Cherilyn "Cher" Horowitz is a good-natured but superficial girl who is attractive, popular, and extremely wealthy. A few months shy of her sixteenth birthday, she has risen to the top of the high school social scene. She lives in a Beverly Hills mansion with her father Mel, a ferocious $500-an-hour litigator; her mother died from a freak accident during a routine liposuction procedure when Cher was a baby. Cher's best friend is Dionne Davenport, who is also rich, pretty, and hip, and understands what it's like to be envied. Though Dionne has a long-term relationship with popular student Murray, Cher claims that it is a pointless endeavor.
Among the few people to find much fault with Cher is Josh, her socially conscious ex-stepbrother, who visits during a break from college. Josh and Cher spar continually but without malice; she mocks his scruffy idealism, while he teases her for being selfish, vain, and superficial, and says that her only direction in life is "toward the mall."
Illustrating that Cher's selfishness is usually innocent and relatively harmless, Cher plays matchmaker for two lonely, nerdy, hard-grading teachers, Mr. Hall and Miss Geist. She achieves her ostensible purpose to make them relax their grading standards so she can renegotiate a bad report card but when she sees their newfound happiness, she realizes she enjoys doing good deeds. Cher decides to give back to the community by "adopting" a "tragically unhip" new girl at school, Tai Frasier. Cher and Dionne give Tai a makeover and initiate her into the mysteries of popularity. Cher also tries to extinguish the attraction between Tai and Travis Birkenstock, an amiable skateboarding slacker, and to steer her toward Elton, a popular rich snob.
Her second matchmaking scheme backfires when Elton rejects Tai and attempts to seduce Cher. When a handsome new student named Christian arrives at their school, Cher takes a shine to him and attempts to secure him as her boyfriend. Eventually, Murray spells it out to her and Dionne that Christian is not interested in her because he is gay.
Despite the failure of this endeavor, Cher remains on good terms with Christian, primarily due to her admiration of his taste in art and fashion. Matters get worse when Cher's "project" works too well, and Tai's popularity surpasses her own. The events afterward reach crisis stage after Cher fails her driver's test and can't "renegotiate" the result. When she returns home, crushed, Tai confides that she's taken a fancy to Josh and wants Cher to help her "get" him. Cher says she doesn't think Josh is right for Tai, and they quarrel. Feeling "totally clueless," Cher reflects on her priorities and her repeated failures to understand or appreciate the people in her life.
After much soul searching, Cher realizes she is romantically interested in Josh. She begins making awkward but sincere efforts to live a more purposeful life, including captaining the school's Pismo Beach disaster relief effort. Cher and Josh eventually admit their feelings for one another, culminating in a tender kiss.
In the end, Mr. Hall and Miss Geist wed; Cher's friendships with Tai and Dionne are solidified; Tai and Travis are in love; and Cher wins a $200 bet for catching the bouquet at the wedding. She embraces Josh, and they kiss as the film closes.