In Victorian London, an ambitious medical student named Max McCandles finds himself working closely with the unconventional surgeon, Godwin Baxter. During his time at the clinic, he becomes infatuated with the enchanting, yet childlike, young woman named Bella, who is under Godwin's care. Godwin reveals a shocking secret: Bella was once pregnant but tragically took her own life by jumping off a bridge. In an audacious medical experiment, Godwin brought Bella back to life, replacing parts of her brain with that of an infant. This astonishing procedure leaves Bella with no memories of her past, forcing her to navigate the complexities of life once again.
With Godwin's consent, Max proposes to Bella, who happily accepts the offer. However, as her cognitive abilities rapidly evolve, she begins yearning for independence, eventually running off with lawyer Duncan Wedderburn and engaging in a tumultuous affair. In response to her actions, Godwin channelizes his focus into a new project involving a more docile young woman named Felicity, who matures at a much slower pace than Bella.
Bella and Duncan embark on a wild escapade beginning in Lisbon, where their relationship ignites through frequent bouts of intimacy and indulgent hedonism. As Bella grows increasingly adventurous, Duncan finds it hard to manage her, prompting him to sneak her aboard a cruise ship to seek a "fresh perspective." Onboard, she discovers companionship with fellow travelers, Martha and Harry, who introduce her to new philosophical ideas. While captivated by Bella's innocence, Duncan tries to suppress her burgeoning intellect, but his efforts prove futile.
One fateful stop in Alexandria leaves Bella heartbroken as she witnesses the devastating poverty surrounding the locals. In a generous attempt to help, she donates Duncan's gambling winnings, only to see the money quickly pilfered by the crew. This event leads to them being forcibly expelled from the ship at Marseille due to dwindling funds, forcing a journey to Paris.
As they face financial ruin, Bella takes a bold step and starts working at a brothel, which escalates Duncan's frustrations to a breaking point. Their relationship deteriorates, culminating in his mental collapse, which drives Bella to finally leave him. At the brothel, she meets Madame Swiney and forms a profound bond with fellow prostitute Toinette, who introduces her to the principles of socialism.
In a twist of fate, a dying Godwin requests to see Bella again. Max successfully locates her after tracking down Duncan, who is now in a mental institution. Upon returning to London, Bella reconnects with Godwin and rekindles her marriage plans with Max. However, their wedding day is disrupted by the arrival of Duncan and General Alfie Blessington, who shocking reveals that he and Bella were married long before her disappearance.
As Alfie, addressing Bella as Victoria, tries to reclaim her, she abandons Max to uncover her previous life. Unfortunately, she soon encounters Alfie's true, abusive nature, leading her to the dark realization that her past entangled her in a suffocating relationship that drove her to suicide.
Confined to Alfie's mansion, Bella faces a harrowing threat when Alfie demands she submit to a brutal procedure. But in a moment of fierce defiance, she throws a chloroform-laced drink in his face, resulting in him accidentally shooting himself in the foot before losing consciousness.
In the aftermath, Godwin passes away peacefully with Bella and Max by his side. Empowered by her experiences, Bella determines to honor Godwin's legacy by continuing his work alongside Max and Toinette, as Felicity's own intellect slowly begins to blossom.
