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Sheriff Matt Doniphon and his deputies, Gene Autry and Frog Millhouse, ride shotgun as rancher Larry's cattle are driven to Box Canyon. Their aid is needed because there has been a rash of "new-fangled" rustlings during which the cattle are slaughtered on the spot and their carcasses taken away. After the lawmen leave the herd, however, the rustlers move in, radio the cattle's location from an airplane, and then bring in refrigerator trucks. Larry and his worker Joe are murdered. As the raids increase, newspaper editor Helen Morgan demands the recall of Matt, whom she claims is too old-fashioned to deal with the modern rustlers. Gene, who was raised by Matt after being orphaned as a young boy, argues with Helen, but she refuses to change her editorial stance. Gene and Frog are suspicious of the Chicago and Western Packing Co., owned by Jack Shannon and run by Jack and his brother Jim, and they go to investigate. They find the carcasses of some rustled cattle and demand that Jim produce the hides so that they can check the brands. Jim lies, saying that the cattle belong to his cohort, Thad Slaughter, and that Slaughter has the hides at his ranch. When Gene and Matt discover Frog locked in one of Jim's trucks as they are on the way to Slaughter's ranch, they chase after the truck and Jim shoots Matt. Matt is not seriously wounded, and later that afternoon, Frog identifies Jim as the one who shot Matt. Jim is jailed, much to the dismay of Jack and Slaughter, who are afraid that Jim will talk and expose their operation. That night, Slaughter beats Jim to death in his cell. The town's citizens then demand Matt's resignation because of the murder and his failure to apprehend the rustlers, and they hire Eustace P. Quackenbush and his detectives, who promise to restore order with their modern, scientific methods. At the welcoming party, rancher Bidwell reveals that all of his men are in town. Jack alerts his rustlers to go to Bidwell's ranch, where Frog and his pal Stubby wear a cow costume and lay in wait for the desperados. Frog sends an emergency message to Gene, and Gene uses the radio to call all local cowboys to defend Bidwell's ranch. The rustlers hear the broadcast and try to flee, while the automobiles of Quackenbush and his men get stuck in the mud. The cowboys ride past the detectives on their trusty horses and quickly round up the gang. Matt shoots Jack as he attempts to use Helen as a hostage, thereby proving to her that old-fashioned methods are still the best. Gene and Helen ride back to town together, passing Quackenbush, who is still stuck, while Frog and Stubby try to outrun a bull attracted by their cow costume.