Willow: So why is this happening?
Xander: Well, that explanation was shorter than usual. It's Billy!
Wow. This is a classic Buffy standalone, and really a great example of what the early seasons are all about.
A mysterious young boy appears and suddenly everyone's nightmares start coming true.
Simple idea, but it's used here to great effect. Wheden and Greenwalt explore some of the absolute archetypal teenage (and indeed adult) nightmare scenarios: forgetting the history test, turning up to class naked, fear of spiders, fear of clowns, fear of turning up to sing soprano and not knowing the words (or how to sing). There's barely a stone of subconscious paranoia left unturned and there are some great laughs along the way.
But like the best Buffy episodes, this one pulls the rug out from under us, and it gets really dark and sinister, with the gradual revelations regarding Billy's attack, as well as the escalating danger to the others. The moment where Giles encounters Buffy's grave is movingly done, and the finale with the "Lucky nineteen" thing still chills me. It's here that Buffy does what it does best; provides an intersection between the comfort of the fantastic and the horror of real life.
One of the highlights of season 1 for my money, it gets five Singing Willows.