A New Man

In which Ethan comes to town and Giles gets really horny as a result.

Official back-of-box synopsis:

Buffy and Riley explore the extent of each other's powers as they become more romantically entwined. Giles is transformed into a demon and gets mistakenly hunted down by an unknowing Buffy.

From the old WB Buffy website:

ANOTHER BUFFY BIRTHDAY DEBACLE - Buffy and Riley explore the extent of each other's powers as they become more romantically entwined. Feeling left out these days, Giles turns to his old buddy Ethan Rayne for camaraderie, but Giles must rely on an unlikely source when he is transformed into a demon and mistakenly hunted down by an unknowing Buffy.

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 featuring Buffy's birthday:

• Surprise
  Buffy is 17, and takes her relationship with Angel to a new level.
• Helpless
  Buffy faces her ultimate trial, and her ultimate betrayal, on her 18th birthday.
• Blood Ties
  Dawn goes missing on Buffy's 20th.
• Older And Far Away
  Buffy is 21, and throws the weirdest, possibly the longest, birthday party ever.
• this episode
  Giles is transformed by Ethan as Buffy celebrates her 19th.

episodes in which our heroes get drunk:

• The Dark Age
  Giles - whisky
• Beer Bad
  A Very Special Buffy
• Something Blue
  Willow's "Not drowning, wading" in light beer
• The Yoko Factor
  Giles - whisky - again
• Life Serial
  Buffy - Tequila - priceless look on her face
• Entropy
  Spike and Anya. Hilarity ensues. Oh, wait, not hilarity.
• this episode
  Giles -- again -- too many beers with Ethan

Cast:

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

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• Anthony Stewart Head as Giles
• James Marsters as Spike
• Robin Sachs as Ethan
• Emma Caulfield as Anya
• Marc Blucas as Riley
• Lindsay Crouse as Professor Walsh
• Amber Benson as Tara
• Elizabeth Penn Payne as Waitress
• Michelle Ferrara as Mother