Army Captain Jim Casey is called home to his ranch by border patrolman Colton to investigate strange happenings. On his way out of town, Casey finds his foreman Karney, who was nearly killed by a Chinese man's axe in Chinatown. When Casey arrives at the ranch he learns that his ranch hand, Slim Elkins, was killed the night before, when Karney was mysteriously absent. Casey's brother Dave had gone with Slim that night to investigate possible contraband being moved along the international road bordering the ranch. They had seen a car turn out its lights and shoot at them. Dave shot back and wounded one man, but Slim was killed. Karney then meets with bunkhouse cook Abner, who has been arranging the smuggling. Abner insists they run more contraband that night, even though Dave's "hair-trigger" shooting killed one of the outlaws the previous night. Jim finds a concha near the border and suspects the culprit is Mexican, and Colton reports that someone will be smuggling Chinese across the border that night. Jim then finds a Mexican named Jose in one of his cabins, where he is hiding three Chinese immigrants for which he was paid $150 each. Although Jose claims to be a rancher, Jim is suspicious and he and his men surround the cabin. One of the ranch hands fires a shot and Jose escapes onto the road, where he hides in Jane Elkins' car. Jim finds him and removes his disguise to discover that he is Karney. Jim then recollects the time he saved Karney's life in the war and is disappointed by Karney's ingratitude. Karney's ability to speak Spanish had made him able to implicate innocent Mexicans in the crime. Karney is holding up Jim and his men when the mysterious Chinese man, angry at Karney for taking his people's money, kills him with an axe and escapes. Jim, certain Colton will apprehend the killer, proposes to Jane, and his servant Snowflake plays the wedding march on his harmonica.