In the last of the "Rocky" Lane series, federal marshal "Rocky" Lane (Allan Lane) saddles up "Black Jack" (Black Jack) and heads for the Mexican border to investigate cattle rustling during the Spanish-American War. He gets a job with feed-merchant Nugget Clark (Eddy Waller), and discovers that Nugget's wizened little helper,Josh Bailey (Edward Clark0, has been giving cattle shipment information to the town dentist, Mason "Doc" Ramsey ('Alden 'Stephen' Chase'), who is the secret leader of the rustlers headed by Floyd Garnett (Roy Barcroft). When Ramsey and Garnett steal some of Nugget's cattle feed, Rocky follows them to their hideout through a waterfall into a hidden valley (El Paso is overrun with such scenic delights.) The outlaws have kidnapped Alice Clark (Phyllis Coates) and Rocky captures Ramsey after Garnett is killed in a gun battle. Alice is rescued, the stolen cattle are recovered and Rocky moves on to his next film assignment, which was more than five years later when he turned up as the sheriff in Universal's "The Saga of Hemp Brown." This film had two working titles, "Stagecoach To Cheyenne" and "El Dorado Gold Rush", which lasted until somebody read the script and found no mention of gold rushes, El Dorado, Cheyenne or stagecoaches-to-anywhere. The waterfall in a hidden valley would have fit Cheyenne better than El Paso, but it would have been a heck of a cattle drive to get the herd across the Mexican border.