Buffy, while looking for the Trio's lair, is attacked by a demon that injects her with something that causes her to hallucinate that she is in a mental hospital.  These hallucinations are telling her that Sunnydale and vampires are all a creation of her mind.  As Buffy is drawn more and more into the hallucination, she thinks that by getting rid of her Sunnydale friends, she can become normal again.