It's Halloween on the Hellmouth - a slow time for vampires, but not for mischievous mystics. Ethan's Costume Shop is the newest store in town, and many buy their costumes from there, including Buffy, who wants to dress up like the noble women Angel would have know two centuries earlier, based on drawings she and Willow see in a purloined Watcher diary.
However, the shop owner, Ethan Rayne, has other things in mind. Invoking the Roman god Janus, he casts a spell that turns everyone who purchased a costume at his store into the persona their costume represents. Xander becomes an army grunt (with real machine gun), Willow becomes a ghost, Buffy becomes an actual eighteenth century maiden, and several kids becomes demons. Spike sees the resulting chaos - and the fact that the Slayer is now useless in that particular role - and decides to party.
Cordelia's lack of transformation reveals that only people who got their costumes at Ethan's were changed. A trip there finds Giles confronted with old friend Ethan - and the ostensibly stuffy and befuddled librarian proceeds to literally beat the method of reversing the spell out of the costumer, just in time to save Buffy from the business end of Spike's teeth.