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 Baby Jane performing with her father while her mother and sister Blanche watch from backstage.

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 used to treat Jane (this detail is absent in the film).

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.

With the loss of income, the sisters move in together, and Jane spends the next three decades as Blanche's caretaker and cook, stoking bitter resentment which consumes her.

Her appearance slowly becomes a grotesque caricature of her childhood self: hideously caked on make-up, greasy hair in curls, and dresses like a 10-year-old girl.

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.