Published on June 27, 2016
During the American Civil War, Col. Cord McNally (John Wayne) has instructions telegraphed to his close friend, Lt. Ned Forsythe (Peter Jason), in charge of the Union troops on a Union army payroll train. However, Confederates led by Capt. Pierre Cordona (Jorge Rivero) and Sgt. Tuscarora Phillips (Christopher Mitchum) hijack the train. Their plan is to listen in on the telegraph wires, grease the tracks to stop the train, disconnect the payroll wagon from the engine so it rolls back down the hill, using a hornet’s nest to force the Union guards to jump off the train, then catch the train with many ropes tied to trees. In the process, Lt. Forsythe is fatally injured.
In the subsequent fighting, they trick McNally and capture him but McNally leads them into a Union camp, pushes a branch forward, and lets it swing back to knock Tuscarora off his horse, then yells out to the camp. As the Confederates flee, McNally jumps Cordona. McNally realizes that a traitor must be selling information to the Confederate States of America, in order for the hijackings to be successful. McNally questions the pair, but they give him no information and are imprisoned.
When the War ends, McNally visits them as they are being released, and asks them to tell him from whom they got their information. When Tuscarora points out that they were the ones who had killed McNally’s friends yet McNally has nothing against them, McNally replies that the killing was an act of war, while the one who sold them the information was a traitor. Unfortunately they don’t know the traitors’ identities, having only seen them from a distance. One was a big, dark-haired, mustachioed man, and the other was very white-haired and pale. McNally then tells Cordona and Tuscarora that if they should ever come across these men again, to contact him through a friend of his, Pat Cronin (Bill Williams), who is the sheriff of Blackthorne in Texas. Tuscarora is on his way to Rio Lobo, Texas where he grew up.
Later McNally is contacted by Cronin on instructions from Cordona. When he arrives in Blackthorne, however, Cordona is sleeping in a hotel room with a woman. Shasta Delaney (Jennifer O’Neill), arrives in the hotel, wishing to report a murder that took place in Rio Lobo. Cronin explains that he cannot intervene because Rio Lobo is outside his jurisdiction. Later a posse from Rio Lobo arrives and wants to take Delaney away. She claims, however, that the leader, “Whitey” (Robert Donner), is the murderer about whom she has been talking.
When one of the posse aims a gun at Cronin, Shasta shoots Whitey under the table, and Cronin and McNally finish off the posse. As the last one is about to shoot McNally, Cordona appears at the top of the stairs and shoots the gunman. Shasta faints from the shock of killing someone. Cordona tells McNally that Whitey was one of the two men for whom McNally was looking. He goes on to tell McNally that Tuscarora had contacted him and had told him that he saw one of the men, for whom McNally is looking in Rio Lobo. He also reports that there is trouble in Rio Lobo and that Tuscarora needs help. His father and other ranchers are being bullied by a man named Ketcham, who installed his sheriff, “Blue Tom” Hendricks (Mike Henry), after Hendricks killed the previous incumbent.
McNally, Cordona, and Shasta go to Rio Lobo, where they discover graft and corruption. Hendricks arrests Tuscarora on trumped up charges. For further help, they go to Tuscarora’s father, Old Man Philips (Jack Elam). When the three sneak into Ketcham’s ranch, McNally learns that Ketcham is really Sergeant Major Ike Gorman (Victor French), the traitor. McNally punches him around and forces him to sign the title deeds of the ranches back to their rightful owners. Philips then wires the triggers back, forming a dead-man’s trap, on his double barreled shotgun so they can order Hendricks and his men out of the jail . They take over the town jail for cover, freeing Tuscarora, while Cordona goes for the Cavalry. Meanwhile, Tuscarora’s girlfriend Maria Carmen (Susana Dosamantes) and her friend Amelita (Sherry Lansing) lend assistance. For that, Hendricks slashes Amelita’s face, and Amelita swears to McNally that she will kill Hendricks.
However, Ketcham’s men capture Cordona. Ketcham’s gang offers to trade Cardona for Ketcham. In the meantime, word spread of the trade and roughly 20 ranchers turn up to help, knowing that, if McNally fails, the town will have gained nothing from the returned deeds. During the prisoner exchange, Cordona dives from the bridge into the river where Tuscarora was hiding. McNally yells out that Ketcham is now bankrupt, having signed the deeds back, so the furious sheriff guns Ketcham down, and in turn, McNally shoots the sheriff in the leg. McNally then gets shot in his leg by one of the deputies, and is dragged back into the cantina by Philips.
After a failed attempt to blow up the cantina McNally’s force was in, the remaining bandits are outflanked and outgunned by the other ranchers who have come to help. Hendricks’s men realize this and they flee. Hendricks shoots at them, but he had been using his rifle as a walking cane and it had become clogged with mud, and it explodes in his face. As he stumbles to his horse, Amelita guns him down, thus keeping her word. McNally tells the ranchers that they have their town back.