Despite Giles' misgivings, Buffy decides to try out for the cheerleading team. During trials, the hands of a girl named Amber spontaneously combust. Amy seems to be under strong pressure from her winning mother, a star cheerleader in her day, and is crushed when she only makes the substitute list after Cordelia and Buffy. We see a brief glimpse of somebody bewitching Barbie dolls over a cauldron. Next thing, Cordelia cannot see, which she proves the hard way during driving instruction, and is just saved in the nick of time by Buffy.
To prove that Amy is a witch, Buffy, Xander and Willow collect eye of newt and some of Amy's hair during science class, and prove that she has cast a spell. Amy comes home and tells her mother to get with her homework - while she goes upstairs with Buffy's bracelet that she stole while Buffy was collecting her hair.
The next morning, a slightly manic Buffy blows her chance at the cheerleading squad when she tosses another girl through the room, ceding her slot to none other than Amy. Buffy turns out to have something more than just a mood disorder: a bloodstone vengeance spell has destroyed her immune system, giving her only about three hours to live. The only way to cure her and break the other spells is to get the witch's spell book. The ailing Buffy and Giles go confront Amy's mother Catherine, and find the real Amy inhabiting Catherine's body: Catherine, months ago, had done a spell to switch bodies, saying that Amy was wasting her youth. Giles finds the witch's book and takes Amy and Buffy back to school to break the spells. Buffy is fading fast.
Catherine is cheering Sunnydale's school basketball team when she starts getting flashes of what Giles is trying to do. Xander and Willow are unable to stop her from storming into the science lab with an axe, but buy enough time for Giles to break the spells: Amy and Catherine are restored to their own bodies, and Buffy feels good enough to fight. However, Amy's mother's power is too great, and it is only by kicking down a steel vent cowling and reflecting her last spell can Buffy win the day; the mother vanishes.
When Amy and Buffy talk in the school hall the next day, they pass by the school's trophy collection where the statue to "Catherine the Great" stands. While both girls wonder where Amy's mother ended up, the camera pulls close to the statue's face, revealing the mother's eyes and a muffled voice pleading for help.