Union leader Cord McNally (John Wayne) is protecting a routine gold shipment when his troops are attacked by Confederate forces. Not only does he lose the gold, but one of his strongest officers is killed in the raid. At the end of the Civil War, McNally learns that the raiders had help from the inside, and he vows to uncover the two traitors. After a chance encounter with one of the turncoats, McNally travels to the town of Rio Lobo and makes an unexpected discovery.