Willow Chance is a 12-year-old girl who is a genius but who sometimes struggles to fit in with the other children at her new middle school. She has a strong interest in the number 7, diagnosing medical conditions, and plants, and she tends to dislike her peers because she finds them loud and unhygienic. One day, after she gets a perfect score on a test, she gets accused of cheating and sent to see a counselor named Dell. Dell isn’t very motivated at his job, but he is surprised by how smart Willow is, and this motivates him to try to help her.
Through going to meet Dell, Willow gets to know two of Dell’s other students: Mai and Quang-ha. Dell tries to impress all of them one day by bringing in a cat he recently adopted named Cheddar, but the cat soon runs away, and Dell gives up searching. Mai and Willow quickly become friends, with Willow starting to learn Vietnamese on her own and then Mai giving her lessons.
One day, Mai convinces Dell to take them all to an ice cream shop. But when Dell takes Willow back to her house afterward, there are police outside. The police inform them that Willow’s mom and dad have died in a car crash.
Willow is shocked and overwhelmed at the death of her parents. Mai manages to convince her mother, Pattie, to pretend to be an old friend of Willow’s family so that they can take temporary legal custody of Willow. Pattie agrees, and Willow spends the night with Mai, Pattie, and Quang-ha in the small garage converted into a nail salon where they live.
Pattie lists Dell’s address on the legal paperwork for Willow’s custody, and soon, the social worker Lenore calls about a visit. In just a few hours, Pattie and her family manage to move with Willow into Dell’s apartment, cleaning up his massive mess, buying new furniture, and getting Dell set up down the hall of his apartment complex with a new roommate named Sadhu. They pass Lenore’s inspection, but the court date to decide Willow’s permanent custody still looms ahead.
While Willow stays with Pattie, she is out of school and often goes to the library. Sometimes she travels by calling a taxi, and she gets to know a driver named Jairo. After Willow helps Jairo identify a potentially cancerous mole and leads him into circumstances where he wins a $20,000 prize, he begins to see Willow as an angel. He vows to use his new money on a college education in order to get out of the taxi job that he has felt stuck in for a long time.
One day, while Mai is out walking, she watches an acorn get run over by a car and remain intact. She retrieves the acorn and hides it for Willow to find, and Willow comes to believe it’s her lucky acorn. Soon after, Willow gets the idea to grow sunflowers in a common area outside Dell’s apartment. This task ends up being surprisingly complicated, requiring a proposal to the bank that owns the property and drawing Dell, Pattie, Mai, Quang-ha, and Jairo into the project. The group works long hours on the sunflowers and faces difficulties, but ultimately the sunflowers bloom. Willow keeps adding more plants until the apartment complex has a full-fledged garden.
Taking care of Willow motivates Dell to turn his life around. He starts by finding his old cat Cheddar, taking him to the vet, then promising that Willow herself can have the cat whenever she finds a new permanent home for herself.
Finally, the day of Willow’s court date comes, and she fears that she will be taken away from her new friends and put into a foster care facility. But she can sense that something unusual is happening right before she’s supposed to meet with the judge. As it turns out, Pattie and Jairo have decided to legally adopt Willow. Willow is relieved, and everyone celebrates this happy outcome.
On the ride back from court, Pattie reveals that she has secretly been saving up a lot of money and plans to buy Dell’s apartment complex from the bank. Willow notes that it’s the seventh day of the month and that her lucky number has saved her. After getting back to the apartment complex, which will be her new permanent home, Willow plants her lucky acorn in the soil of the garden that she helped oversee.