Before visiting her new client, a mentally retarded young man known as “Baby” Wadsworth, social worker Mrs. Ann Gentry scrutinizes a stack of photographs of Baby, showing him from infancy to adulthood. In each picture, Baby Wadsworth is swathed in diapers and confined to a crib. At the Wadsworth house, Ann, stating that she specifically asked for the assignment because she found the case fascinating, introduces herself to Baby’s mother and his two sisters, Germaine and the sultry, teenaged Alba. Although the family is taken aback by Ann’s interest, Mrs. Wadsworth explains that her husband left them right before Baby was born, and they have been on their own ever since, subsisting on Baby’s welfare payments. Ann asks to see Baby, and is taken to his nursery, where he is sleeping in his crib. Ann watches as Mrs. Wadsworth feeds Baby while he is seated in his high chair and then massages the muscles in his legs, which have become atrophied because he is unable to stand up or walk. When Ann later visits the Wadsworth home three times in a week, her superior, Mr. Foley, summons her to his office and criticizes her for spending too much time with one client. In her defense, Ann states that she suspects criminal negligence in the case and needs the time to ascertain the situation. On another visit to the Wadsworth house, Ann observes that Baby seems intelligent and alert, a claim immediately disputed by Germaine. To deflect Ann’s questions, Germaine asks about her husband, and Ann says that Roger was her husband’s name, then quickly changes the topic. That night, Ann, who lives in a large house with her mother-in-law Judith, is viewing slides of the happy times she spent with Roger, then blames herself for what happened to him. One day, Ann arrives at the Wadsworth house while Mrs. Wadsworth is away playing bridge. Alba, suspicious that Ann has come because she knew that her mother would not be home, nevertheless leaves Baby alone with Ann while she goes to answer the phone. Just as Ann coaxes Baby to stand up, Mrs. Warren returns and angrily informs Ann that Baby is too weak to stand. When Ann suggests admitting Baby to a day clinic for retarded children, Mrs. Wadsworth challenges her to prove that Baby is capable of making progress. To prove her point, Ann prods Baby to respond to her instructions, but sensing his mother’s disapproval, he is afraid to respond. After Ann departs, Alba punishes Baby by shocking him with an electric prod while she repeats that Baby does not stand, Baby does not walk. That evening, while Baby is sleeping, Germaine slips into the nursery, disrobes and climbs into his crib with him. Ann finally convinces Mrs. Wadsworth to visit the Greenview School for Exceptional Children, and while Ann awaits her arrival, she discusses the case with the school’s doctor. Ann confides that she believes Mrs. Wadsworth is punishing Baby because his father deserted her. The doctor advises Ann to turn the case over to the public guardian, but Ann admits that she is unable to prove her allegations. When Mrs. Wadsworth fails to keep her appointment, Ann phones the Wadsworth house and Germaine informs her that her mother has changed her mind and directs her to call Mr. Foley, who explains that Ann has been removed from the case at the Wadsworths’ behest. Furious, Ann drives to the Wadsworth house, and when Mrs. Wadsworth orders her off the property, Ann threatens to fight for legal custody of Baby and inform the public guardian’s office of her suspicions. To placate Ann, Mrs. Wadsworth agrees to cooperate with her and invites her to a party being held for Baby’s birthday the next day. At the party, Germaine drugs Ann’s punch, and once she passes out, the Wadsworth women drag her into the cellar where they gag and tie her up. They then return to the party, and while they are distracted, Baby crawls into the cellar, and upon finding Ann, tugs off her gag. Inching her way over to a toolbox, Ann upends a saw and uses the blade to cut her bonds, then pulls Baby upstairs to the kitchen. Once the party guests depart, the Wadsworths return to the basement and discover that Ann is missing. They hurry upstairs to see Ann drive off with Baby, and when they try to follow, find that Ann has punctured one of their car’s tires. The following day, Ann, who is building a pool in her backyard, instructs Judith to tell the construction workers to hold off pouring the Gunite into the pit they have dug until she calls them. Ann then dresses Baby in a suit, coaxes him to stand up and takes his photograph, which she then sends to the Wadsworths with a note boasting that she is going to transform Baby into a grown man and that their Baby will be lost to them forever. Determined to retrieve Baby before it is too late, the Wadsworths drive to Ann’s house that night, where Mrs. Wadsworth instructs her daughters to search the grounds while she waits in the car. When her daughters fail to return, Mrs. Wadsworth slips into the house and finds Germaine hanging from the banister, her throat slit. Alba then staggers out of one of the rooms with a knife protruding from her back. When Ann accosts Mrs. Wadsworth with a knife, she runs up the stairs and is confronted by Judith, who is weilding a hatchet. Judith is about to hack Mrs. Wadsworth to death when Ann stops her, after which they tie Mrs. Wadsworth up, haul her into the yard and after tossing her into the open pit with her daughters, bury her alive. Ann then takes Baby to the nursery, where her brain-damaged husband sits on the floor, and introduces Baby to Roger as his new playmate. Some time later, Ann, Roger and Baby frolic in their newly completed swimming pool.