Episode 68 of "Buffy, the Vampire Slayer"
s04e12 A New Man
Title: A New Man
Original Air Date: 01/25/2000 on WB
Director: Michael Gershman
Writer: Jane Espenson
Guest Stars: Robin Sachs (Ethan Rayne), Elizabeth Payne (Waitress)
Plot Outline:
At Buffy's 19th birthday party, Giles finally realises that he is a middle-aged man hanging around with a bunch of teenagers and starts to get depressed about his life. He feels even more like the third wheel when he finds out about Riley and the Initiative, which Buffy neglected to tell him about. When Ethan Rayne returns to town, instead of kicking his butt back to England, Giles goes for a drink with him. The next morning Giles wakes up and notices that he has turned into a demon.