Red Ryder and Little Beaver ride into a sleepy Arizona town, and are seen by Blackie Blake. Red and his young Indian pal go to the office of Marshal McGaw and Blake sneaks up to a window and is about to shoot Red from ambush, but "Thunder", Red`s horse, bears down on him with pounding hoofs. The hostile townspeople gather and are bent upon carrying out the law that says a killer horse must be shot. Red demands that "Thunder" be given a trial, and then (in flashback) tells the story of how he first found "Thunder." It was six months ago at the recruiting office for Teddy Roosevelt`s Rough Riders, where the horse was pointed out as a potential killer that no one would ever be able to ride. Red accepted the challenge and was given the horse by Teddy Roosevelt after a wild ride. Red wants to join the Rough Riders, but Roosevelt tells him that horse thieves in Red`s home state of Wyoming are disrupting the Army`s supply service and that Red is the only man capable of stopping them.