Dicey Tillerman and her three younger siblings, James, Maybeth, and Sammy, have just arrived at the home of their grandmother, Abigail Tillerman. They had traveled all the way from Massachusetts, where the children's mother abandoned them, having become mentally ill. Gram is a terse, eccentric, guarded, and poor woman, but she takes the children in almost in spite of herself, and the book opens with the entire family looking forward warily to the first stage of their life together. Dicey, hardworking and independent, quickly gets a job cleaning the local grocery store and refurbishes an old sailboat in the barn. A few days into the school year, Gram writes the children's names into the family Bible and talks seriously to them about the possibility of adopting them. Gram does not often show emotion other than anger, and the children understand what a powerful statement of her love for them she is making when she does show those other emotions.
Dicey sits stony-faced and uninvolved in school, trying her hardest not to let anyone know anything about her, for fear that they will gain some sort of hold over her. Dicey and all her siblings are used to being loners, as growing up in Provincetown, their classmates teased and mocked them because of their mother's untraditional lifestyle. Dicey meets Jeff, an upperclassman who plays guitar, and though she regularly stops to listen to him and even sing with him while she collects her bicycle, she rebuffs all of his friendly advances. Similarly, she rejects the friendliness of Mina, a clever and popular girl in her class, who, despite Dicey's coldness, often engages her in conversation and even works with her as a partner on a project. As the days pass, Dicey notices the beginnings of problems in her siblings' lives. Maybeth continues to struggle with reading and fractions, James worries about whether or not his classmates will like him, and Sammy withdraws into himself in school. However, before long, Maybeth's music teacher has noticed the girl's gift for music and offered to give her piano lessons, James gets a paper route to help out, and Sammy agrees to help James. Dicey, used to acting like a mother, begins to feel isolated. Letters from Boston, where Momma is in a mental hospital, arrive regularly, but Gram only tells Dicey that there is no change in Momma's condition, adding to Dicey's sense of isolation.
Shortly after parent-teacher conferences, Gram and Dicey go shopping for the family. During their trip, Gram announces the problems her siblings are facing: James is dumbing down his work to fit in better, Maybeth is failing, and Sammy is wearing a mask to school every day. Gram demands that Dicey help solve these problems, explaining that this is part of holding on to them. Together, they decide to let Sammy know he should be himself and to ask James to help Maybeth with her reading. Sammy, after talking with Dicey while working together with her on the boat, begins to get in fights at school, causing Gram and Dicey a new set of worries. When Dicey receives her report card, she finds that she herself has problems to face: she is failing home economics and getting a C in English. She dismisses the home economics grade, as she despises the class, and decides to wait until their English essays are returned to ask her teacher about her grade. On the day he returns their essays, the teacher, Mr. Chappelle, reads Dicey's paper, which is about Momma, aloud as an exemplar of an excellent paper, but announces that the paper is plagiarized. Mina stands up and defends Dicey, and manages to prove that Dicey wrote the paper. The teacher, shamefaced, returns the paper and promises to change her grade.
The family has a happy Thanksgiving celebration; they invite Maybeth's music teacher over and eat heartily. The following Saturday, Mina and Jeff, as well as one of James's friends, come over, and they all spend the afternoon singing. The next Monday, Gram, who has deduced that Sammy is fighting because his classmates are teasing him about her, visits the school and beats all the second graders at their new favorite game, marbles. Many of the children are impressed by Gram's skill and friendliness, and Sammy no longer feels the need to fight. That same day, Jeff asks Dicey to a dance. Dicey, anti-social as always, declines, but feels flattered that Jeff asked her. That night, Gram announces that the legal procedures have been completed and the children are adopted.
Shortly after, Gram receives a call from Boston, and without any explanation, she calls Mr. Lingerle to stay with the younger children while she takes Dicey with her by plane to Boston. The following morning, Gram leads Dicey into the mental hospital, and when Dicey sees Momma, she knows she is dying. Gram sits with Momma all day, and after sitting with her for a while, holding her hand and talking, Dicey ventures out into the streets with money Gram has given her to buy presents for her siblings. Dicey buys gifts, uplifted by remembering her siblings and finding things they will like. She returns to the hotel, and in the morning Momma is dead. Gram and Dicey, determined to bring Momma home with them, decide to have Momma cremated. They bring her home with them on the train in a wooden box Dicey found in a store the day before. In Wilmington, the children and Mr. Lingerle meet them and drive them home.
At home, Gram and the children bury Momma under a tree in the front yard. Dicey struggles to let go of the past, of the young girl she was before, and of what became of Momma. That night, Gram goes into the attic and, for the first time, brings down family photo albums. Together, the new family looks into their past.