Two old friends, Matt Morgan (Douglas) and Craig Belden (Quinn), now find themselves on opposite sides of the law. Belden is the de facto ruler of the town of Gun Hill, a rich cattle baron. Morgan is a U.S. marshal living in another town with his Native American wife (played by Ziva Rodann) and young son, Petey.
Two young drunken cowboys rape and murder Morgan's wife while she is returning with their son from a visit to her father. The boy escapes on one of the killers' horses, bearing a distinctive, fancy saddle.
Morgan sets off to find the killer. His one clue is the saddle, which he recognizes as belonging to Belden. Assuming it was stolen from his old friend, Morgan travels to Gun Hill to pick up the trail, but once there he quickly realizes that Belden's son Rick (Holliman) is the killer.
Belden refuses to turn over his son, forcing Morgan to go against the entire town. He vows to capture Rick and get him on that night's last train from Gun Hill.
Rick is taken prisoner and held at the hotel. Belden sends men to rescue his son, but Morgan manages to hold them off. In the meantime, Belden's former lover (Jones) decides to help Morgan. She sneaks a shotgun to his hotel room. The second rapist, Lee, sets fire to the hotel to flush out Morgan.
Morgan presses the shotgun to Rick's chin on the way to the train depot, threatening to pull the trigger if anyone attempts to stop him. Lee tries to kill Morgan but shoots Rick instead. Morgan then kills Lee with the shotgun. As the train prepares to leave, a devastated Belden confronts Morgan in a final showdown and is gunned down.