Gene Autry, in the role of Gene Autry, rounds up a large herd of horses in Texas before a telegram arrives from his brother Ken Cooper as Tex Autry. Tex has fled to South Africa, where he and his partner, Edward Barclay, have reportedly struck it rich in the Valley of Suspicion and urge Gene to bring as many horses as possible to Dunbar, South Africa, so he can contact LeRoy Mason as John Cardigan, the saloon owner who grubstaked Tex and Barclay. With his loyal sidekick Frog Millhouse, played by Smiley Burnette, Gene quickly arranges passage overseas and sails with the horses toward a new frontier.
On the way back from the mine, Tex and Barclay are ambushed by Cardigan’s men. Barclay is killed, and Tex is wounded but manages to escape. Cardigan, hungry for control of the mine, watches and waits as Gene’s journey unfolds. When Gene and Frog finally reach Dunbar, they hire the wily Earle Hodgins as Barkey McCusky, a small-time English con artist, to act as auctioneer for the horse sale. Cardigan, meanwhile, keeps a close eye on Gene, who is unaware that he’s being looked at as a rival not only by Cardigan but also by Cardigan’s henchman, Dick Wessel as Henchman Craig Johnson.
The tension deepens as Gene grows suspicious of Cardigan after spotting manipulation around Tex’s messages. Cardigan’s jealousy grows when Gene becomes friendly with Gwen Barkley, Maxine Doyle who is Barclay’s daughter. The plot thickens when a servant named Namba, played by Corny Anderson, is seen in town wearing the belt buckle Gene gave to Tex, confirming Gene’s suspicions about Cardigan’s duplicity.
Johnson schemes to eliminate the Texan threat by tricking Frog into a trap, even offering uncut diamonds to Frog in exchange for a horse. The local constabulary explains that it’s illegal to possess uncut diamonds without a license, and Gene and Frog are taken to Kimberly for trial. The Texans, however, manage a daring escape and soon rediscover Cardigan’s jungle movements as he edges closer to the diamond mine.
The group is captured by natives and brought before Chief Bosuto, a formidable ruler played by Buddy Williams. Frog wins the chief’s favor by teaching his children to sing, and the chief agrees to release the others if Frog stays behind to continue teaching music. Frog and Cardigan remain with the natives while Gene, Barkey, and Gwen are led away by Namba.
When the police arrive and free Cardigan, the trio heads to Cardigan’s jungle hideout, where Tex is being kept as a slave. Gene bursts in, frees his brother and the other workers, and a tense chase ensues. Cardigan and Johnson attempt to escape, taking Gwen hostage. Frog catches up with Johnson, and Gene engages Cardigan in a climactic struggle that ends with Cardigan plummeting from a cliff to his death. The rest of Cardigan’s gang are captured as the law closes in.
On the voyage back to Texas, Frog serenades Bosuto’s children, and Gene shares a quiet moment with Gwen as they kiss, bringing a hopeful end to a perilous journey.
