Posing as a hangman, Mace Bishop (James Stewart) arrives in town with the intention of freeing his brother Dee (Dean Martin) from the gallows. Dee had been arrested for a bank robbery in which Maria Stoner's (Raquel Welch) husband was killed. After freeing his brother, Mace successfully robs the bank on his own after the gang has fled with the posse in pursuit.
Dee has taken Maria as a hostage. The posse, led by local sheriff July Johnson (George Kennedy) and deputy Roscoe (Andrew Prine) chase them across the Mexican border into territory policed by bandoleros, whom Maria describes as men out to kill any gringos (foreigners) that they can find. Maria further warns Dee that the sheriff will follow, because they have taken the one thing that July Johnson has always wanted: her.
Despite initial protestations, Maria falls for Dee and finds herself in a quandary. She had never felt anything for the sheriff, nor for her husband, who had purchased her from her family. The posse tracks them to an abandoned town and captures the gang. The bandoleros also arrive, so the sheriff releases the outlaws so that everyone can fight in defense.
In this final showdown, almost everyone is killed. Dee is fatally stabbed by the leader of the bandits, El Jeffe, and then Mace is shot by another. Maria grabs a pistol and shoots El Jeffe dead. Maria and the sheriff, with little left of the posse, bury the Bishop brothers and dead posse members without markers, after which Maria notes that no one will know who was there nor what had happened.
The film was shot at Alamo Village, the movie set originally created for John Wayne's "The Alamo." Alamo Village is located north of Bracketville, Texas.
During the filming, Raquel Welch complained on the set about being asked for autographs. James Stewart chastised her, reminding her that her salary was paid by the movie fans.
Larry McMurtry, the author of the novel "Lonesome Dove," reportedly paid homage to Bandolero! by using similar names for the characters in his book. Both tales begin near the Mexico border and involve bandoleros. Both have a sheriff named July Johnson and a deputy Roscoe who travel a great distance in search of a wanted criminal and the woman who has rejected the sheriff's love. Both stories have a charismatic outlaw named Dee, who is about to be hanged and who wins the love of the woman before he dies. In the Lonesome Dove miniseries, the main characters twice pass directly in front of the Alamo -- or at least a set built to replicate the Alamo.