Angel
Original air date: April 14, 1997
After Angel saves Buffy's life from a group of vampires that the Master has sent to kill her, she takes him. When the two kiss for the first time, Angel accidently transforms into a vampire, horrifying Buffy. Darla, Angel's lover for Centuries enters the picture and tries to get Angel to kill Buffy and vise versa, but in the end, Angel kills Darla.
After Buffy realizes Angel is a vampire, her friends try to get her to realize that she can't date a vampire, meanwhile Darla ries to make Angel see he can't date a mortal, especially a slayer. This is the first episode where Buffy and Angel face off with eachother after Darla tricks Buffy to believe Angel attacked her mother.
Character Study: Angel
Angel is very tortured in this episode, first his feelings for Buffy grow, then she learns he's a vampire, Darla returns to him and makes him realize there can never be anything between him and the slayer for she will kill him. But the look on his face when Darla presents to him a bleeding Joyce, he looks at the wound on her neck with such longing, but resisting the temptation makes you see just how much he sufferes from having a soul.
Episode Highlight:
At the end, after Buffy and Angel kiss goodbye, she unintentionally imprints a scar on his chest by the burn of the  Cross necklace she is wearing that he had given her in the first episode.