In 1900, after being released from military service in the Spanish-American War, Rough Riders Roy, Rusty and Tommy head home to Arizona, where they join the Border Patrol under Grover Harrison. The American side of the border is being besieged by robberies perpetrated by Arizona Jack and his gang, who are taking their loot across the border to Mexico, where the Border Patrol is forbidden to pursue them. Harrison assigns Roy, Rusty and Tommy to detain Dorothy Blair, a headstrong young American who has run away from home, and to guard a shipment of gold arriving on the stage. At the local cantina, they meet Dorothy, who has run away from home to join her lover, George Lanning, the foreman of her father's mines, but the trio fails at both of their assignments when the outlaws outwit them by starting a fight and Dorothy escapes on the stagecoach as it hastily leaves town. After killing Tommy, the outlaws rob the stage and take Dorothy captive. Suspended for dereliction of duty, Roy and Rusty ride across the border, where they are captured by the outlaws and imprisoned with Dorothy. Meanwhile, Lanning, who is in league with the bandits, arranges for Dorothy to escape. Once freed, Dorothy returns to rescue Roy and Rusty, and Roy follows Lanning to the gang's hideout, where he sees Arizona Jack kill Lanning and then ride off to collect the gold from its hiding place in the mines. After dispatching Rusty to round up the Border Patrol, Roy rides to protect Dorothy and her father from the outlaws. They then barricade themselves in the mines until the Patrol rides to the rescue and arrests the bandits.