This story starts out in the Civil War. Col. Cord McNally (John Wayne), of the Union Army is waiting at a train station for a shipment of gold, but it never shows up. A band of Confederate Soldiers led by Pierre Cardona (Jorge Rivero) decide they need the gold worse. During the hold up a young officer in the Union Army, a close friend of McNally's, is killed. McNally tracks down the Rebels, but is captured by them. He then escapes and captures them, but never finds out who the informer was. After the war is over, he offers to buy Cardona and his friend Tuscarora (Chris Mitchum) a drink. When they part, Cord tells them if they ever find out who the informer was to let him know. They told Cord that they were the ones that killed his friend and asked him why he wasn't mad at them, and he answered, "what you did was an act of war, what the informer did was an act of treason". Cardona gets word to Cord that he thinks the man he is looking for is in Rio Lobo, Texas. They plan to meet a short distance away in Hawthorne, Texas. While there a young lady Shasta (Jennifer O'Neill) comes into town claiming her friend had been murdered in Rio Lobo by deputy's from Rio Lobo.