(Plot synopsis from the official Buffy website
Buffy has a nightmare about being attacked by the Master. Later in class, a student named Wendel opens his book and spiders emerge and attack him, while a young boy stands outside the classroom window. In his lair, the Master explains to the Anointed One that fear is the most powerful force in the world, and changes are about to take place.
Buffy, Willow and Xander question Wendel, who says he's had recurring nightmares about such attacks. Later in class, Buffy finds out there is a test she's totally unprepared for. As soon as she starts writing, she glances at the clock and sees almost the entire hour is up. Again, the boy appears. Later, a girl goes into the school basement and is attacked by a huge Ugly Man. Questioned in the hospital, she's able to provide one clue: the attacker said the words "Lucky Nineteen."
Xander enters a class to find everyone staring at him he's only wearing boxer shorts. Buffy finds a newspaper picture of the boy who keeps appearing. The article says he's in a coma after falling down the steps after a little league game. Buffy notices his uniform number: 19. Giles says even though the boy's in the hospital, perhaps Buffy has been seeing his astral body, which can travel through time and space. Buffy's father picks her up at school and explains he left home because Buffy caused so much trouble and was a disappointment to him. He leaves Buffy in shock and the boy appears again. Willow and Xander talk to Giles, who explains that their nightmares have been coming true, leaving everyone vulnerable to all sorts of danger. Buffy finally confronts the boy, who explains that Ugly Man wants to kill him. Suddenly, the Ugly Man appears and starts to attack Buffy, who is no match for him.
Giles warns Xander and Willow that Buffy's nightmares could be fatal, and they must find her. Willow's attempt leads her into her own nightmare: she is onstage in an opera for which she is woefully unprepared. Xander's search leads him to yet another nightmare: a clown he feared from childhood chases him with a knife. Buffy and the boy manage to escape Ugly Man by literally jumping from school during the day into a cemetery at night. The Master appears and shoves Buffy into an open grave. Willow, Xander and Giles need to find Buffy unless Billy is awoken, all reality will become nightmares within hours. They notice a cemetery in darkness is across the street, and go to investigate. Seeing Buffy's headstone, Giles laments not protecting her better, until Buffy emerges as a vampire. Giles says it's reversible, if only Billy could wake up. They go to the hospital and Ugly Man appears again. Buffy manages to attack him. She asks the little boy to finish the job he walks up to him and peels off his face. Buffy returns to normal, and the real little boy awakens from his coma. His little league coach enters and is shocked to find the boy awake. Buffy confronts him about beating up the boy for losing a game when he tries to flee, Xander grabs him. Buffy's dad shows up after school and cheerfully greets her another happy ending at the Hellmouth.