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. Red returns home to find the Duchess arguing with fellow-rancher Major Harding 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 and Luce 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.