Inca Mummy Girl

In which Buffy and Cordy host exchange students, Willow dresses as an Eskimo, Giles suffers a head wound, and Xander woos a girl with Twinkies. (Exhibit C in the 'Xander is Hot' debate)

Official back-of-box synopsis:

A Peruvian exchange student stirs up Buffy's and Giles's suspicions when a student goes missing on a field trip to the Natural History Museum, but Xander is the one who's really in jeopardy when he falls for the ravenous beauty.

From the old WB Buffy website:

MUMMY MAY I -- Buffy and Giles are suspicious of Ampata, a Peruvian exchange student staying at Buffy's house, when a fellow student goes missing during a school field trip to the Natural History Musueum and a broken Peruvian pictogram-cover plate is found in his place. Meanwhile, after Xander falls head over heels for Ampata, he faces certain death when his love interest is revealed to be an ancient mummy who must kill to stay alive.

Cross-reference:

episodes Featuring Major Moments In Xander's Love Life:

• Teacher's Pet
  The teacher he's hot for is a preying matis
• The Zeppo
  he loses his virginity: everyone's looking the other way
• The Harsh Light Of Day
  Anya, basement, cran-apple juice everywhere
• this episode
  Ampata nearly sucks the life out of him

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
• Kristine Sutherland as Joyce
• Seth Green as Oz
• Jason Hall as Devon
• Danny Strong as Jonathan
• Ara Celi as Ampata
• Henrik Rosvall as Sven
• Joey Crawford as Rodney
• Kristen Winnicki as Gwen
• Gil Birmingham as Peru Man
• Samuel Jacobs as Peruvian Boy