A woman suspects that her perfect little girl is a ruthless killer.

Angelic Rhoda Penmark, an eight-year-old with blonde pigtails, seems the perfect child. An excellent student, she is extraordinarily well-mannered and never looks unkempt in the party dresses she always wears. To make her shoes last longer, she thoughtfully asked to have metal plates put on the soles, and she charms her happily married parents, Kenneth and Christine, with cloyingly sweet affection. When Kenneth, an Air Force colonel, is ordered to the Pentagon for several weeks, Christine and Rhoda stay behind, watched over by Monica Breedlove, their middle-aged landlady. Beguiled by Rhoda's quaintness and old-fashioned curtsies, childless Monica gives her many gifts, which Rhoda hoards in a "treasure box." Only Christine is aware of Rhoda's greediness, although Rhoda demonstrates it by turning angry and willful when another student, Claude Daigle, is awarded her private school's gold penmanship medal that she covets. While Rhoda attends a school picnic, Christine lunches with Monica, her brother Emory and mystery writer Reginald Tasker. Their conversation about female murderers makes Christine uncomfortable, until her interest is piqued by the name of serial killer Bessie Denker. Intrigued, Monica, who is endlessly fascinated by psychology, convinces Christine to use "word association" to explore her feelings, causing Christine to recall that she always felt she was adopted, despite a happy childhood. While they are talking, a radio broadcast reports that Claude drowned in the bay during the picnic, and when Rhoda returns, she seems strangely unmoved by the tragedy. Her lack of compassion is noticed by Leroy, the half-witted, neurotic apartment building janitor who has never been fooled by Rhoda's pretensions. A few days later, Rhoda's teacher, Miss Fern, visits and tells Christine that Rhoda was the last to see Claude alive and was with him on the pier under which he drowned. She says Rhoda repeatedly tried to snatch away Claude's medal and, because of her lack of fair play, is no longer welcome at the school. The conversation is interrupted by the arrival of Claude's parents. Drunk and belligerent, Mrs. Daigle says the medal is missing, and wonders aloud about a moon-shaped injury to Claude's head and scratches on his hands. After Miss Fern and the Daigles leave, Christine discovers the medal hidden in Rhoda's treasure box. While questioning Rhoda about it, she remains impervious to her daughter's attempts to manipulate her with demonstrative affection. Christine then remembers that Rhoda was bequeathed a beautiful trinket from an elderly woman who died falling down some stairs when Rhoda was alone with her. On another day, Leroy, wanting to frighten Rhoda, accuses her of killing Claude and tells her that blood traces cannot be washed off a murder weapon. Rhoda later asks Christine if this is true, thus increasing Christine's suspicions. That evening, when Christine, pretending to write a book, asks Tasker about child murderers, he tells her that the "greatest" killers often start young and come from advantageous backgrounds. The idea of inherited criminal tendencies, he explains, is popular with many criminologists and behavioral scientists. However, Christine's father, Richard Bravo, a highly respected journalist who is also present, nervously dismisses the "bad seed" theory as unsubstantiated. Tasker then suggests that Bessie Denker, about whom Richard wrote several articles years earlier, proves the theory, but Richard claims to have forgotten the articles. Tasker explains to Christine that Bessie's beauty and sweet disposition convinced jurors to acquit her of three separate indictments and that some criminals, having no capacity for guilt, present "a more convincing picture of virtue than normal folk." Tasker recalls that Bessie, who had a child, escaped to South America where she continued killing. After Tasker leaves, Christine describes her recurring dream to Richard, in which she hides from a beautiful mother who scares her. Eventually, Richard reveals that she is Bessie's daughter, and was adopted by he and his wife after she was discovered in a field at the age of two. Christine fears that Rhoda has inherited Bessie's criminal tendencies, but Richard refuses to believe it. Later that evening, Christine catches Rhoda discarding her shoes in the incinerator chute and realizes that a steel plate made the half-moon mark on Claude's head. After Rhoda confesses that she was responsible for Claude's drowning, Christine, torn between justice and a mother's love, asks Rhoda how the elderly lady fell down the stairs and Rhoda admits that she pushed her. The next day, Leroy notices that Rhoda is without her tap shoes and tricks her into admitting that she burned them in the incinerator. To torment her, he falsely claims that he retrieved them and, when she furiously demands them back, realizes that his malicious teasing has uncovered the truth about Claude's death. Meanwhile, Monica, who is concerned about Christine's haunted look, offers her vitamins and sleeping pills. Later, Rhoda sneaks some matches from Christine's purse, locks Leroy in the cellar after starting a fire there and sits down at the piano to practice. Although rescued by Emory and Tasker, Leroy dies from the burns. That evening, Rhoda asks for the medal, but Christine says she dropped it into the bay at the end of the pier. Christine then gives Rhoda several sleeping pills, calling them vitamins, and after Rhoda falls asleep, tells her that she is saving her from imprisonment and other indignities. Then, Christine goes to her own room and shoots herself. Two days later, the hospitalized Christine lies comatose, while Kenneth, Emory and Monica, who saved both mother and daughter from death, ponder her attempted suicide. Although Richard guesses Christine's motive, he says nothing. At bedtime, Rhoda tells Kenneth that she is to inherit Monica's pet lovebird and that she and Monica will sunbathe on the roof soon. During the stormy night, Christine regains consciousness and, by phone, tells Kenneth she must pay for her "dreadful sin," to which Kenneth says they will solve their problems together. Meanwhile, Rhoda sneaks out to search for the medal in the bay, heedless of the rain. When lightning strikes the pier, she goes up in smoke.