A Civil War veteran searches for the traitor behind a friend's death.

As the Civil War nears its end, a band of Confederate guerrillas led by Capt. Pierre Cordona and his scout, Tuscarora, steal a shipment of gold from a Union train. Although they also capture Union Col. Cord McNally, he eventually outwits them and retrieves the gold. When the war ends and the men meet again, McNally asks Cordona for the names of the traitors who informed the Confederates of the gold shipments, but Cordona replies that he never knew their names. Shortly thereafter, McNally rides into a Texas town and aids Shasta, a young woman whose medicine show partner has been murdered by a sheriff's deputy from nearby Rio Lobo. Cordona is also in town, and he tells McNally that the Union traitors have taken over Rio Lobo. Accompanied by Shasta, the two former enemies ride into Rio Lobo and learn that Deputy Ketcham and Sheriff Hendricks have confiscated land from the people, with the exception of old man Phillips, Tuscarora's grandfather. To force the issue, Hendricks arrests Tuscarora on trumped-up charges and throws him in jail, but McNally, Cordona, and Phillips take Ketcham hostage, force him to sign back the stolen land, and make him order Hendricks to release Tuscarora. They then barricade themselves inside the jail while Cordona goes to the nearest Army post for help. He is captured, however, by Hendricks' men and brought back to Rio Lobo; Hendricks then demands that Ketcham be exchanged for Cordona. Forced to comply, McNally releases his hostage but informs Hendricks that his partner signed back all the stolen land. Enraged, Hendricks shoots Ketcham and starts a massive gunfight. Hendricks is killed by his former mistress, and McNally's forces emerge victorious as the timid townspeople rally to defeat their common enemy.