The Wish

In which Cordy makes a wish and gives us a whole new world. And what a world it is. (Exhibit F in the 'Xander is Hot' debate)

Official back-of-box synopsis:

Cordelia finds out you must be careful what you wish for. When she tells her new friend Anya that Buffy is to blame for all her problems, Cordelia unleashes a demon that only Buffy can save her from.

From the old WB Buffy website:

BE CAREFUL WHAT YOU WISH FOR - Hell hath no fury like a woman scorned-Cordelia, wanting to be rid of all things Xander and his spooky entourage, decides that all of her troubles started when Buffy moved to Sunnydale and wishes that historic event never actually happened. Not knowing that a an evil genie would grant her fateful wish, Cordelia is transported into an altered reality in which the Hellmouth is open and The Master is alive and ruling, with Willow and Xander as his undead minions.

Cross-reference:

episodes featuring alternative realities:

• Superstar
  where Jonathan is Keanu, Michael Jordan and Al Gore all rolled into one
• Normal Again
  is the entire 'verse just insane-Buffy's hallucination?
• this episode
  Sunnydale sans Buffy? Be careful what you wish for...

episodes in which Angel suffers chest-trauma:

• Angel
  Buffy's cross burns him but he takes it without flinching
• What's My Line? (part 2)
  Dru sprinkles him with holy water to see the pretty colours...
• this episode
  Vamp!Xander and Vamp!Willow torture him with matches

episodes in which major characters die:

• Prophecy Girl
  Buffy drowns, if not permanently
• Becoming (part 2)
  Buffy sends Angel to hell to save the world
• The Body
  Joyce
• The Gift
  Buffy saves the world a lot, rather than let Dawn die
• Seeing Red
  Tara
• Chosen
  Anya. Spike.
• this episode
  Another reality in which Xander, Willow, Cordelia and Buffy all die

Cast:

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

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• Anthony Stewart Head as Giles
• Charisma Carpenter as Cordelia
• Mark Metcalf as The Master
• David Boreanaz as Angel
• Mercedes McNab as Harmony
• Seth Green as Oz
• Danny Strong as Jonathan
• Larry Bagby as Larry
• Emma Caulfield as Anya
• Nicole Bilderback as Cordette #1
• Nathan Anderson as John Lee
• Mariah O'Brien as Nancy
• Gary Imhoff as Teacher
• Robert Covarrubias as Caretaker