 Pony Express rider Gene Autry (Gene Autry) has his horse shot out from under him when nearing his California destination and is knocked unconscious. His attackers rob his mail pouch of a money package and ride away. Gene finds the torn wrapper, bearing the California address of Delores Moreno (Ann Rutherford), to whom the cash was going, to meet delinquent taxes on her vast ranchero. At the ranch, Delores with Matt Ford (LeRoy Mason),scheming owner of the adjoining rancho,is watching Frog Millhouse (Smiley Burnette) and the other cowhands trying to break a beautiful but viscious wild horse known as El Diablo (Champion.) The horse, trailing a rope from its neck, breaks away and heads for the hills. Gene, struggling through the mountains afoot, rescues the horse from a wolfpack and wins its confidence and, the next morning, rides El Diablo to the Moreno ranch. Caldwell (Raymond Brown), owner of the Pony Express system, is in California to buy horses and a race is proposed between Ford s stock of thoroughbreds and Miss Moreno s mustangs, with Caldwell buying stock from the winner for use as Pony Express mounts. Ford has the Moreno horses stampeded and this forces Gene---who is riding for Delores---to race only a few mustangs against Ford s twenty thoroughbreds. And Ford pulls another trick to ensure his stock will win.