In
Victorian London,
[n 1] medical student Max McCandles becomes an assistant to eccentric surgeon Godwin Baxter. He falls in love with Godwin's ward, Bella, a childlike young woman. Godwin reveals that the woman, who was pregnant, killed herself by leaping off a bridge. He replaced the woman's brain with that of her unborn fetus, resulting in her having an infant's mind, and named her Bella Baxter.
With Godwin's encouragement, Max asks for Bella's hand in marriage. Bella accepts, but as her intelligence rapidly develops, she becomes curious about the outside world and herself. By exploring her own body, she discovers masturbation and sexual pleasure. She runs off with Duncan Wedderburn, a debauched lawyer whom Baxter hired to overhaul the nuptial contract. Deciding to let her go, Godwin starts a new experiment with a young woman, Felicity, who matures much more slowly than Bella.
Bella and Duncan embark on a grand journey, starting in
Lisbon, where they have frequent sex. When Bella becomes difficult for him to control, Duncan smuggles her onto a cruise ship. Bella befriends the passengers Martha and Harry, who open her mind to philosophy. Duncan attempts to stunt her growth to no avail. He becomes exasperated and indulges in drinking and gambling. During a stop at
Alexandria, Bella witnesses the suffering of the poor and becomes distraught. She entrusts Duncan's winnings to unscrupulous members of the crew, who falsely promise to give it to the needy. Unable to afford the rest of the trip, Bella and Duncan are dropped at
Marseille and make their way to Paris. Seeking money and accommodation, Bella begins working at a brothel. Duncan, enraged, has a breakdown and Bella abandons him. At the brothel, she comes under the tutelage of Madame Swiney and befriends another prostitute, Toinette, who introduces her to
socialism.
Godwin, now terminally ill, asks Max to bring Bella to him. Max locates her after tracking down Duncan, who has been institutionalized. In London, Bella reconciles with Godwin and renews her plans to marry Max. The wedding is interrupted by Duncan and General Alfie Blessington. Alfie, addressing Bella as Victoria, declares that they were married before her disappearance and that he has come to reclaim her. She abandons Max to learn of her past life, but discovers Alfie's violent and sadistic nature and realizes Victoria killed herself to escape him.
Alfie confines Bella to his mansion. He threatens her at gunpoint to submit to
genital mutilation, demanding she drink a sedative. She tosses the sedative in his face, and after a struggle Alfie accidentally shoots himself in the foot before passing out. Godwin dies peacefully with Bella and Max at his side. Bella decides to follow Godwin's footsteps by becoming a surgeon with the help of Max and Toinette, while Alfie has a goat's brain transplanted into his head.