A crazed, aging star torments her sister in a decaying Hollywood mansion.

In the 1920's, 6-year-old Baby Jane Hudson becomes an enormously successful child star in vaudeville while her older sister, Blanche, is forced to remain quietly in the background. As the two reach maturity, however, Jane loses both her appeal and her talent, and Blanche develops into a beautiful and renowned film actress. Then, at the height of her career, Blanche is crippled in an automobile accident for which the alcoholic Jane is held responsible. As the years pass, the two sisters become virtual recluses in an old mansion, where the slatternly and guilt-ridden Jane cares for the helpless Blanche. When she learns Blanche is planning to sell the house and perhaps place her in a home, Jane plots a diabolical revenge. She serves her sister trays of dead rats and parakeets, tears out her phone, and keeps her a prisoner in her bedroom. She even resorts to killing their black maid, Elvira, with a hammer when the woman becomes suspicious and threatens to go to the police. Jane is also planning to make a comeback and has hired the obese pianist Edwin Flagg to accompany her. But when Edwin discovers Blanche gagged and bound to her bed, he runs hysterically from the house. Realizing he will go to the police, Jane drags Blanche into a car and drives to a nearby beach. There Blanche confesses that she had arranged the automobile accident 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.