 Red Ryder (Bill Elliott) and Little Beaver (Bobby Blake) ride into a sleepy Arizona town, and are seen by Blackie Blake (Roy Barcroft). Red and his young Indian pal go to the office of Marshal McGaw (Hal Price) 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 (Ed Cassidy) 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. Red returns home to find the Duchess (Alice Fleming) arguing with fellow-rancher Major Harding (Eddy Waller) over a line-fence dispute, and concludes that someone is using them as dupes to cover some activity. In the town saloon, Blake, the Major s ranch foreman, and two henchmen, Dow (Kenne Duncan) and Luce (Edmund Cobb) are discussing how they let one of the Duchess  horses, with the Major s brand newly burned on it, wander back onto the Duchess  ranch to throw suspicion on the Major. Red beats Blake in a fight and calls a meeting to settle the dispute. Dow steals the Major s horses and Red finds "Thunder" missing and assumes he was also rustled. In reality, "Thunder" had followed the stolen herd to Box Canyon and was captured. Blake tries to ride "Thunder" but is thrown and then beats the horse unmercifully. "Thunder" gets away, Red finds him, sees the whip marks, and vows to find the owner of the saddle that "Thunder" is wearing. The trail leads to the Arizona town where Red is now telling the townspeople what prompted "Thunder" to attack Blake.