Stargirl, living now in Pennsylvania, tells her own story this time, in "the world's longest letter," which is actually a series of journal entries. New in town, homeschooled, and feeling rejected by Leo, the 16-year-old narrator of the first book who had fallen under her spell, she is lonely and sad -- her "happy wagon," where she keeps stones representing her level of happiness, is almost empty. She befriends Dootsie, a loud but lovable 6-year-old who takes a shine to Stargirl and her pet rat.
Dootsie introduces her to Betty Lou, an agoraphobic elder woman. She is quite nice and Stargirl soon becomes friends with her as well.
With the arrival of autunm, Stargirl's life is affected when, arriving at her Enchanted Hill to plant another spatula in her solar calendar, a calendar counting down the days to the Winter Solstice, she sees a house on fire, and in her attempt to break in to warn any possible residents, she ends up in the hospital with smoke-damaged lungs and a sprained ankle. She stays in the hospital for five weeks, getting visited by Dootsie (in her Halloween costume), Alvina, a grumpy young girl who delivers donuts to Betty Lou, Perry, a teen boy who she is falling in love with, and his harem: The Honeybees. She is also getting calls from Betty Lou. Once Stargirl recovers, she returns to her Enchanted Hill to plant the next spatula, only to find Perry has been planting them in her absence. Perry and Stargirl share a sunrise kiss, ending Stargirl's confusion over having feelings for both Leo and Perry, but leaving her to deal with the reality of living with uncertainty.
As winter sets in, Stargirl turns herself to planning her Winter Solstice party, inviting all of the people she has encountered in her new town to celebrate the beginning of winter by joining her at sunrise on the Enchanted Hill, which she nows calls Calendar Hill. Stargirl also discovers the truth about Perry. He has been very mysterious about his family and personal life. She learns his mother has a new baby, whom Perry has been trying to support by working several jobs last summer and by resorting to "stealing" to avoid burdening her with feeding him. In the end, Stargirl becomes worried that no one will show up for her solstice party, but is reassured by Archie, her former teacher and friend from Arizona, who arrives to attend her celebration and comforts her with his wisdom.
On the morning of the Winter Solstice, Stargirl is overwhelmed and surprised when a huge crowd of her friends and acquaintances, and several other people she's unfamiliar with, flock to Calendar Hill, including her friend Betty Lou who hasn't left her house in nine years. The magic moment of sunrise is magnified by a special tent her parents have built, allowing the sunlight to stream in through a hole in the tent, forming a single beam that cuts through the crowd of people and pierces the back wall. Everyone is profoundly affected by the start of this new day, and returns home to the start of cold winter.
In the end, Stargirl asks Archie what she should do about missing Leo, and also what is happening with Perry. He tells her to remember who she is and do what her heart tells her.