John Wayne, in the last of his Civil War characterizations, portrays Cord McNally, a Union Army colonel who loses a gold shipment in a Confederate raid, during which a devoted young officer is also killed. After the end of the war, McNally bears no ill-will toward the leaders of the raid, Pierre Cordona (Jorge Rivero) and Tuscarora Phillips (Christopher Mitchum), who were acting as soldiers, but he still wants the two unknown men on the Union side who they say sold them the information about the gold shipments. A year later, McNally crosses paths with one of the men, now a deputy from Rio Lobo, who is about to take Shasta Delaney (Jennifer O'Neill), a seemingly innocent young woman, out of a neighboring town at gunpoint. A shootout ensues, in which McNally's man and three other Rio Lobo deputies are killed, with help from Cordona -- this makes McNally very interested in what's going on in Rio Lobo, and he decides to go there with Cordona and Shasta. They find a whole community under siege from their own sheriff, a sadistic ex-outlaw named Hendricks (Mike Henry). What follows is a series of confrontations and revelations that are alternately suspenseful, sadistic -- with maimings worthy of a spaghetti western and characters even getting blown to bits -- and even occasionally comical. But the pieces all tie together very neatly, despite a convoluted plot that's sort of Rio Bravo (made 11 years earlier, also starring Wayne and directed by Hawks, and scripted by Leigh Brackett) turned sideways and readjusted to a more cynical era.
by Bruce Eder synopsis