When cowboy Sunset Carson rides into the town of Cimarron looking for Ted Cartwright, he meets Dr. Pinky Snyder, who tells him that Cimarron has recently been plagued by an outbreak of lawlessness. Sunset then goes to the Express office and meets manager John Burton and his daughter Helen. While there, Sunset prevents a hold-up. When Sheriff Duncan rushes to help, he is wounded. As the bandits flee, Sunset follows, captures one and brings him back to Cimarron, thus impressing Frank Holden, the president of the mining corporation. Meanwhile, the stage, which is driven by Ted, is held up by bandits. Later, as the sheriff's wounds are bandaged, he tells Sunset about the excessive hold-ups and killings in town. When the stage returns to town and Ted tells the sheriff that the gold shipment was stolen by bandits, the sheriff quits. Burton and Holden then plead with Sunset to stay on in Cimarron as the new sheriff, and he agrees to fill in until a new state marshal arrives. Later, Sunset and Ted talk and it is revealed that Ted is Sunset's brother and that Sunset has recently been released from serving a three-year prison term for cattle rustling. Although Sunset is innocent of the charges, both believe it is better to keep their relationship a secret, to help them find out who framed Sunset. Later Ted goes to an abandoned mining cave where he joins the men who held up the stage. The gang, of which Ted is a member, is led by Brace McCord, who plans another robbery immediately. The next day, while most of the townsmen are fighting a fire outside town, McCord's gang makes another attempt on the Express office. Sunset, who suspects the fire is a ruse, waits atop a building across the way and surprises the robbers, killing two and wounding another before the rest get away. Sunset's quick action impresses Helen, Holden and the town and they convince him to stay on as sheriff, despite the imminent arrival of a new marshal. Ted is now furious at Sunset's intervention in the robbery and tells McCord that the gang should try the Express office again that night, as a town dance is scheduled and no one will expect another attempt so soon. That night, Helen pauses from dancing with Sunset to take her father some food at the Express office and Ted offers to deliver it. When Burton lets him into the office, the rest of the McCord gang follow and take the strong box from the safe. Ted kills Burton, knowing he can link him to the robbery, but pretends to have tried to stop the robbery. Later at the mine, the gang opens the strong box and finds it filled with old horseshoes. Knowing that Sunset is responsible, Ted determines that his brother must be eliminated. McCord decides the gang will stop the stage bringing in the new marshal and that he will assume his identity. He can then expose Sunset's criminal past and accuse him of the recent robbery attempts. The plan works, but when McCord tries to arrest Sunset, he escapes, but is nicked by a bullet. McCord then asks Helen about the next gold shipment, which is an exceptionally large one. McCord proposes that they send the shipment early and he will ride along as guard. Ted and McCord then depart with the gold as Holden, Pinky and Helen sadly reflect on Sunset's apparent betrayal. When Sunset turns up at Pinky's to have his wound treated, he is surprised when Pinky tells him that the marshal is taking the gold shipment early and goes after the stage. Unknown to McCord and Ted, Sunset hides on the back of the stage where he overhears McCord tell Ted they should cut out the other gang members. When they meet the others at a prearranged spot, McCord kills them as Sunset, listening, realizes the similarities to his own frame-up. Knowing finally that it was Ted who set him up, Sunset jumps out and accuses him, then shoots McCord in self defense. After a fistfight with Ted in the mine shaft, Sunset takes Ted back to Cimarron to confess to his part in the holdups and in framing Sunset. With Cimarron free of the McCord gang, Sunset plans to leave until Helen convinces him to stay.