First-person narrator Leo Borlock recalls how, as a child, he loves his Uncle Pete’s special tie which features a porcupine. When Leo is 12, his family moves from Pennsylvania to the small town of Mica, Arizona. Uncle Pete gives Leo the porcupine necktie as a going away present. Leo determines to start his own collection of porcupine ties, but two years later, he still only has the one from his uncle. Leo’s mother mentions his aspirations to collect porcupine ties in a birthday section of the local town newspaper. A few days later, Leo receives a mysterious birthday present from an unknown sender: a porcupine necktie.

On the first day of their junior year at Mica Area High School (MAHS), Leo and his friend Kevin Quinlan hear rumors about a new student named Stargirl. They finally see her at lunchtime. Stargirl is wildly different from the other kids at school. She wears a long, ruffled dress and carries a ukulele strapped to her back. They learn that Stargirl is in tenth grade and was previously homeschooled. In the cafeteria, she takes her ukulele and begins to sing and dance among the tables while the other students stare in silent amazement.