 As they return from their secret mine in the desert, prospectors Juan Valdez (Josef Swickard) and Burke (Frank Lanning)are ambushed. Burke escapes but Valdez is shot and killed. A notorious bandit, The Cactus Kid, is suspected of the crime. Ramon, actually a Justice Department Agent named Marco Roberti, (Walter Miller), a mysterious figure, overhears a conversation between two townsmen, Halkey (Lee Shumway) and Jenkins (Bob Kortman) in which they discuss their failure to get the location map of the mine from Valdez. Rinty (the top-billed Rin-Tin-Tin), Valdez s dog,recognizes Jenkins as his master s killer and attacks him. Halkey, actually "The Cactus Kid", decides to capture Rinty as he thinks the dog can lead him to the mine.