The novel Stargirl by Jerry Spinelli is told from the first person point of view of Leo Borlock. The first chapter entitled "Porcupine Necktie" explains how Leo's Uncle Pete owned a tie with a porcupine on it that Leo admired when he was little. When Leo was twelve, he moved from Pennsylvania to Arizona, and his uncle gave him the tie as a going away present, so Leo decided to start a collection. After two years he still only had one tie in his collection. On his fourteenth birthday, there was an article about Leo in the local Mica paper, and it stated that Leo collected porcupine neckties. Several days later he found one wrapped up on his front porch with no sign of who sent it.

Chapter one begins with the first day of Leo's junior year in high school. Everyone at school, including his best friend Kevin, was talking about a new girl named Stargirl Carraway. Leo saw her at lunch, wearing a ruffled gown with long sandy hair carrying a ukulele. She was a sophomore who had previously been homeschooled. Kevin wanted to put her on the school television show Hot Seat, which Kevin hosted and Leo acted as the director. When she started playing her instrument and singing at lunch, students weren't sure how to react, a few clapped, Kevin and Leo slapped hands, agreeing she would be an interesting person to interview on their show.

In chapter two Hillari Kimble, a popular junior, was spouting off theories about Stargirl, such as that she was a an actress hired by the administration or that she's a narc. At lunch Stargirl walks up to people who are having birthdays and sings to them. Leo knows that Mica Area High School is not a place of nonconformity; most people tend to look, dress, and act the same. Therefore Stargirl's odd outfits, including flapper dresses, kimonos, or overalls, stand out. She constantly raises her hand in class and says hi to people in the halls. She tried to join cross-country, but at the first meet, she turned the wrong way and never arrived at the finish line. One day she was spotted toting her pet rat in the sunflower backpack. People didn't know what to think, Leo, however, believed she was real and wonderful.

Chapter three starts with Kevin Quinlan asking Leo repeatedly if he has recruited Stargirl to be on Hot Seat yet. Leo wasn't sure why he kept putting it off. The theories about Stargirl had grown and included that she was an aliens, her parents were witches, and she lived on a bus. Stargirl was such a contradiction in the way that she was the friendliest person he knew, but she had no friends. He couldn't believe she had no television in her house.

One day after school Leo decided to follow her. They walked all over town. At one point she consulted a piece of paper, left something in a mailbox, and walked away. He looked inside and found a card that said "Congratulations!" but was unsigned. She took the rat out of her bag and put him on her shoulder, strummed her ukulele and sang. She walked on toward the desert and the saguaros. Since it was past dinnertime, Leo thought of calling out to her but just turned and ran home.

Chapter four begins by describing more about Hillari Kimble, who was famous for a hoax that she pulled in her sophomore year. She tried out for cheerleading, made it, and then quit. She is dating Wayne Parr, her handsome popular boyfriend. Wayne was put on Hot Seat due to his popularity, and when Kevin asked him who his role model was, Wayne answered, "GQ" as in the magazine. Wayne said he wanted to become a model and be on the cover someday. He had led the school ever since.