After buying a flashy wig, chain-smoking misfit Beatrice Hunsdorfer picks up her daughters Matilda and Ruth from high school. On the way home, painfully withdrawn Matilda listens as her older, outgoing sister Ruth talks about her boyfriend, and her mother recalls her high school love interest who died in Korea. By the time they return to their squalid house, Beatrice has progressed to carping about their father, who deserted the family and later died. Beatrice next complains about the mess made by Matilda’s rabbit, given to her by her science teacher, Mr. Goodman, and orders the girl to stay home the next day to clean the house, to which Matilda replies that the school does not like them being late so often. The next morning, Beatrice makes phone solicitation calls for a dance studio with little success and after giving up, goes outside to flirt with her next-door neighbor, Charlie, who politely rebuffs her. Meanwhile at school, Ruth’s boyfriend watches her at drill team practice, while Matilda is enthralled as Mr. Goodman tells the science class about atoms and how parts of distant stars make up their bodies. That night, when Matilda enthusiastically shares this information with an uninterested Ruth, Beatrice ridicules Mr. Goodman, prompting Matilda to leave the room. Beatrice follows her, reminding her about cleaning the house because she wants to rent the spare room. Late that night, when Ruth wakes up screaming from a terrifying dream, Matilda helps Beatrice settle her, then listens as her mother comforts her sister. When a frail, elderly woman who is to be their boarder, is dropped off by her daughter, Beatrice patronizes her and nicknames her “Nanny.” The next morning, as Beatrice scans the ads in the paper for opportunities, Matilda explains her science project, exposing Man-in-the-Moon marigolds to radiation to see how it changes them. That afternoon, Beatrice regales them all with how popular she was in school, recalling that she was a wonderful dancer who made great cheesecake and mourning the fact that it now amounts to nothing. Later, in drama class, Ruth wears Beatrice’s wig and imitates her reading the ads while everyone laughs. When Ruth returns home, Beatrice angrily confronts her about the missing wig, asking about the skit she performed in class. Although Ruth denies it, Beatrice realizes that Ruth impersonated her and asks if people laughed. The next day, Beatrice is excited by an ad featuring a sandwich shop for sale, seeing the opportunity to open a tea shop and sell her cheesecake. She goes to the empty shop, returning full of enthusiasm for the prospective business, which she has named “The Man in the Moon Teashop,” but when she calls the bank for a loan, she is rejected. Still determined, Beatrice goes to see Floyd, her late husband’s brother, interrupting his wife’s bridge game and haranguing Floyd, who is home with flu. Beatrice presses him to use his influence at the bank where he works to get her a loan or, failing that, to give her the money. When he demurs, however, she is so unpleasant that he finally throws her out. Stopping at a bar, Beatrice is picked up by an antiques dealer and goes with him to his shop, ostensibly to sell him a lamp. He offers her eight dollars for it, plying her with more whisky. As he begins to kiss her, he raises his offer to thirty dollars, which so insults Beatrice that she hits him and breaks his merchandise. She wakes up the next morning in her car parked on a roadside and has walked into a nearby field when a policeman spots the car and begins questioning her. They soon recognize each other from high school and he asks about her life. After she has sourly recounted her failed marriage, he offers a half-hearted invitation to have dinner with his family and asks her to move her car. Beatrice is met back home by a worried Matilda, to whom she delivers a bitter diatribe about her horrible life, ending with “Don’t you hate the world, Matilda?” Moments later, Mr. Goodman calls and Beatrice rails at him about the messy rabbit and how the radiation used in Matilda’s experiment is dangerous. When he responds by inviting her to school for a talk, she hangs up on him, and another teacher sees his reaction. The teacher recalls that when she and Beatrice were classmates, Beatrice was considered crazy and was nicknamed Betty the Loon. Ruth, who is standing nearby, overhears the slight against her mother. After school, Ruth informs Beatrice that Matilda is one of the five finalists in the science fair, adding that Beatrice will be expected to be onstage during the ceremony to be held that night and speak if Matilda wins. Beatrice is unnerved by the possibility, worried that people will laugh at her. She questions Matilda about the event, who reassures her through sniffles that she probably will not win. Beatrice concludes that if Matilda is the winner, she will simply saying “my heart is full.” When Beatrice demands that Ruth stay home with Nanny that night, Ruth, who planned to attend a dance with her boyfriend, is infuriated. Beatrice is upstairs dressing for the science fair when Matilda leaves with her tray of blooming marigolds, telling Ruth she is sorry she cannot come and urging Beatrice not to be late. When the self-conscious Beatrice asks Ruth how she looks, the girl responds by calling her Betty the Loon. Beatrice shuts herself in her room as Ruth pounds on the door, viciously repeating the epithet and finally running out of the house. The only child onstage unaccompanied by a parent, Matilda explains her experiment as Mr. Goodman beams and Ruth, who has come to the auditorium, listens attentively. Meanwhile at home, Beatrice gets drunk, makes a crank call to the high school and threatens Matilda’s rabbit. She then drives Nanny to her daughter’s apartment and tries to leave her there, but finds that the woman is out of town. Driving to the school with Nanny in the backseat, Beatrice staggers into the auditorium to find that the ceremony is over and Matilda has won. When Mr. Goodman sees Beatrice coming up the aisle, he tries to avoid a scene by rushing to her, but when he loudly trips, all eyes turn to him and Beatrice. Everyone watches as Beatrice approaches the stage to announce dramatically “my heart is full.” When one of the girls laughs at her, Beatrice screams the phrase into her face to the embarrassment of everyone. After they return home, Matilda goes up to her room as Beatrice holds court outside, spinning plans about renovating the yard to Ruth and Charlie. As Matilda puts her award away, she sees that her rabbit is dead. Silently taking it downstairs, she lays it on the ground in front of Beatrice, who says nothing. Matilda then sits on the steps as the words of her acceptance speech are recalled, about her love of the atom and the wonder of life, concluding with “No, Mama, I don’t hate the world.”