Kenneth and Christine Penmark dote on their eight-year-old daughter, Rhoda, pristine and proper in her
pinafore dress and blonde
pigtails. Kenneth leaves on military duty. Neighbor and landlady, Monica Breedlove, is fond of Rhoda, who tells her about a
penmanship competition that she lost to her schoolmate Claude Daigle. Rhoda then leaves for her school picnic at a lake. While lunching, Christine and friends hear a radio report that Claude has drowned in the lake. Christine worries that her daughter might be traumatized, but Rhoda is unfazed by the incident. Rhoda's teacher, Miss Fern, visits Christine, revealing that Rhoda was seen grabbing at Claude's penmanship medal and was with the boy just prior to his death. She hints that Rhoda might have some connection to Claude's death, adding that Rhoda will not be welcome at the school after the current term ends. Claude's parents barge in; Mrs. Daigle is distraught and drunk, accusing Miss Fern of withholding information. When Christine finds the medal hidden in the lining of Rhoda's jewel case, she demands an explanation. Rhoda tells her that she paid Claude fifty cents to wear it for the day and only kept it in the confusion after Claude's death. During a visit from her father, journalist Richard Bravo, Christine confronts him about haunting and confusing memories of her own childhood. She tells him that she always suspected that she was adopted. Initially he denies it, but when Christine recalls details, he reluctantly confirms that she was adopted as a toddler. Christine is horrified to discern that she is the biological daughter of a notorious Australian
serial killer. She worries that her origin is the genetic cause of Rhoda's
sociopathy. Richard tries to convince her that it is
nurture, not nature, that primarily influences such behavior. Christine catches Rhoda trying to dispose of her
tap shoes in the apartment incinerator. She realizes that Rhoda must have hit Claude with the shoes, which had left odd crescent-shaped marks on his face and hands that could not be identified. Alternately feigning tears and angrily blaming Claude, Rhoda admits that she killed the boy for his medal. Rhoda also confirms Christine's suspicion that, to acquire a keepsake, she had previously murdered an elderly neighbor when they had lived in
Wichita, Kansas. Christine orders Rhoda to burn the shoes in the incinerator. The next day, janitor Leroy Jessup, who has previously seen Rhoda's malice behind her demure pose, teasingly tells Rhoda that he believes she killed Claude. After Rhoda angrily tells him that she burned her shoes, Leroy opens the incinerator and finds the remains. A drunk Mrs. Daigle returns and tells Christine that she believes that Rhoda knows what happened to her son. Realizing Leroy knows the truth, Rhoda sets his
excelsior bedding ablaze and locks him in the basement. After men break open the basement hatch, Leroy runs into the yard aflame, ultimately burning to death. From the window, Christine and Monica see him die; Christine becomes hysterical. That night, a strangely calm Christine tells Rhoda that she dropped the penmanship medal into the lake where Claude's body was found, then gives her daughter a lethal dose of
sleeping pills. Christine shoots herself in the head; however, the gunshot alerts the neighbors, and Rhoda and Christine are rushed to the hospital. They both survive, although Christine lingers in a coma. Kenneth arrives and takes Rhoda home. At bedtime, Rhoda excitedly tells Kenneth that she will inherit Monica's pet
lovebird. She mentions that she and Monica plan to sunbathe on the roof. When Kenneth tells her that lovebirds don't live as long as people, Rhoda becomes pensive about Monica's lifespan and her chances of getting the lovebird. Christine regains consciousness and is expected to make a full recovery. She calls Kenneth and tells him that she must pay for her "dreadful sin," but Kenneth assures her that they will work on their problems together. At night, Rhoda sneaks away during a thunderstorm and attempts to retrieve the medal from the lake using a
dip net. A sudden bolt of lightning strikes her, ultimately causing her death. ==Cast==