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A land grant is signed by the United States Supreme Court bequeathing to Betty Castelar 100,000 acres of Arizona territory known as El Rio Rancho. The grant is sent by Pony Express, but its rider, Buck Carson, is intercepted by landowner Clem Porter and his gang. Clem steals the letter, then leaves Buck alone in the desert to die. Back at El Rio Rancho, Betty is ordered off her land, as it is to be sold at public auction unless the land grant arrives. Betty is offered shelter by a priest of the nearby San Paul mission, but she plans to stay with her good friend, June Grey; however, when June's calvary officer brother Bob arrives, he is ordered by Captain McCarthy to confiscate all Betty's horses, including her favorite, "Dynamite." Betty turns her back on the Greys and moves into the mission. "Dynamite" manages to elude Bob and ends up in the desert, where the horse finds the dying Buck. Meanwhile, Clem is blackmailed by the local marshal into keeping the public auction "private." Needing 5,000 pesos, Clem plans to rob the mission's gold shipment. Buck is brought to town by "Dynamite," and pledges to help Betty find her stolen letter. At a fiesta at the mission, Buck meets Clem, who is introduced to him by Betty as "an old family friend." Buck recognizes Clem as the leader of the robbers by the way he nervously spins his pocket watch chain. The next day, the mission's gold shipment is prepared, with Clem offering to "personally" protect it. Buck and Pedro, Betty's trusted servant, go to Clem's ranch in search of the letter. In the meantime, the gold shipment is attacked by Clem's men, who encounter more resistance than expected when Bob and some cavalry men arrive. Bob is captured by Clem's men and taken back to the ranch, where they discover the gold shipment is nothing but worthless rocks. While hiding in the attic, Buck sets fire to a bedroom as a diversion, then makes his escape with the letter, with Clem's gang in hot pursuit. Clem takes the kidnapped Bob to his hideout in Sackalow Canyon, then returns to the mission, telling Betty and June that the outlaws are holding Bob for ransom. The two women agree to accompany Clem to learn the outlaws' demands. Buck learns from Pedro where the women have gone, and rides to their rescue, while Pedro summons the cavalry. At the hideout, Clem reveals his true character, agreeing to let Bob go only if Betty signs her rights to El Rio Rancho to him. Before she can sign, Buck arrives, quickly followed by the cavalry. With Clem and his men captured and Betty's grant safely secured, Captain McCarthy prepares to be best man for a double wedding: Betty to Bob and June to Buck. As they prepare to marry, Buck asks to be introduced to his future brother-in-law.