When Jim Mallory returns home to Texas after the Civil War, he learns that his father, Colonel Mallory, has become the leader of an unscrupulous band of land owners. These land barons, who have refused to adhere to a Congressional decision which removed their titles from vast land holdings, have formed their own "independent" territory known as "The Republic of Rio Grande." Jim and his friend, "Happy" T. Snodgrass, enter the territory just in time to save newspaper editor Jonathan Taylor from the murderous hands of his father's henchmen, Peabody and Idaho. Jonathan and his daughter Nan then tell Jim that the colonel has used such outlaws to force the newly arrived settlers from their legal lands. Brent Gordon, a special representative to the Regional Governor, under an appointment from Congress, arrives in Rio Grande to investigate the homesteaders' complaints. He openly goes to the colonel and accuses him of mistreating the new ranchers. Jim then defies his father and joins ranks with the settlers. Jonathan and Nan are wary of Jim's offer of help, but their suspicions are put to rest when the younger Mallory kills one of the colonel's would-be assassins. Sneed, the colonel's top henchman, then has Jim arrested and convicted of murder. When the colonel orders his condemnmed son's release, Sneed revolts against his employer, hoping to take the elder Mallory's place as leader of the republic. With nowhere else to turn, the colonel goes to Brent, Jonathan and Nan for help, and they agree to take arms against Sneed, on the condition that the colonel disband the new "republic." He agrees, and with Happy's explosives expertise, Brent frees Jim from the jail. In the ensuing gunfight, the colonel is mortally wounded, and Jim avenges his father by killing Sneed. With his dying breath, the colonel then leads his "rebel" forces in their first uttering of the Oath of Allegiance to the United States of America.