Sixteen-year-old Leo Borlock arrives at Mica Area High School in Arizona for the first day of eleventh grade. Leo’s best friend, Kevin, asks Leo if he’s seen “her” yet. When Leo questions whom he’s talking about, Kevin tells Leo he’ll know her when he sees her. At lunchtime, Leo realizes the “her” everyone is talking about is a new girl named Stargirl Caraway. The students at Mica Area High School see Stargirl as different from them not only because of her name but also because of her quirks, such as playing the ukulele in the cafeteria, giving out anonymous gifts to people she doesn’t know, and carrying around her pet rat, Cinnamon. Some students, such as Hillari Kimble, think Stargirl must be faking her personality to get attention. But others, like Leo, are intrigued by Stargirl. Kevin wants to interview Stargirl on their school TV show, Hot Seat, but Leo is hesitant.
Stargirl draws even more attention for her behavior at a football game. She dances around with the marching band and the players themselves, and the crowd cheers for her. The head cheerleader invites Stargirl to join their squad, and as a result the games draw more crowds than they ever have before, as people are curious to see what Stargirl will do. Instead of being looked down on, Stargirl becomes more popular due to her performances at the football games. As Stargirl remains a mystery to Leo and Kevin, they decide to seek advice from Archie, a nearby retired professor whose house operates as an informal school for students of all ages. Archie knows Stargirl from helping out Stargirl’s mother when Stargirl was homeschooled, and he advises Leo and Kevin to pay attention to their own reactions to Stargirl.
By the time Leo and Kevin finally bring Stargirl on as a Hot Seat guest, Stargirl’s popularity has begun to dwindle. Some students become suspicious of Stargirl’s unusual behavior and of how she knows so much about certain people. Well into basketball season, the school’s team is undefeated. Stargirl feels bad for the teams that lose and ends up cheering for them as well as for her team. During her Hot Seat interview, Stargirl is interrogated with accusatory questions by her classmates who resent her behavior. Later, the school blames Stargirl when their team loses in the playoffs.
Leo becomes aware that Stargirl has feelings for him when he finds a note from her that says, “I love you.” They begin spending time together both in and out of school, and Leo learns more about the way Stargirl sees the world. However, everyone in school has been giving Stargirl the silent treatment due to her cheering for other schools’ teams, and they begin extending the same treatment to Leo. Leo grows frustrated as he wants to be with Stargirl but also does not want to be ostracized.
Leo tells Stargirl that no one likes her because she dresses and behaves so differently from everyone else. As a result, Stargirl starts calling herself Susan and begins dressing and acting like all the other kids in school. She leaves her ukulele and pet rat at home and begins wearing makeup and jewelry like the other girls. Leo feels thrilled by this change. However, Susan realizes after a few weeks that her classmates still refuse to talk to or even look at her. She explains to Leo that if she wins the state’s upcoming oratorical contest, she believes she will become popular again.
Leo accompanies Susan to the state’s oratorical contest, and she ends up winning with a speech she invents on the spot. On their way back to the school, Susan expects to see a crowd of students waiting to congratulate her, but only two teachers and her friend Dori are there. The next time Leo sees Susan, she has gone back to being Stargirl, and she tells Leo that she doesn’t expect him to ask her to the upcoming Ocotillo Ball.
Leo doesn’t go to the ball at all. Stargirl does, and she starts a spectacle that people will talk about for years to come. At the ball, Stargirl leads students in a bunny hop dance out the door, across the tennis court, and all around the golf course for hours. The only couple who doesn’t join is Hillari and her boyfriend, Wayne. When the group returns, Hillari accuses Stargirl of ruining everything and slaps her, to which Stargirl responds by kissing Hillari on the cheek and leaving.
No one at the school sees Stargirl after that night. That summer, Leo goes to her house to find a “For Sale” sign in the yard. Leo goes to college, becomes a set designer, and moves to the East Coast. He occasionally talks to Kevin, who informs him that their former classmates still talk and wonder about Stargirl. Leo finds himself doing things he learned from Stargirl, such as dropping change for other people to pick up or buying cards for strangers. He doesn’t know where Stargirl is or what has become of her, but on his birthday one year, he receives a gift that he knows is from her.