The Civil War is over, but former Union Colonel Cord McNally (John Wayne) still has battles to fight. During the war, marauding Rebel thieves captured Union gold from trains, and McNally knows they succeeded only because some traitor or traitors on the Union side gave them help and inside information. Spurred in part by the death in one such raid of a young lieutenant he saw as a sort of adopted son, McNally is determined both to find the thieves and to reveal the traitors, a task that takes him to his home, Rio Lobo, Texas. Also searching for the same evildoers is a vulnerable woman named Shasta (Jennifer O’Neil), and eventually an unlikely ally in a former Confederate fighter, “Frenchy” Cordova (Jorge Rivero). A local character (Jack Elam) adds a bit of wry humor as the good guys and bad guys end up in the inevitable conclusion and face off.