In late-Victorian London, the medical student Max McCandles is recruited by the mad scientist Godwin Baxter to record the behaviour of a strange, nearly mute woman living in Godwin's house. Godwin reveals that her body is that of a woman who was pregnant and committed suicide by leaping off a bridge; Godwin replaced the woman's brain with that of the fetus, giving her an infant's mind, and named her Bella Baxter.
As weeks pass, Bella's vocabulary improves and she exhibits childlike wonder at the world. With Godwin's encouragement, Max falls in love with Bella and proposes marriage. Bella accepts, but soon discovers masturbation and sexual pleasure, leading her to run off with Duncan Wedderburn, a debauched lawyer, to Lisbon. There, they have near-constant intercourse as Bella lives purely for pleasure. She is mistakenly addressed as "Victoria Blessington" by a fellow hotel guest.