The story centers on a new 10th grade student at Mica Area High School in Arizona: Stargirl Caraway, otherwise known as Susan Julia Caraway, an eccentric and compassionate non-conformist vegetarian who has spent her previous years in homeschooling. Eleventh-grader Leo Borlock narrates. Between the story and the epilogue, the narrator implies that 15 years have elapsed, and the references imply that the novel is set in the 1980s. The sequel Love, Stargirl was released August 14, 2007.
This book begins with some background information on the narrator, Leo Borlock. Leo moves to Arizona from Pennsylvania at the age of 12, and, for a going-away present, his uncle Pete gave Leo a porcupine necktie. Leo liked it so much that he decided to start a porcupine necktie collection, however he couldn't ever find a store that sold them. Two years later, the family section of the local newspaper runs an article on him for his birthday, and mentions that he collects porcupine neckties as a hobby. A few days later, Leo finds a plastic bag on his front step with a package inside. The card says "Happy Birthday!," but doesn't say who its from. Inside is a porcupine necktie.
The novel goes on to recount Leo's junior year at Mica Area High School, when a new student, "Stargirl" Caraway, sends the school, which up until that point had nothing unusual about it, into an uproar. Her unusual antics include wearing kimonos, Indian buckskin, 1920s flapper dresses and pioneer dresses to school. She carries her pet rat, Cinnamon, around with her in a bag with a life-size sunflower painted on it. During every class periods she puts a table cloth and a vase with a daisy in it on her desk. When it rains, instead of running inside like the other students, she dances in the rain. She strums a ukulele during lunch every day and sings "Happy Birthday" to kids she has never met.
Baffled by the new girl, Leo and his friend Kevin seek out the advice of Mica's local retired paleontologist, A.H. (Archibald Hapwood) Brubaker. Archie (as he prefers the children of the town to call him) is a retired college professor who never gave up teaching, despite not being certified to teach in Arizona. Students come to his house every weekend for meetings of the "Loyal Order of the Stone Bone." The boys ask Archie about Stargirl's name, and he tells them that her real name is Susan. When she had first turned up at Archie's house she had called herself Pocket Mouse, then went to Mudpie, then Hullygully and, finally, Stargirl. She changes the name according to her current likes and dislikes.
Leo begins to innocently "stalk" Stargirl, he follows her around town, and sees her put something into a mailbox. It is a unsigned handmade greeting card with the words: "CONGRATULATIONS" on it. He continues to follow her into the desert and they walk for miles. In the end, he gets anxious about where she might be taking him and runs back home
Stargirl's enthusiasm for life infuses itself into the students of MAHS. One boy comes to school with purple hair, students copy her desks' tablecloth and vase decoration and, one day, when it rains, a dozen girls run outside to dance. When Stargirl starts showing up at the school's football games, so do the rest of the students. She's made an official cheerleader at the end of the football season and never stops cheering; she cheers for everyone--the spectators out of the cheerleader's usual area, the parents at the hot dog stand, even the opposing team. During a game, after the star player from the other team gets injured, Stargirl consoles him in the middle of the field before help arrives. This event, combined with her tendency to cheer for even the other team, causes almost the entire student body (save Leo and a freshman named Dori Dilson) to shun her.
Leo eventually becomes Stargirl's boyfriend, as well as her partner in her attempts to make everyone happy. She leaves coins on the floor for people to pick up, and she sends cards to people by looking in the announcements in the newspaper. Leo discovers that it was her who sent him the porcupine necktie a few years earlier. Stargirl then takes Leo to her house to show him her 'happy wagon'. The wagon is compiled of 20 stones. For each time Stargirl feels sad, a stone is taken out of the wagon; for each time she feels happy a stone is put back in. Stargirl tells Leo that the lowest number of stones was 3 and the highest was 18. The 'happiness' (Stargirl said) that Leo had brought her made the total of the stones 18. However, he is still unsettled by her ways and suggests that the student body shuns her because of her strangeness. They argue, and two days later Stargirl disappears.
Stargirl returns to school as Susan Julia Caraway; a jean-wearing, toenail-painting, gum-chewing, average teenager, and becomes rude and cruel just like Stargirl's arch enemy, Hillari Kimble. Stargirl and Leo go out for pizza; Stargirl orders anchovies and Leo says everyone hates anchovies and she throws away the anchovies. Stargirl gives up her idiosyncrasies in the hope that the other students might accept her again. But as the weeks go by, and she sees that not even being normal can make the other students like her. When Leo notices her happy wagon, he sees that there is only 2 stones in it.
One day Stargirl has a vision that if she wins the Speech contest everyone will stop ignoring her. So she invites her friends Leo and Dori to the event, but only Leo goes with her because Dori thinks Stargirl (Susan) betrayed herself. They stay in a fancy hotel while Stargirl practices her speech. On the stage, Stargirl is very nervous to perform to so many people so she has a small conversation with everyone and just like that wins. When they return to MAHS Stargirl only finds a teacher and Dori with a banner saying: "Congratulations Susan! We're Proud of You" in the parking lot. Her vision failed. She finally gives up and reverts back to being "Stargirl." Leo retains his affection for her, but does not display it. He is tired of being an outcast and desires, once more, to be part of school society. There is the dance called the Octillo Ball at the end of the year, but Leo does not ask Stargirl to go with him. He ends up not going to the dance.
At the Ocotillo Ball, Stargirl arrives with Dori in a fabulous yellow gown, even the parents stop to stare. Leo did not ask her to the Ocotillo Ball. She lead of the school in a bunny-hop train around the country club. Hillari Kimble, gets angry and slaps Stargirl in the face saying that she ruins everything. Stargirl responds with a kiss on the cheek and says goodbye and leaves never to come back, she and her family move away.
But, Leo informs us, that she has left her mark. For example, Leo himself has become less egocentric and more introspective. The school band now features ukuleles; and the people at school are more inclined to be like themselves. Archie believes that Stargirl has a quality of being closer to the original nature of the universe than other people. As the years go by, Leo keeps watch for Stargirl's silver lunch truck; but, she never does return. He wonders if he'll ever get another chance. At the end Leo receives a porcupine necktie, implying that Stargirl has never stopped thinking or caring about him. She will always be watching him.
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