In 1891, Sunset Carson, a special operative for the governor of the New Mexico territory, is ordered to join the border patrol in an undercover investigation of jewel smugglers operating in the town of Tres Piños, near the Mexican border. Sunset goes to Tres Piños, where he meets Aloysius J. Philpots, a city slicker being picked on by a group of toughs led by Spur Brannon. Upon entering the saloon, Sunset then bests Spur in a fistfight in order to stop him from pestering singer Ann Morton. Ann, who hides Sunset's dropped badge for him, later pleads for his help and reveals that her uncle, Fred Loder, is the owner of the Slash J ranch, which is the suspected base of operations for the smugglers. Ann believes that her uncle is the victim of foul play, because he did not meet her when she arrived in town from the East, but Spur, who is the ranch's foreman, maintains that Loder is away on business. Sunset promises to find her uncle, and when he and Aloysius ride to the ranch, Sunset sees a frightened man, whom he suspects is Loder. Spur orders Sunset and Aloysius to leave and after they return to town, the body of a murdered border patrol agent is brought in. The killers consult with saddle maker John Gant, a prominent citizen who is the secret leader of the smuggling gang. Gant has designed saddles with hidden compartments in their pommels, in which the outlaws transport jewels to Smiley, Gant's Mexican contact, who fences the stolen goods. Sunset and Aloysius return to the saloon, where Aloysius finds a letter to Spur from Smiley, who is sending hired killer Brazos Kane to help him. Pretending to be Kane, Sunset returns to the ranch, and Spur hires him. Sunset then contacts Loder, who explains that he has been held captive so that the outlaws can use his ranch, which included land on both sides of the border. Sunset follows Denver and Mills, two of Spur's men, to Smiley's hacienda, where they are told to prepare for a big shipment. Sunset shoots the two men for resisting arrest during their return journey, then tells Spur that the border patrol was responsible. Spur takes Sunset to meet Gant, who reveals that he is the outlaws' leader. Fearing that the border patrol is aware of the pommel compartment, Gant constructs a hollow wagon wheel for the next trip to Smiley's. After they leave, however, the real Brazos Kane arrives at the saloon. Ann, who is really a reporter and not Loder's niece, warns Sunset, who outdraws Kane in a shootout and then frames him with his own badge. When Sunset accompanies Spur and Gant to Smiley's, however, Smiley reveals Sunset's true identity, and Spur and Gant attempt to escape with the latest shipment of jewels. Sunset shoots Spur during the chase, and although Gant reaches town and his secret cache of loot, Sunset finds him and overpowers him. Soon after, Sunset makes plans to spend his upcoming vacation with Ann.