Halloween

In which Buffy becomes a lady, Xander becomes a man, Willow becomes a ghost, and Giles becomes Ripper. (Exhibit D in the 'Xander is Hot' debate)

Official back-of-box synopsis:

There are lots of tricks but not many treats when Buffy's costume causes her to lose her powers and the kids she, Willow and Xander are trick-or-treating with turn into their costume characters. Could Giles have something to do with all this?

From the old WB Buffy website:

TRICK OR TREAT -- Buffy, Willow and Xander are recruited by Principal Snyder to trick-or-treat with the neighborhood kids, but the night doesn't turn out as they had planned when Buffy's costume causes her to lose her slaying ability and the other kids transform into their costume characters. Meanwhile, when Giles begins to paste together the night's events, he learns that it's his past that is making this a very haunted Halloween.

Links:

This episode's page at http://www.restlessbtvs.com/, the Restless BTVS Trivia Guide

This episode's page at http://www.vrya.net/bdb/, the Buffyverse Dialog Database

Cross-reference:

episodes in which people wear amusing clothing:

• Inca Mummy Girl
  The Cultural Exchange costume dance
• Reptile Boy
  Xander in drag
• Fear, Itself
  Mexican Giles, Bunny Anya...
• Doomed
  Spike in Hawaiian shirt and boardshorts. Somehow he's still sexy.
• No Place Like Home
  Giles is in a Wizard costume. Buffy is speechless.
• All The Way
  Pirate Xander, Charlie's-Angel Anya
• Hells Bells
  Those bridesmaid dresses...
• this episode
  Some of the clothing is deadly serious, but Caspar!Willow and Pussycat!Cordy are unforgettable

Cast:

• Sarah Michelle Gellar as Buffy
• Alyson Hannigan as Willow
• Nicholas Brendon as Xander
  

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• Anthony Stewart Head as Giles
• Charisma Carpenter as Cordelia
• David Boreanaz as Angel
• Armin Shimerman as Principal Snyder
• James Marsters as Spike
• Juliet Landau as Drusilla
• Seth Green as Oz
• Larry Bagby as Larry
• Robin Sachs as Ethan
• Abigail Gershman as Girl