The novel Stargirl by Jerry Spinelli is told from the first-person point of view of Leo Borlock. The majority of the book takes place when he is sixteen. A new girl, named Stargirl Carraway, appears at school the first day of his junior year; she is a sophomore. Everyone talks about her because she is so unique. She dresses differently from the other girls, she carries a ukulele and a pet rat, and she was previously homeschooled. Very friendly and kind, people are embarrassed to sit with her because she is so odd: singing happy birthday to people at lunch and dancing outside in the rain. Leo and his best friend Kevin Quinlan talk about putting her on their school television show called Hot Seat to find out more about this strange student, but Leo puts off talking to her for a long time.

One popular senior girl, Hillari Kimble, does not like Stargirl and threatens to drop her rat down a flight of stairs, but she doesn't do it. Stargirl gains attention at a football game on weekend when she joins the cheerleaders on the field and does a bunch of crazy stunts. The cheerleading captain, Mallory Stillwell, officially asks Stargirl to become a cheerleader the following week as her antics drew a crowd. Stargirl's popularity continues to rise as a few brave students, including a freshman named Dori Dilson, agree to sit with her at lunch. Then some students begin mimicking her style by carrying instruments or wearing interesting backpacks. It is clear that Leo has feelings for Stargirl in the way that he is embarrassed whenever she notices him.

However, sometimes Stargirl's kindness crosses the line. She shows up to the funeral of a man she doesn't know then buys a child who broke his arm a new bicycle. These gestures are meant to show her affection, but people aren't used to such love from strangers. Then when she is cheering at a basketball game, a student on the opposing team gets hurt, and Stargirl rushes to comfort him. Her teammates are upset that she would support the opposing team in this way, especially since their basketball team has gone undefeated all year. They play a trick on Stargirl, leaving her behind at the following away game to show their disapproval. After the next game when she keeps smiling and cheering, despite the fact that their team is losing badly, they kick her off the squad.

In the meantime, Leo has scheduled her to appear on Hot Seat, which turns out to be a disaster with the jury bullying Stargirl, asking questions about why she changes the words to the pledge of allegiance or why she came to their school to begin with. Due to their harsh judgments, the episode is never aired. The next day, Valentine's Day, she gives her homeroom candy hearts as if nothing happened. She also gives Leo a special card, telling him that she likes him.

Leo shows up in her driveway one evening, and they talk, and it's clear they want to date. They begin walking together in the hallways and hanging out on weekends; however, when they are at school, Leo notices that people ignore them. They punish him because he is associating with her by pretending he doesn't exist. Leo doesn't like it and talks about it to Archie, who is a retired paleontologist friend. Archie tells Leo he needs to decide whose affection is more important to him: Stargirl's or everyone else's. Leo doesn't want to have to choose. He enjoys his time with Stargirl playing strange card games and helping her practice her speeches for the oratorical contest, but he doesn't like not being acknowledged at school. In an attempt to gain favor, Stargirl decides to change herself into Susan, her actual name, and dresses and acts like everyone else, but they still don't like her. She doesn't like herself this way, and neither does her true friend Dori.

Leo accompanies her to the state speech competition in Phoenix, which Stargirl wins. She thinks that when she returns the entire student body will be waiting in the parking lot to support her, but only Dori and two teachers show up. Stargirl returns to acting like her old self and again tries to get people to become her friend by starting a band and offering back rubs, but no one is interested, and even Leo is too embarrassed to be seen with her anymore, so they break up.

Stargirl attends the Ocotillo Ball by herself. One boy asks her to dance, and then she leads a line of students doing the Bunny Hop off onto the golf course before returning back from the woods to the tennis court. People talk about it for years to come. After that dance, they never see Stargirl again. When Leo talks to Archie, he finds out that Stargirl's family moved back to Minnesota. She left an impression on people though, especially Leo. He notices little things and watches for random acts of kindness even fifteen years later. He knows that Stargirl is still keeping an eye on him when he receives an unsigned birthday present on his front porch containing a porcupine necktie, which is the same gift she gave him when he first moved to Arizona.