Aroused by the alarming growth of espionage activities along the border, Sue Lambert confronts Thorpe, the leader of a gang working for a sinister group of foreigners newly arrived to the border badlands. Dissatisfied with the explanation of Schmidt, Thorpe's boss, that the airplanes and parachute jumpers are working on an irrigation project, Sue writes a letter to Commander Jim Moore of the American Legion, who wires back that five of his agents have been murdered while investigating that section of the border, and that he is sending crack manhunter Trigger Tim to investigate. Realizing the danger of the situation, Tim leaves headquarters with his friend Lanky to meet agent Charley Hall and his men at Three Forks. Learning of the rendezvous, Thorpe orders Hess and Mario to ride into camp and murder the legionaires. After leaving Hall's body where it will be found by Tim, Thorpe and his gang blow up the Lambert cabin as a mysterious figure wearing a Mexican costume watches from the distance. When Tim finds Hall's corpse, he is infuriated, and as he kneels to examine the body, he sees a rider in a Mexican costume. Tim and Lanky pursue the rider, and learn that he is Captain Juan Rodriguez of the Mexican National Police who has been posing as a bandit named "Gringo" to investigate the border activities. Joining forces, the three discover that wild horses are being used to smuggle defense secrets across the border. Just as they are about to close in on Schmidt, Thorpe accuses them of blowing up the Lambert cabin, and rather than reveal their true identities, they submit to arrest by Sheriff Dan Kirk. After escaping from jail, the trio find Lambert and convince him to surrender to Schmidt so that they can trail him to the gang's hideout. Their plan works, and as the trio lays siege to Schmidt's shack, the sheriff and his posse hear the gunshots and arrest Lanky and Rodriguez. After killing Thorpe, Schmidt escapes and gallops off, and Tim, freeing his friends, follows to capture Schmidt and his men.