Margaret Simon, eleven years-old, has lived her entire life in New York City. Yet one day her parents decide to move out to the small suburb of Farbrook, New Jersey. Margaret suspects that her parents want to get her away from her father's mother, Sylvia, who pampers her. For her own part, Margaret is about to enter the sixth grade at a brand new school, and much like the typical adolescent girl of her age, she begins to worry about issues such as boys, bras, and periods. She often speaks to God about these things, but privately—she has her own personal connection to God yet does not follow a formal religion.
After making friends with three other girls her age—Nancy Wheeler, Gretchen Potter, and Janie Loomis—Margaret joins their secret club, the Pre-Teen Sensations (PTSes) in order to discuss topics such as boys and puberty, and to experience the thrill of growing up at last. Over the course of her sixth-grade year, Margaret reaches many milestones, notably getting her first bra and having her first kiss with the handsomest boy in the class, Philip Leroy, during a game at a class dinner party.
But Margaret has additional concerns, ones that other children her age typically aren't forced to deal with. Because Margaret's mother is Christian and her father is Jewish, Margaret was not raised in a particular religion, and instead her parents told her she could choose a religion for herself once she was old enough. When Margaret's sixth-grade teacher, Mr. Benedict, assigns each student an individual yearlong project, Margaret decides to do hers on religion, and accompanies her grandmother to temple and two of her friends to church in her attempts to understand different faiths. To her dismay, though, she can't feel her connection to God in the context of any formal religion; it's only when she's alone, talking to God, that she feels a divine presence.
Margaret is tested even more when, towards the end of the novel, her maternal grandparents visit for the first time. (As devout Christians, they had refused to speak to Margaret's mother after she married a Jewish man.) Since these grandparents ruin Margaret's plans to visit Sylvia in Florida, Margaret is determined not to like them—and she doesn't, especially when they try to insist that Margaret was born a Christian girl. By the end of the school year, Margaret has reached a stalemate on religion; she has decided that she still isn't ready to choose a faith. The novel ends with Margaret getting her first period at last and realizing that, when she's patient, the things she wants will always come.