The film opens with a brief prologue about the film's setting: the Old West and the town of Tombstone, Arizona, in the year 1879. After the prologue, members of the outlaw gang known as the Cochise County Cowboys, led by William "Curly Bill" Brocius, ride into a Mexican town and interrupt a local police officer's wedding. They then proceed to massacre the assembled policemen in retribution for killing two of their fellow gang members. Shortly before being shot in cold blood, a local priest warns them that their acts of murder and savagery will be avenged, referencing the biblical fourth horseman. Read more on Wikipedia.