A rash of strange cattle rustlings has occurred in which cattle are slaughtered on the range and their carcasses taken away. Sheriff Matt Doniphon (William Farnum) and his deputies, Gene (Gene Autry) and Frog Millhouse (Smiley Burnette), watch over one rancher's cattle as they are driven to Box Canyon. After the sheriff and his men leave, the rustlers move in, radio the cattle's location from an airplane, and then bring in refrigerator trucks. The rancher and one of his workers are murdered, the cattle killed, and the carcasses taken away.
Newspaper editor Helen Morgan, responding to the increased cattle raids, demands that Sheriff Doniphon be replaced, claiming he is too old-fashioned to deal with modern rustlers. Gene defends the sheriff against the editor's attack, arguing that the man’s character and abilities have kept a delicate balance on the range. Having been raised by the sheriff after being orphaned by outlaws as a young boy, Gene knows the man’s methods and limits, though Helen refuses to change her stance.
While investigating the raids, Gene and Frog grow suspicious of the Chicago and Western Packing Co., owned by Jack Shannon and run by Jack and his brother Jim (House Peters Jr.). The deputies uncover cattle carcasses and demand to see the hides to verify brands. Jim lies, claiming the cattle belong to his partner, Thad Slaughter, and that the hides are at Slaughter’s ranch.
On their way to Slaughter's ranch, Gene and Sheriff Doniphon rescue Frog, who is trapped in one of Jim's trucks. They pursue the truck, and Jim shoots the sheriff, though the injury is not serious. Later that afternoon, Frog identifies Jim as the shooter. Jim is taken to jail, and Jack and Slaughter worry Jim may talk and expose their operation. That night, Slaughter summons Jim to the jail window and beats him to death.
The next day, the town’s citizens demand that Sheriff Doniphon resign, blaming him for the murder and the ongoing raids. Eustace P. Quackenbush (James C. Morton) and his uniformed private detectives are hired to end the raids and restore order with their modern, scientific methods. At the welcoming party, Jack learns that rancher Bidwell's men are in town and alerts his rustlers to head toward Bidwell's ranch, where Frog and Deputy Stubby (Frankie Marvin) lie in wait, disguised in cow costumes. When the rustlers approach, Frog sends an emergency message to Gene, who uses the radio to call all local cowboys to defend Bidwell's ranch against the gang. Hearing the broadcast, the rustlers try to flee. Frog and Stubby also have to dodge an amorous bull.
Meanwhile, as Quackenbush’s detectives motor toward the expected showdown, their automobiles and motorcycles get bogged down in the mud, while the cowboys ride past on rugged horses to round up the gang. Sheriff Doniphon shoots Jack as he attempts to seize Helen as a hostage, proving that old-fashioned grit can still outmatch modern schemes. As Frog and Stubby flee from the cow-powered chase, Gene and Helen ride back to town together, passing Quackenbush and his tediously persistent detectives who remain stuck in the mud and suffering the effects of tear gas grenades that detonate with a noisy misfire.
