At the start of the book, Baby Jane Hudson is a highly successful child star in vaudeville, billed as "The Diminutive Dancing Duse from Duluth". In the film version, a prologue set in 1917 shows her performing with her father while her mother and sister Blanche watch from backstage. She is favored and spoiled by her father, while her mother attempts to soothe Blanche's envy and resentment by promising that one day she will achieve stardom. The novel reveals that, when both parents die in the late-1910s flu pandemic, the sisters move to Hollywood to live with an aunt who favors Blanche the way their father prefers Jane. By the mid-1930s, Blanche and Jane are Hollywood actresses. Blanche is a successful star, while Jane gets film work only because her sister's contract demands it. Blanche is the leading lady of her era, but Jane is widely seen as a has-been, and her films are critical and commercial failures. One night, an inebriated Jane mocks and humiliates Blanche at a party, provoking Blanche into leaving in tears. That night, Blanche is paralyzed from the waist down in a mysterious car accident that is unofficially blamed on Jane, who is found three days later in a drunken stupor with no memory of the party and its aftermath. The accident ends Blanche and Jane's careers. Jane spends the next three decades living with and caring for Blanche, stoking bitter resentment which consumes her. Over the years, Jane sinks into alcoholism and mental illness. She is now a grotesque caricature of her childhood self, wearing hideously caked-on make-up, her hair in greasy curls, and dressing like a 10-year-old girl. A television retrospective honoring Blanche's old films sends Jane into a jealous rage as she realizes that she is no longer the ingenue she once was. Jane is driven to desperation by the combination of the increased attention towards Blanche and her discovery that Blanche plans to sell the house and have her committed to a mental hospital. Delusional and stuck in the past, she clings to hopes of reviving her childhood act, even though she is approaching old age. She steals Blanche's money to pay for an accompanist and for adult-sized versions of her little-girl costumes. She removes Blanche's telephone, making her a virtual prisoner in her room. Jane gets drunk and pathetically sings her signature song, "I've Written a Letter to Daddy", a maudlin song from her childhood act. However, upon seeing her reflection and the ravages of age and alcoholism, Jane snaps and destroys the mirror. She kills Blanche's pet parakeet and serves it to her on a dinner platter; she later serves Blanche a dead rat the same way. When Blanche's cleaning woman, Elvira Stitt, threatens to report Jane's abuses, Jane kills her with a hammer and disposes of the dead body. After the police call about Elvira, whose family has reported her missing, Jane worries that she will be caught. When Edwin Flagg, the accompanist she hires to revive her Baby Jane act, discovers Blanche bound and gagged in the bedroom, he calls the police and Jane flees to the seashore with Blanche. On the beach, a near-death Blanche reveals that she is paraplegic by her own fault: she intended to kill Jane by running her down, but her car struck the metal gates outside their mansion instead, snapping Blanche's spine. Blanche blamed the accident on Jane, who was too drunk to remember the incident. This revelation destroys what little remains of Jane's sanity, and she regresses to her childhood and becomes "Baby Jane" once again, dancing for startled on-lookers at the beach. When two policemen notice the Hudsons' illegally parked car nearby and link it to Elvira's death, they spot Blanche lying on the sand and rush to her aid. Jane is too caught up in her delusions to notice the policemen's presence.
Acting
In this film, director Robert Aldrich mined the careers of his two stars, Bette Davis and Joan Crawford, by using their early film clips when the story called for examples of their characters' work. The other characters react to clips of Bette Davis's early roles with dismay at her "bad acting", and when clips of Joan Crawford's old movies are shown the other characters speak with praise for her acting.
