As a child, Baby Jane Hudson, "the diminutive dancing Duse," (Julie Allred) is the spoiled and sole support of her father, Ray (Dave Willock), her mother Cora (Ann Barton), and her older sister, Blanche (Gina Gillespie). As Jane outgrows her appeal, Blanche blooms into a beauty and becomes a top film star, reversing the sisters' relationship. But, as all Hollywood knows, Blanche is loyal, even to insisting that, for every picture she makes, Jane must be starred in one of her own. Since Jane's "cuteness" is now grotesque, her movies are flops. At the height of Blanche's career, an accident cripples her for life. The details are vage. The sisters had left a party where Jane had been embarassingly drunk, with Jane driving their car. When Blanche was found unconscious in the wrecked car, Jane was missing. When she was located, uninjured, she was unable to explain what had happened. Years later, those who recall the tragedy agree that if Jane (Bette Davis) was responsible, she's done penance ever since. She and Blanche (Joan Crawford) live alone in the old mansion, Jane caring for her helpless sister. Their isolation is broken only by the weekly visits of the cleaning woman, Elvira Stitt (Maidie Norman) and the sporadic efforts of Mrs. Bates (Anna Lee), from next door, to meet Blanche, attempts which are always thwarted by Jane. Learning that her sister plans to sell the house and perhaps place her in a "home" of some sort, Jane threateningly reminds Blanche of her helplessness. Thereafter, Jane forestalls Blanche's every effort to get help, cuts of her upstairs phone, intercepts messages, keeps her a virtual prisoner and contrives sly, macabre persecutions until Blanche fears for her own safety and sanity. With addled dreams of a comeback, Jane orders ludicrous adult versions of her Baby Jane costumes and advertises for an accompianist. Her ad is answered by Edwin Flagg (Victor Buono), a huge unstable man, whose mother (Marjorie Bennett) has supported and pampered him and whom he loathes. Appalled by the "act" Jane plans, Edwin feigns enthusiasm in hopes of getting funds so he can desert his mother and agrees to a deal when Jane promises him a salary advance the next day. Catching Blanche downstairs in a painful attempt to phone for help, Jane slaps and kicks her and drags her back to her room. She fires Elvira who, fearful for Blanche, waits until Jane leaves the house, then enters to investigate. Blanche's door is locked. Getting no response, Elvira is trying to force open the door when Jane returns and, when Elvira threatens to call the police, unlocks the door. As Elvira freezes in horror at seeing Blanche, lips taped, trussed to the bed, Jane kills the cleaning woman with a hammer. That night, Jane gets the body downstairs and into the car trunk. Edwin, somewhat drunk, is brought to the door by the police as a prowler suspect. They leave Edwin when Jane says he has an appointment with her. Edwin demands his promised advance. Upstairs, hearing voices, Blanche upsets her bedside table in a frantic effort to attract help. Hearing the sound of the crash, Edwin rushes up to investigate. At the sight of the haggard, helpless Blanche, he runs from the house and attempts to find help. Dawn finds the sisters at the beach, Blanche lying on the sand, covered in a blanket. Fearing that she will shortly die, Blanche confesses that she had been responsible for the automobile accident that had disabled her and had intended to kill her sister to avenge herself for the years of humiliation she had spent in the shadow of Baby Jane. As the police arrive upon the scene, the now totally deranged Jane goes into her song-and-dance routine of long ago.