Cher Horowitz (Alicia Silverstone), a wealthy, popular 15-year-old, lives in Beverly Hills with her widowed father, Mel (Dan Hedaya), in an enormous house, enjoying the luxurious life that Mel provides as a very successful lawyer. Though good-natured, Cher is naive and caught up in a superficial lifestyle revolving around expensive clothes and the social hierarchy of her school. Her best friend Dionne (Stacey Dash) has a similar outlook on life, though she dates a high school boy (Donald Fasion), which Cher claims is a pointless endeavor.
Cher's debate teacher, Mr. Hall (Wallace Shawn), gives her a C grade for the semester, which starts a dramatic chain of events. Dionne, who got a lackluster grade as well, hatches a plan with Cher to find a romantic interest for Mr. Hall, thus making him happy enough to consider boosting their grades. They select another teacher, Miss Geist (Twink Caplan), as the chief candidate, and slip a fake love letter into her mail cubby. She soon begins dating Mr. Hall, and both teachers become much more lax with their grading. Cher works her grade up to an A minus, greatly impressing her father.
At home, Cher interacts with Josh (Paul Rudd), the college-age son of one of Mel's ex-wives, who often stays at the Horowitz home to avoid his mother and new stepfather. Josh and Cher usually have a typical sibling relationship, with mild arguing and teasing. Josh is nearly the polar opposite of Cher: an earthy, environmentalist, do-gooder..
Tai Frasier (Brittany Murphy), is a grungy artist with a trusting disposition and a clueless grasp of high school popularity, transfers to Cher's school. Cher and Dionne befriend Tai, and decide to give her a makeover to improve her social standing. However, Tai sparks up a romance with cheerful, skateboarding stoner Travis (Breckin Meyer). Cher attempts to steer Tai toward the rich and popular Elton (Jeremy Sisto), and leads her in after-school workout sessions. Josh disapproves of Cher's project with Tai, and accuses her of attempting to turn an innocent girl into a superficial brat.
Through a series of subtle hints, Cher gets Tai and Elton to notice each other. Tai is giddy with excitement, and while surprisingly receptive, Elton pays much more direct attention to Cher. To present the newly made-over Tai to the high school elite, Cher and Dionne bring her to a party. Once again they attempt to steer her toward Elton, and believe that they made a connection when Tai is knocked unconscious with a projectile shoe, and Elton revives her with an ice pack. However, when departing, Cher winds up being driven home by Elton, who makes a pass at her, revealing that he had been interested in her all along. Cher is shocked and repulsed, and Elton abandons her in a parking lot far from home, where she's immediately mugged, and is forced to call Josh from a payphone to come pick her up. On the drive home, Cher suggests that they pick up food for Mel and his colleagues, who are working on a difficult case at the house. Impressed by Cher's consideration, Josh begins to form a new opinion of her.
Another new student, suave and handsome Christian, transfers in. Cher is taken with him immediately, and considers breaking her own rule of never dating a high school boy. The two become friends, and attend a party along with Tai. After Christian picks Cher up, Josh is clearly envious and tells Mel that he will trail them to the party for Cher's protection. Cher and Christian bond on the dance floor, while Tai is self-consciously standing alone in a corner (watching Elton dancing with his new girlfriend, Cher's social rival Amber). Josh arrives and dances with Tai to cheer her up, which Cher notices and is surprised at his thoughtfulness.
Cher decides to move things forward with Christian, and invites him over one evening when her father is out. To her disappointment, he ignores her advances in favor of watching Tony Curtis movies. When driving with Dionne and her boyfriend Murray the next day, Cher laments her situation, causing Murray to burst out laughing and point out that Christian is obviously gay. Cher is embarrassed at her misconception, but remains good friends with Christian.
While at the mall, Tai experiences a brush with death as some male acquaintances nearly drop her over the rail from the upper level. Though she escapes unscathed, Tai becomes something of a celebrity at school, surpassing Cher in popularity and rebuffing her love interest, Travis, troubling Cher.
Cher fails her long-anticipated driving test, and returns home dejected. Tai confides in her that she has a crush on Josh after he had danced with her at the party. Cher is confused at the feelings that arise in her at Tai's confession, and tries to convince Tai that she and Josh do not mesh well together. Tai is affronted and insults Cher ("You're a virgin who can't drive!"). A miserable Cher goes shopping to clear her head, and realizes that she herself is in love with Josh.
In an effort to impress Josh, Cher collects donated items for victims of the Pismo Beach disaster, surprising him with her dedication and selflessness. When Josh and Cher help a stressed Mel out with his case by doing paperwork at the house, Cher makes an error and is insulted by an angry associate of Mel who leaves in a huff. Josh defends her, and they admit their feelings for each other.
Soon afterward, Miss Geist and Mr. Hall are married. Cher, Josh, Dionne, Murray, Tai, and Travis attend the wedding, with Cher skillfully catching the bouquet.