The film delves into the complex life of Georgiana, Duchess of Devonshire, an 18th-century aristocrat, navigating the intricate web of marriage, relationships, and desires. At just 17 years old, Georgiana (Keira Knightley) is ecstatic to attract the attention of the much older Duke of Devonshire (Ralph Fiennes), and she marries him amidst soaring familial expectations. However, her excitement is quickly stifled as she realizes her husband is a deeply reserved man, more interested in his beloved dogs than getting acquainted with her. He does little to ease her anxieties on their wedding night, fulfilling his duties with a grim sense of obligation and scant tenderness. He firmly establishes that her primary role is to produce a male heir soon.
Within the lavish circles of British nobility and political figures, Georgiana shines as a socialite who leverages her beauty, intellect, and connections to support the Whig party, particularly championing the aspirations of the charming young nobleman Charles Grey (Dominic Cooper), whom she finds herself drawn to before her wedding. While her peers revel in her company, playfully declaring that Georgiana is adored by everyone in England except her husband, a sense of isolation subtly festers beneath her glamorous exterior.
The plot thickens when shortly after their wedding, Georgiana's world is shaken by the arrival of a young girl, revealed to be the Duke's illegitimate daughter. Though initially taken aback, she embraces her new role as the child's mother. Yet, her hope of bearing a son for the Duke is dashed as she delivers two daughters instead. The Duke’s infidelity further complicates matters, as he continues to pursue other women.
While seeking treatment in Bath to conceive a son, Georgiana befriends Bess Foster (played by Hayley Atwell), who faces her own domestic grave issues, including abuse from her husband. Georgiana suggests Bess come to live with them, but when she learns that the Duke has taken an interest in Bess, conflict arises. Confronted by Bess, who seeks the Duke's influence to regain custody of her children, Georgiana's frustration mounts. When the Duke refuses to send Bess away, their precarious living arrangement becomes a source of torment for Georgiana.
Amidst this turmoil, she begins exploring her mutual attraction to Charles Grey, eventually proposing to the Duke a "deal" whereby she could also pursue her affection for Grey if he continues his affair with Bess. The Duke brutally rejects her conditions, stating he does not make "deals," asserting his control and chastising her for failing to provide him with a male heir and remaining loyal. Outraged and distressed, Georgiana flees, only to be violently pursued and raped by the Duke.
In the aftermath, Georgiana navigates her profound emotional pain, gradually drifting through social events until a significant revelation: she is pregnant. After birthing a son, she visits Bath, where she finally connects intimately with Grey. Their bliss is short-lived as the Duke discovers their affair, threatening her with dire consequences if she doesn’t return with him to Devonshire.
Upon her return, she reveals her pregnancy with Grey’s baby to the Duke, and at his command, relocates to the countryside to await the birth. Once again, tragedy strikes as she gives birth to another daughter, whom she must relinquish to Grey's family. Heartbroken, Georgiana resumes her life at Devonshire, but the vivacious spirit she once embodied fades away.
Ultimately, Georgiana remains trapped in her role until her untimely death. In a twist of fate after her passing, it is revealed that the Duke marries Bess, concluding a poignant tale of sacrifice, loyalty, and unfulfilled aspirations.
