Ronald Wilby [Scott Jacoby] is a socially inept, awkward high school youth with budding artistic talent and a strong predilection for fantasy, often ridiculed for his mannerisms. His overprotective mother, Elaine [Kim Hunter], needs gallbladder surgery and envisions Ronald following a medical path to cure her illness. Ronald's father has been out of the picture for years after divorcing his mother and agreeing to terminate parental rights in exchange for not paying child support.
One afternoon, while asking out Laurie Matthews [Shelley Spurlock], Ronald is rejected and ridiculed by her friends. As he returns home, he accidentally knocks over her younger sister Carol [Angela Hoffman]. Carol, like Laurie, taunts Ronald. He pushes her, and she strikes her head on a concrete block, killing her inadvertently. He buries the body and confesses to his mother. Fearing the police will not believe that it was accidental, Ronald and his mother wallpaper the door frame to the downstairs bathroom and convert the closed-off space into a hidden living quarters for Ronald, with a concealed trapdoor in the pantry through which he can escape in an emergency. Mrs. Wilby tells the police that Ronald ran away.
Mrs. Wilby dies soon after, following gallbladder surgery, leaving Ronald to cope with the aftermath. In the meantime, Ronald has created a fantasy world in his head—a prince and a princess who live happily until an evil duke appears and a struggle begins.
Shortly after Mrs. Wilby's death, the Wood family [Mrs. Wood] [Pippa Scott], [Mr. Wood] [Dabney Coleman] and their three teenage daughters—Babs [Cindy Fisher], Althea [Cindy Eilbacher], and Ellen [Lisa Eilbacher]—move into the former Wilby residence. Ronald, who now needs food and human interaction, begins to notice odd noises and the disappearances of food. Babs identifies with Ronald's princess, and he, in turn, identifies himself as the prince. Ronald's fantasy deepens as Duane Matthews [Ted Eccles], Ellen's boyfriend and brother of the Matthews girls, steps into the role of the evil duke threatening their happiness. Ronald's aim becomes to reclaim his princess and eliminate anything, including members of her family, that stands in the way of his dream becoming reality. During this tense period, neighbor Mrs. Schumacher [Linda Watkins] witnesses Ronald and later dies of a heart attack from the shock; fearing the police will blame him, Ronald buries her as well.
Mr. and Mrs. Wood plan to go out of town for a couple of days, leaving the girls on their own. Ronald confronts Babs when she is alone in the house and tells her she is his princess. She flees to Mrs. Schumacher's house next door (unaware of her demise), but Ronald locks her in the basement and forges a note for her sisters to find, claiming she has run away. Both Ellen and Althea are skeptical of the note, but the police refuse to take action.
Shortly after the police leave, the girls hear noises downstairs as Duane tries to free himself and fights with Ronald. Althea notices one of the many peepholes Ronald has drilled; when she approaches it, she sees Ronald's eye staring back at her and screams in terror as Ronald breaks through the wall. The police, who have been monitoring the house, rush back. Ronald is captured as he tries to flee, crying out for his mother. Babs is found after escaping from Mrs. Schumacher's basement, and Duane is discovered in the living space, both shaken but not seriously harmed.
