London, 1668. Samuel Pepys writes in his diary of Nell Gwyn and her rival, King Charles II's current escort, the French Catholic Louise de Kérouaille, Duchess of Portsmouth, who is much disliked by the British public.

As a diversion from ruling the country, Charles spends an evening at the Drury Lane Theatre to see 'The Vestal Virgin', the title role being played by 18 year old actress Nell Gwyn. Nell winks at the King from the stage, and he is enchanted. Visiting backstage, he invites Nell to supper at The King's Arms alehouse, where he is greeted by his subjects, including two disabled ex-soldiers. The King flirts with Nell, and seduces her as they eat lobsters.

As Nell spends more time with the King, she and the Duchess become rivals for his affections. The Duchess and Charles attend another performance, during which Nell throws her hat into the royal box. Afterwards (with his tongue firmly in his cheek), Charles reprimands Nell for humilating the Duchess in public.

Pepys comments on Nell's low background, and the Duchess spreads scandal about her. But Nell brings vitality to the court, and happiness to the King.

At a Whitehall Palace reception, Nell is stared at and ignored by guests, but after the King enters and approaches her, the courtiers follow suit, leaving the Duchess isolated. At dinner, Nell and the Duchess sit on either side of Charles. Nell finds the courtiers a bit stuffy, and gives the king a nudge and a wink. He askes her to dance for the court and the courtiers reluctantly applaud.

The Duchess examines a large hat from Paris, in a style that proved successful in seducing King Louis of France, and decides to wear it to the premiere of Nell's new play. During the performance, Nell appears from under a giant hat similar to that worn by the Duchess. Humilated again, the Duchess leaves the royal box with the King.

The Duchess is visited by the French ambassador, who tells her not to lose the king's ear, as a royal marriage would be good for France. After her arrival, Nell causes embarassment by displaying her petticoats to the ambassador.

Nell is visited by two alehouse friends, Robin and Ben, diasabled war vetrans who fought for the King. They tell her about their plight, having no home or pension. Nell has a 'vision' and introduces them to Charles and asks him to help such men; in response, the King founds the Chelsea home for ex-soldiers.

Charles suggests that Nell makes friend with the Duchess, but Nell, encouraged by her ever-faithful maid Meg, is defiant. Pepys meanwhile, records the 'love and constancy' of Nell and Charles, who suggests that Nell be given the title 'Duchess of Greenwich'

In 1685 Charles is taken ill, feeling pain in his 'overworked heart'. The Duchess and Nell are informed, and both dash to his bedside as the dying king lies in state, surrounded by priests and courtiers. Nell arrives at the gates and is barred by officials. Unable to deliver a message, she signals to kindly courtier Chaffinch, who is able to deliver a message to the King that Nell sends him 'her dying love'. The King's death is announced, and flags are lowered. A tearful Nell turns away from the court in sorrow, alone and isolated.