NOTE: This spoiler sent in by Thomas.
The movie opens as Georgiana (Keira Knightley) has a game on the grounds of an estate among her high-born girlfriends wagering which amongst the young gentlemen will win a race while inside the manse her mother Lady Spenser (Charlotte Rampling) makes a deal with the much older Duke of Devonshire (Ralph Feinnes) for her hand in marriage. As Charles Grey ("Mamma Mia's Dominic Cooper) wins the race for her, her Mother seals the deal with the notion that every woman in her family is fertile and surely to give the Duke a male heir.
The pair are married and the marriage consummated, an experience that leaves something to be desired for Georgiana. Her mother informs her that this is something to be borne and that after she produces a male heir the husbands interest in carnality will wane.
A child arrives at the Manor. It's explained that the child is the Duke's and since the mother is dead the child will be raised by them. Georgiana is appalled, especially when he cavalierly explains that since she is only a girl child she will have no claim on the inheritance, but will be good practice for her motherly instincts toward his heir.
Georgiana is not happy with her husband, who seems more interested in his dogs than with her. She sees one evening a servant girl fleeing his room, naked except for the clothing clutched to her busom. She goes to confront him and sees the crystal flacon of wine on his bed table (true to the time, water was suspect then and everyone drank wine, even at breakfast) that he has clearly been drinking from. They sleep together. In time she is pregnant.
At a political dinner the Georgiana engages the men in a discussion of freedom. The Duke walks out. She follows him and he tells her that the meeting bored him; he wishes that they wouldn't give such long speeches. She returns to the dinner and charms them all.
Georgiana is now the toast of London, dragging her dullard husband behind her into the limelight. Their marriage is even satirized in Sheriden's play "School for Scandal" and people say that the only man in London who is not in love with the Duchess is the Duke. They are sketched at parties and the opera (18th century Paparazzi). She does become pregnant by him; at one party, G (as she is called by her loved ones) goes into labor. Her husband proposes a toast to his heir as the screams of her labor pains trail off as she's borne away.
The next scene opens as Lady Spenser, dressed in black, visits. There is mourning, but only because G has produced a girl child. The scene jumps several years as G has produced another child also a girl. Her husband takes her to the mineral baths at Bath for a cure for her "problem". At Bath, G meets Lady Elizabeth (Beth) Foster, whose husband has thrown her over, keeping their three boys. Since this is the 1700's and women have literally no rights she has no recourse against him and nowhere to go. G takes her into her home as her friend.
Beth tells G that she should take a lover, that sex can be a wonderful thing. She tells her to close her eyes and imagine that Charles Grey (about whom they had been talking) was doing to. Beth brings G to an orgasm, most likely her first. G looks shocked, but Beth treats it as more like schooling than anything sexual.
The Duke has allowed Beth to come and live with them, knowing she has no place to go. Once more at her husbands door G hears the sound of a man and woman making love. Loudly. Demanding of the servants to know who is in there she realizes that it's Beth and her husband.
She confronts the Duke and demands that he send Beth away. He refuses. She goes to her mother, who tells her that she must return to him, that is her only option.
Beth comes to her and explains that she needed the Duke to get access to her children; the only man powerful enough to help her was he. G curses her, but Beth responds that she would do that and more to get her children back, as would G if it came to that.
G becomes more and more involved in politics, and closer to Charles Grey, who is making a name for himself in the field. She is a rock star to Londoners, incredibly popular with the hoi polloi and the gentry alike: her appearance at a rally in London boosts Grey's popularity enormously. Eventually they cannot help but admit that they love one another and consummate the relationship. Finally G is fulfilled.
At a meal with the Duke and Beth (who has become a permanent part of the house as the Dukes mistress and has meals in the center position of the long table with G and the Duke at either end) G proposes a deal: she will accept completely the relationship between the Duke and Beth if he will do the same with her feelings for Grey. He falls into a rage, telling her that she's in no position to bargain for anything being a woman and basically getting lathered up at the idea that she's attracted to another man. She runs from the room, her follows her and rapes her. The servants are of course unable to help but are visibly shocked and sickened by the screams coming from her room.
At a party, G seems drunk. She dances oddly and seems disconnected with reality. She knocks over a candelabra and sets her wig on fire, walking for several moments before realizing it. Someone knocks the wig off her head and puts it out. A doctor is called who prescribes complete bed rest until the child comes. Finally the Duke will have his male heir.
The child is born and G goes back to bath to take a cure. A Charles Grey cure. Neither of them think that this will be noticed by anyone. One evening when G is leaving her toilette to meet Charles she is confronted by her Mother and husband. The Duke gives her an ultimatum: stop seeing Charles immediately or he will make sure that his career as a politico will be over, and she will be cut off from her children. The Duke shows her a packet of letters to her by her kids. She defiantly refuses him. They leave and she tearfully reads the letters.
She arrives back in London, embraces her children and acquiesces to him. At dinner, Charles shows up to claim her, stating that he doesn't care who knows that he loves her. G goes to him and firmly tells him that she cannot go with him, that she can't leave her children. She then tells her husband that she is going to have Charles' child. He decides that she will go away and have the child in secret and the Grey family will take her. Bess forcefully insists that she will go with her as her friend. G has the child and gives her over to the Grey family, telling the General that the child's name is Eliza before collapsing into near hysterics into Beth's arms.
At home, G sits stiffly with her husband. He admits, as best he can that he hasn't been the best of mates. But in his way he does love her, as best as he knows how. He tells her that there will be a party that night to which they are invited and the whole of London will be there. She says she will go. He clasps her hand, and she briefly does his.
Beth, the Duke and G arrive at the party together (it's intimated that this is completely accepted as normal at this point). G greets several people at the party and sees Charles Grey across the room. She goes to speak to him, as she would to any party guest. They make small talk and he mentions that he in engaged. She congratulates him. He mentions also that he is a new uncle, he has a new niece named "Eliza" who is "much loved"
G returns to her extended family. She sits on a sofa as her husband looks out the window at the children of their blended family play. He turns to her and says "wouldn't it be wonderful to be that free?"
Subsequent titles mention that the three lived together for years, and that upon her death and with her blessing, the Duke married Beth. They also mention that G secretly met Eliza often, and that Eliza named her girl Georgiana