In 1862, cavalry soldiers Lt. Steve Nolan and Corp. Banjo Bonners are sent to Benton County in the Oklahoma Territory to stop the theft of Union horses by an outlaw gang led by a masked bandit known as "El Vengador." Steve goes undercover as a cowboy looking for work and, soon after his arrival in Benton, finds himself wounded and falsely accused of robbing the express office. While the unconscious Steve is being transported to a doctor in a nearby town, Higgins and Duggan, the real thieves, are ordered by their boss, land commissioner James Prescott, to kill Steve before he can testify against them. They mistakenly kill Deputy Sheriff Williams instead, and Steve escapes the two henchmen when he pretends to be dead after his runaway wagon overturns. The injured Steve is then tended to by Donna Ross, better known as "El Vengador," and her gang. Upon his recovery, Steve is told by gang member Todd Wingate that Donna's father and the other ranchers in Rainbow Valley had their lands taken from them at the outbreak of the war, and that they formed this outlaw band as an act of protest. After Steve leaves their hideout, Donna learns that a letter to Judge Clem Masters concerning the ranchers' appeal was among the things stolen in the express robbery, and Todd correctly surmises that Prescott was behind the hold-up. That night, Banjo arrives in Benton in the guise of a medicine man, and learns that Arnold Drew, Prescott's partner, is planning a wild horse roundup the next morning. After Banjo points out Drew to him, Steve recognizes the crook as the man who shot him during the robbery. Steve then meets with Prescott and Drew to offer his official help for the roundup, but they are interrupted by Donna and Todd, who have come to Prescott's office to demand the judge's stolen letter. After Donna and Todd are forced to escape empty-handed, Masters attempts to arrest Steve, as he alone recognizes Steve as the "wounded robber," but the judge is quickly briefed by the two undercover agents as to their mission in Benton. Steve then tells Masters to inform Donna and her men that if they will allow him to round up the necessary horses for the Union cavalry, he will see to it that their case is heard in Washington. Donna's men stampede the herd anyway, and Todd is captured by Steve in the ensuing shootout. After Steve explains to him the importance of the horses to the Union war effort, Todd agrees to put his personal grievances aside, and promises to convince Donna and the others to do the same. Before he can leave, however, Prescott's men arrive and place Todd under arrest. While Todd sits in the jail, Steve meets with Donna, and he agrees to break Todd out of jail if she agrees to deliver the rustled horses to the cavalry. Steve then returns to Benton just as Prescott's men prepare to hang Todd. Steve can do little to stop it, however, as he is caught going through Prescott's desk by the land commissioner and Drew. Banjo, however, manages to free Todd from the lynch mob, while Steve escapes himself with the judge's letters. Prescott and Drew are then captured, arrested and convicted of land fraud, while Donna and her men have their lands rightfully returned to them.