(Plot synopsis from the official Buffy website)
Buffy, in an attempt to bring some normalcy into her complicated life, decides to try out for cheerleader. When she goes to the big tryout, she meets a quiet girl named Amy. As one girl auditions, she gets more and more into her routine, and suddenly her hands begin to smoke. Buffy knocks her to the ground and puts the girl's smoking hands out. Amy tells Buffy that her mother was a cheerleading star and she feels pressure to make the squad. When Amy leaves, Willow says it's a lot more than pressure — she thinks Amy's mom is a tyrant. Neither Amy nor Buffy is chosen as part of the regular squad — both are alternates. Amy is distraught when she is only chosen to be third alternate — with Buffy as first alternate.
In a spooky attic with a cauldron, someone is casting spells with dolls. The next day, snooty Cordelia Chase, who has made cheerleader, is temporarily blinded while taking her driving test. Giles thinks Amy's a witch, but the only way to prove it is to form a potion and pour it on her — if her skin turns blue, she is a witch. Buffy, Xander and Willow join forces to test this theory in chemistry class, and Giles is proven correct. Then another cheerleader has a mysterious accident, and when Amy reacts to it with horror, it makes Buffy doubt whether Amy realizes what she's doing. Amy comes home in a huff to find her mother on the couch watching TV. She tells her mother to write her history report. We see that Amy is holding an ID bracelet which Xander gave Buffy for good luck at the cheerleading tryouts — Buffy will be Amy's next victim.
As first alternate, Buffy is now on the cheerleading squad, and starts acting strangely during practice. When she tosses another cheerleader across the room, it's time for the next alternate to come in — Amy. Buffy suddenly becomes weak, and Giles says the only way to keep her from dying is to get Amy's spell book. Willow and Xander stay at school to keep an eye on Amy. Giles and Buffy force their way into Amy's house, and Buffy discovers that the woman they're speaking to is not really Amy's mother but Amy herself. The mother switched their bodies so she could relive her glory days as a cheerleader.
Giles finds the spell book, and they all troop off to school, where "Amy" is getting ready to cheer. As Giles chants, "Amy" becomes weaker as the transformation begins. "Amy" charges out of the gym to find Buffy and kill her. Just as she's about to get her, the transformation is complete. "Amy" is now the real Amy, and the mother goes berserk. She starts to cast an evil spell on Buffy, but Buffy deflects it with a mirror and sends it back to the mother, who disappears. Everything seems to be back to normal, until we catch a glimpse of the mother's cheerleader trophy in a school display case. Its eyes are moving...
