Buffy the Vampire Slayer's first season veered between monsters of the week that allowed the characters to get to know each other and smaller adventures that led to the climactic battle between Buffy (Sarah Michelle Gellar) and the Master (Mark Metcalf). Season 1, Episode 10, "Nightmares," incorporated both elements. The main conflict of the episode surrounded a comatose boy named Billy (Jeremy Foley) whose trauma caused his astral body to manifest other people's nightmares in the real world.
Buffy and her friends, found themselves at the center of this. One of their classmates was swarmed by tarantulas (as he later explained, he wasn't afraid of spiders but hounded by guilt because he inadvertently let his own spiders die); another was accosted by his embarrassing mother when he was trying to be cool in front of his friends. Cordelia (Charisma Carpenter) was forced to be a stereotypical nerd, Giles (Anthony Stewart Head) couldn't read, Xander (Nicholas Brendon) was naked in school and Willow (Alyson Hannigan) was forced to go on stage and fail at singing opera in front of a crowd. It was all fairly straightforward and many of the nightmares were very typical.
The episode begins with Buffy having a nightmare about battling, and losing to, the Master -- though this takes place before the dreams and reality start to melt together. This isn't the first time she expresses trepidation about her upcoming showdown with the Master, though she doesn't learn that she's prophesied to die until the finale two episodes later. She also has a waking nightmare where her father Hank (Dean Butler) tells her that she was responsible for her parents' divorce, which is traumatizing in its own right.
Having noticed Billy's astral body in the vicinity of many of the waking nightmares, Buffy and Giles begin to investigate what happened to the boy. He was brutally attacked, but when Buffy speaks to his astral body, he can only explain that the "ugly man" did it to punish him for his poor performance in a recent baseball game. The nightmares continue to appear and intensify around them as Buffy tries to figure out what to do.
Before she can reach any conclusion, though, she and astral-Billy are confronted by the Master, who proceeds to bury Buffy alive. When she's found by her friends, Giles sadly explains that this is his nightmare: failing Buffy and letting her get killed. She claws her way out of the grave and the nightmare quickly becomes her own: she's been made into a vampire, presumably by the Master, which is one of her greatest fears.
They theorize that if Billy's nightmare ends, everyone else's will as well, so vamped-out Buffy accompanies her friends to the hospital, where she confronts the "ugly man" that haunts Billy. They fight and Buffy manages to incapacitate him, but, as she explains, Billy has to be the one to end him and therefore the nightmare world he's accidentally created.
When Billy does this, everything returns to normal. Buffy is no longer a vampire and they discover that Billy's baseball coach is the one that beat him for not doing well enough. This contrasts with Giles' fears that his guidance or lack thereof will get Buffy killed. Giles blames himself, while the coach blames Billy and takes it out on him in a cruel and inappropriate way.
Though Buffy was only a vampire for a few minutes, this was the first instance of a Slayer-turned-vampire (or "slaypire") appearing in the canon and paved the way for future vampire alternates to appear in later episodes. This vision of Buffy's death also preceded her actual canonical deaths, one of which would happen only two episodes later in the Season 1 finale.