The book begins with the introduction of the character Leo Borlock. He already gives some unique characteristics, including his love for porcupine neckties. He received a porcupine necktie for his fourteenth birthday and doesn't know who sent it.
The plot of the book begins when the character of Stargirl is introduced to Mica High. This school is in a very small city in Arizona causes much uproar when a new student joins. Except, Stargirl isn't just a new student. She's a flashy, expressive girl that flaunts around in skirts and sings with her ukelele, walking around Mica High with her pet rat Cinnamon. At first, the whole school is confused and taken aback, who is this girl? And why does she call herself Stargirl? Leo Borlock questions the same thing with his friend Kevin, and they contemplate putting her on their show known as the "Hot Seat", a show where they question their fellow peers.
As the book goes on, the students seem to become more accepting of her presence. She always emits positivity and radiance, and after cheering enthusiastically at a football game, the cheerleaders let her in. Leo falls in love with her attitude and kind personality as well, wanting to slowly be around her more and more. However, this seems to affect Stargirl's popularity as she cheers for both teams, not just Mica High. Students suddenly shun her for what she has done and she takes a major fall in popularity. Leo takes a sudden notice as well, noticing that by him dating Stargirl that his fellow students are shunning him as well.
Stargirl tries to please Leo by going back to her actual name, Susan, and act normal just like all the other students around her. Leo coaches her how, yet Stargirl realizes that none of the students are warming up to her causing her to revert back to her normal self. After a disheartening relationship with Leo, Stargirl ends up going to the dance alone. she successfully leads dance after dance, but as the night goes on she suddenly disappears. Students look for her the next day, but she has disappeared from Mica High.
