns again and closes the saloon. Cecille arrives, announcing that Rourke and the Indians are about to attack; Dynamite rides off to get reinforcements while the others take refuge in the hospital car.
Cecille's injured father is brought in and tells them that they are considerably outnumbered. Andrews goes after Rourke and Cagle and, as the Indians prepare for a final attack, shoots Cagle. Rourke is killed by a falling branch from a tree that had been set on fire as a signal to the Indians. The Indians attack but reinforcements arrive by train at the last moment, and they are driven off. Later, the Indian chief walks into the camp and sues for peace. Dismayed by the violence she has experienced in the west, Edith takes the train back east.
Cecille looks on as Andrews joins the train and it moves off, but after saying goodbye to Edith, he jumps off and comes back to her.Randolph Scott as Tom Andrews
Jane Wyatt as Dr. Edith Cabot
J. Carrol Naish as Dynamite Dawson
Victor Jory as Dirk Rourke
Nancy Olson as Cecille Gautier
Robert Barrat as Cornelius Van Horne
Walter Sande as Mike Brannigan
Grandon Rhodes as Dr. Mason
Don Haggerty as Cagle
John Parrish as Mr. Gautier
Mary Kent as Mrs. Gautier
John Hamilton as Pere Lacombe
Howard Negley as Mallis