Louise Elmore Carrie Snodgress is a depressed, alcoholic spinster who works as the head librarian in Wichita, Kansas. Since her fiancé, Robert, disappeared the day before their wedding nineteen years ago, Louise has devoted herself to caring for her infirm, abusive father, Wendell Ray Milland, who is confined to a wheelchair. After a nervous breakdown and a fire started inside the library, Louise resigns and begins training her younger incoming replacement, Emily Perkins Ruth Cox. Following the fire, Louise attempts suicide by slashing her wrists, but survives. Louise and Emily quickly become friends, and Emily shows genuine empathy toward Louise. At home, Louise harbors fantasies of revenge against her father, sometimes even imagining murder.
One night, Louise goes to the movie theater alone, where she meets Richard, a sailor. After the film, Louise accompanies him to a hotel, and the two have sex. The next day, Louise and Emily spend the morning shopping. When Emily notices Louise's affection for a chimpanzee in a pet store window, she secretly buys the animal as a gift. Louise brings the chimp home and names him Dickie. Emily confides in Louise about her own boyfriend who has moved to California and wants her to join him there; Emily is reluctant to leave Kansas, but Louise urges that she should go, citing the state of Louise's life as evidence.
Emily invites Louise to dinner at her mother's house; the dinner goes awkwardly as Mrs. Perkins scolds Emily's younger brother for his manners. Louise responds by breaking one of Mrs. Perkins' cherished figurines, feigning an accident. The next evening, Emily and the library staff throw a small party to bid Louise farewell. When Louise returns home, she finds Dickie missing, and her father claims the chimpanzee ran away. Louise grows distraught. The next morning, Louise buys a plane ticket to Los Angeles for Emily and mails it to her at the library with a short note. Emily decides to seize the opportunity and heads to the airport, calling Louise from a payphone to thank her.
The following day, Louise brings her father to the local park. As she wheels him up a hill, the wheelchair tips over and Wendell falls out. He stands up, revealing that he has feigned disability to keep Louise under his control. In a burst of rage, Louise pushes him down the hill, and he dies when his head strikes a rock. Louise flees the house to gather her belongings and search for her father's hidden money. In the attic, she finds a key to a long-locked space, and, once inside, discovers Dickie’s body alongside the long-decomposed remains of Robert, both victims of her father's murderous acts.
