Bob Crandall, a corporal in the Texas Rangers, is pleased when his friend, Tim Martin, the brother of Bob's sweetheart Sue, is also appointed to the rangers on the same day that Tim pays off the mortgage of his ranch. The mortgage is held by banker Hatfield, the secret leader of a smuggling gang run by saloon owner Largo, who is disappointed to lose the ranch, as it lies along the Texas-Mexico border. Tim confides in Bob that he got the money for the mortgage by briefly working for Largo, but Bob tells him to put the past behind him and become a good ranger. That night, however, as Tim is riding patrol alone, he is told by Hawk, one of Largo's men, that Largo is injured and needs his help. After Tim leaves his post, he is ambushed by Hawk, while the rest of the smugglers attempt to get a wagonload of guns across the border. Bob and the other rangers stop the wagon, and the injured driver curses Hawk, who told him that the ranger on patrol had been bought off to insure the shipment's safety. Tim is taken to ranger headquarters, where he refuses to answer Captain Banning's questions about his complicity. Bob visits Tim when he is sent to jail to be held for trial, but after Bob leaves, Sheriff Warde, who is one of Hatfield's stooges, shoots Tim and makes it look as if the young ranger was trying to escape with a gun slipped to him by Bob. Banning dishonorably discharges him, and even Sue believes that Bob is responsible for Tim's death. Soon after, Bob joins Largo's gang, and Sue is told by Hatfield that Tim leased the ranch to him. Sue is mystified but allows Hatfield to fill the barn with supplies. Largo gets Bob to retrieve the confiscated shipment of guns from ranger headquarters, and although Bob is captured, he is released by Banning, who did not really discharge Bob and is his contact during his undercover work with the gang. Sue and her cook, T-Bone Jones, discover that Hatfield's crates are filled with guns and ammunition, but Bob releases them after they are captured by the gang. The nervous Hatfield and Largo decide to move the contraband across the border that night, and while the rangers, alerted by Sue, chase the gang, Bob forces Warde to confess that he killed Tim on Hatfield's orders. Meanwhile, Hatfield attempts to escape with the gang's money, and Largo, realizing that he is being double-crossed, shoots the banker. Largo is then appprehended by Bob, and later, at a celebration, Sue, Bob and the other rangers enjoy a meal prepared by T-Bone.