The port of Calais, at the beginning of the 19th century. A shabby woman, no longer young and down on her luck, enters a wine shop and steals a bottle from one of the racks. She is caught by the police and thrown into gaol, where she is befriended by the other female prisoners. Claiming to be That Hamilton Woman who was once famed throughout the courts of Europe for her beauty - and notorious for her affair with the British naval hero Lord Nelson, she tells them the story of her rise and fall, beginning with her arrival in Naples in 1786, at the British Embassy.

A humble blacksmith's daughter of great beauty, Emma Hart has taken English society by storm, with at least two liaisons with powerful men already to her credit. Arriving in Naples with her widowed mother, she imagines that the Ambassador Sir William Hamilton will give his approval for her marriage to his nephew Charles Greville, with whom she is very much in love and whom she expects will join her later. She learns however that she has been handed over, to Sir William in settlement of Charles' considerable debts and that her erstwhile lover has now married an heiress. At first disillusioned and broken-hearted, Emma forgets her grief amid the hospitality and glamour of the Embassy. She enchants Sir William and soon becomes Lady Hamilton, her former dubious reputation forgotten. She is even presented at Court.

In 1793, cannon fire from a British warship salutes the Embassy. The ship's captain brings news of Britain's declaration of war on France and the urgent necessity of securing a force of Italian soldiers to bottle up Napoleon's fleet at Toulon. Sir William declares this to be an impossibility, the King of Naples will never agree. Lady Hamilton however, knows that the real ruler in Naples is the Queen, with whom she is now great friends. She takes the captain along to meet the royal couple while they are at play with their numerous children, and amid the bedlam the captain is introduced to the King and able to make his request for the soldiers.

Back at the Embassy Sir William is astounded to learn that the King has agreed to release 10,000 soldiers for the enterprise. Emma is elated by her success and plans a party on board the captain's ship to celebrate, at which she will perform her famous dances and wear a long tunic of pale blue cashmere, but the captain insists, with regret, he must return to his duties. Emma is attracted to the captain and is somewhat dismayed to learn that the young man who accompanies him is his step-son, and that a wife is alive and well in England. As he leaves, she asks his name - it is Nelson. His seriousness about the coming war has affected her, and her mood changes. Solemnly she asks Sir William to explain the situation to her, which he does by pointing out the countries concerned in the conflict, on a large globe which stands in the room, emphasising that up to now England has 'fought alone, with no allies'.

Five years of war pass before Nelson and Lady Hamilton meet again, on board his ship at Naples. Nelson is complaining bitterly to Captain Hardy of the King of Naples' refusal to allow his ships into the harbour to take on food and water, when Emma is winched on board. She is shocked by his appearance - since their last meeting he has become a great naval hero but at the cost of an eye and an arm, and he is utterly exhausted by the ceaseless campaigns. Emma has brought good news - for the second time she has interceded with the King and obtained the royal approval for the victualling of the fleet in the harbour. Nelson sails away to meet Napoleon's ships on the Nile - but he is already falling in love, and when he returns in victory it is to Emma.

There is a great reception at the Royal Palace in Nelson's honour, but he is ill and collapses. While he recovers at the Embassy under Emma's care, their bond deepens. They are seen in public together, at the theatre and the tavern, to the dismay of Nelson's step-son, who writes home in concern. After a ball which Emma arranges at the Embassy, Sir William confronts his wife with her infidelity. Emma retorts that she has never felt more than an ornament in his house, like his statues and paintings. To wound her, Sir William tells her that Nelson has already left for an important campaign in Malta, and Emma thinks she may never see him again. But he is waiting to say goodbye to her on the balcony and she rushes out to him. They no longer deny their love.

While on route to Malta, Nelson - now Admiral of the Fleet - learns that revolution has broken out back in Naples, endangering the Royal Family - and all those at the Embassy. Against orders he sails to their relief, remaining in the city with Emma. Sir William tries to persuade her to leave Nelson, on the advice of the Admiralty - she is to winter in Egypt while he returns to England alone. But when he returns it is with the Hamiltons, after a triumphant tour through Europe. Lady Nelson has travelled to London with Nelson's father to greet him, and the Admiralty has planned a low-key reception, but the London public have decreed otherwise and give Nelson a tumultuous welcome.

The two lovers realise the futility of their position, as Lady Nelson reasserts her status and her hold over her husband. Now Lord Nelson, he gives his maiden speech in the House of Lords, while public rumour is rife over his relationship with Lady Hamilton. Both women attend for the speech, and afterwards Emma faints - she has been concealing her pregnancy with Nelson's child. A daughter, Horatia, is born in secrecy and sent to live with a family in the country.

Attempts are made to relieve Nelson of his command but when danger threatens in Denmark he is despatched to deal with it. Sir William loses all his treasures and antiquities when the ship bringing them to England is wrecked. He becomes gravely ill and dies, leaving his estate to his nephew, Charles Greville. When Nelson returns victorious from Denmark he finds Emma penniless and beset with debts. He warns the Admiralty that despite a recent peace treaty Napoleon remains a threat and is not to be trusted. He resigns his command and takes Emma away to live with him in seclusion in the country. Emma exchanges the glamorous lifestyle of her Neapolitan existence for domestic bliss.

The tranquil idyll is broken when Captain Hardy arrives to beg Emma to release Nelson - war is looming again, as Napoleon has made himself Emperor of France and plans to invade England. Only Nelson can stop him. Knowing it means the end of her happiness, Emma persuades Nelson to return to his ship. The French are defeated at Trafalgar but Nelson is wounded during the battle and dies. His last thoughts are of Emma and what will happen to her.

Back in the French prison, Emma finishes her story - without Nelson, her life has no future and no meaning