The episode parodies the show's own melodrama. In the background of Xander’s antics, the "main" plot (the apocalypse) is treated with over-the-top, cheesy seriousness, including dramatic orchestral music and slow-motion scenes between Buffy and Angel.
Getting roped into a night of crime and chaos with Jack’s undead friends, who plan to blow up the high school. [S3E13] The Zeppo
Meeting Jack O'Toole, a delinquent who turns out to be leading a group of zombies. The episode parodies the show's own melodrama