The movie explores the marriage, relationships, and passions of 18th century aristocrat Georgiana, Duchess of Devonshire. 17-year-old Georgiana (Kierra Knightly) is delighted to have excited the notice of the much older Duke of Devonshire (Ralph Fiennes), and marries him amid high personal and family expectations. Unfortunately for Georgiana, the Duke is an undemonstrative and tight-lipped man who is far more interested in his dogs than in getting to know his new wife. He does nothing to alleviate her wedding night fears, and does his husbandly duty with few words and a noticable lack of tenderness. He makes it clear at the outset to both Georgiana and her mother, Lady Spencer (Charlotte Rampling), that it is Georgiana's duty to bear him a male heir in short order.
Georgiana becomes the center of a glittering social circle, attended not only by the British noble elite but also by the political figures of the time. She uses her beauty, wit, charm, and social connections to advance the Whig party -- especially the policies and ambitions of a young nobleman, Charles Grey (Dominic Cooper), with whom she socialized, and to whom she was attracted, before her marriage. She indulges her love of parties and gambling, to the apparent delight of her peers, who joke that Georgiana is adored by all of England, except her husband.
Shortly after Georgiana marries the Duke, he has a young girl brought to live with him and Georgiana. Initially shocked by his admission that the child is his illegitimate daughter, Georgiana soon takes on the role of the child's mother. In the meantime, Georgiana gives birth to two daughters, but fails to provide the Duke with the expected male heir. The Duke continues to have affairs with other women.
While "taking the cure" at Bath in the hopes of being able to conceive a boy, Georgiana befriends Bess Foster (Hayley Atwell), whose husband, Georgiana is appalled to learn, beats and even tortures Bess. Because Bess has left the marital home, the husband refuses to allow her to see their three young boys. At Georgiana's suggestion, the Duke invites Bess to live with the Duke and Duchess in Devonshire. Some time later, Georgiana discovers that the Duke and Bess have begun sleeping together. When Georgiana confronts Bess, Bess begs Georgiana to understand that she is in the affair only in order to have the Duke pressure her husband to release her sons to her. When Georgiana demands that the Duke remove Bess from the house, he refuses. Bess remains in the house, and her sons come to live with them. While Georgiana adores her children and is obviously a devoted and loving mother, she chaffs at the presence of the Duke's lover and her sons. She explores her attraction to Charles Grey, and finds that the attraction is mutual. She brings the situation to a head by proposing a "deal" to the Duke: she will not object to his continuing to have Bess live with them, if he will not object to her engaging in an affair with Grey. The Duke, not surprisingly, rejects the proposal, pointing out that he does not make "deals," and that all of the power in the situation belongs to him. He upbraids Georgiana for failing in the two things that he demands of her: bearing him a male heir, and remaining loyal to him. She is outraged by his attitude and refusal to break off his affair, and flees from him. He chases her down and rapes her.
After the rape, Georgiana appears at her social gatherings in a fog -- alcohol induced? -- until it is learned that she is pregnant. She delivers a boy. Having done her "duty," Georgiana goes to Bath, where she finally consummates her passion for Grey. They live blissfully in Bath until the Duke arrives unexpectedly. He accuses of her of being indiscreet and demands that she drop Grey and return with him to Devonshire. He warns her that, if she does not, both she and Grey will be cut off entirely from their wealth and positions in society, Grey will lose his political position, and she will never see her children again. After the Duke leaves, Georgiana races back to Devonshire, realizing that she cannot live without her children.
Bess and her children are still at Devonshire. Georgiana informs Bess and the Duke that she is pregnant with Grey's child. At the Duke's command, she moves to the countryside to await the birth. She delivers another girl, Eliza, and immediately is forced to give the baby up to Grey's family. Heartbroken, she returns to Devonshire, where she continues her social circle, but with none of the vivacity and verve for which she was known before. She remains at Devonshire with the Duke and Bess and their children, until her death. After her death, the viewer is told, the Duke marries Bess.