In 1646, a beggar-woman with a hump on her back set out from Oatlands in Surrey. She was travelling with another woman, two men and a child. One of the men was her husband, and the child was her lively two-year-old son, Pierre. The little party walked for a hundred miles along the muddy roads and tangled lanes of south-east England, through countryside scarred by the wounds of the Civil War. Their destination was Dover.

No one they met on the way paid much attention to the prattle of the little boy. He informed all and sundry that his name was not Pierre, and that the ragged suit he wore did not belong to him. He was the Princess, he said, but fortunately nobody was listening.

At Dover, the party boarded a French ship bound for Calais. Two of the men and one woman were servants in disguise. The hunchbacked beggar was Lady Dalkeith, the governess of the Princess Henrietta, the fifth and last child of Charles I and Henrietta Maria. Born in Exeter on 16 June 1644, Henrietta had lived most of her short life under siege by Parliament’s armies. When the city eventually surrendered, Parliament placed her in protective custody in Oatlands, a royal dower house. Lady Dalkeith decided to flee when she learned that the government intended to move the child to London.

Henrietta’s flight from her father’s enemies was the opening chapter in the story of a life that often feels like a fairy tale, the sort that ends badly.  The princess never saw her father again. In France, she was reunited with her mother, Queen Henrietta Maria, who by birth was a French princess. Henrietta became known as Henriette Anne, as a compliment to their hostess, Anne of Austria, who was Queen Regent to her young son, Louis XIV.

During her short life, Henriette had several names and titles. After her marriage she was generally known as Madame. Nowadays, she is someimes called Minette. That was the name her brother Charles used for her – certainly in one of his letters, and perhaps often between themselves.

At her father’s command, Henriette had been baptised into the Church of England. Her mother, a fervent Catholic, soon changed that. In the seventeenth century, religion always had a political dimension. For the rest of her short life, Henriette clung to her Roman Catholic faith. Its hierarchical and authoritarian structure made it the natural spiritual home for many of those who believed that a monarch’s right to rule, like the Papacy itself, had been personally ordained by God.

A French artist, Claude Mellan, sketched the princess when she was a child. The drawing is said to date from about 1650, when she would have been five or six years old; but to me she looks more like nine or ten. It shows a plainly dressed girl leaning on a table covered with a fringed cloth. She’s wearing what looks like a sort of pinafore. Around her neck is a necklace of what might be beads but are probably pearls, those reliable indicators of seventeenth-century status.

Henriette is standing in three-quarter view, with her eyes looking not at the artist, it seems, but something over his shoulder. The small mouth is unsmiling. Her hair is tousled – some of it has escaped from the ribbon or whatever ties it back behind her head. There’s a gleam of light reflected in the pupil of the nearer eye: perhaps she’s looking through a window or through an invitingly open door.

To me, she looks as if she has been playing in the garden. And then authority in the form of her governess or her mother’s femme de chambre has dragged her into the house and told her to stand still while the artist draws her. She doesn’t want to stand still and be drawn. She wants to be in the garden.

Henriette saw little of her brother Charles throughout the 1650s, He was elsewhere, trying to whip up support for his flagging cause. When, late in 1659, he arrived on a visit to his mother and sister at Colombes, the queen’s country house, the princess had not seen him for five years. She was 15, he was 29. A rapport rapidly developed between them, a deep affection that lasted for the rest of their lives. It’s probable that Charles trusted her more than anyone.

Charles II

After the Restoration in 1660, Henriette’s value in the marriage market rose steeply. It soon became clear to everyone that her cousin Philippe, Louis XIV’s only brother, was infatuated with the vivacious 16-year-old princess. It was a brilliant match, welcomed by their relations.

But Madame de Lafayette, who was very close to Henriette, struck a warning note:

“Monsieur, the King’s only brother, was by inclination as much disposed towards the pursuits of women as the King was averse to them. He was well-made…but with a stature and type of beauty more fitting to a princess than a prince…His vanity, it seemed, made him incapable of affection save of himself.”

By his own admission, Philippe fell out of love with his new wife within a fortnight of their marriage. By this time, his brother had made him the Duke of Orleans, the traditional title for the younger brother of a French king, and he was known as Monsieur; and Henriette became Madame.

Philippe submitted his wife to persistent psychological abuse. His own emotional life revolved around strong attachments to male friends, particularly the ruthlessly manipulative Chevalier de Lorraine. With the active encouragement of the Chevalier and other friends, he came to hate her.

He did his dynastic duty, however, and slept with her when necessary. On occasion, he used sex maliciously as a means of control – notably during the weeks before Madame’s last visit to England: if he had succeeded in making her pregnant, she would not have been able to travel. By the time she died, Madame had had a number of pregnancies. Two of her daughters survived into adulthood.

As Madame’s husband, Monsieur was legally her master. His cruelty to his wife was widely remarked and widely condemned. But even his brother, Louis XIV himself, was limited in what he could do to prevent it. Monsieur was untouchable.

During the 1660s, Madame played a leading part in the social and cultural life of the wealthiest and most glamorous court in Europe. Her life revolved around the great palaces of France. There were rumours of affairs – including one with Louis XIV himself – though the evidence is sketchy. Nevertheless Monsieur was violently jealous, and he took it out on his wife. During the decade, her health slowly declined, partly perhaps because of the endless pregnancies.

Madame had many close and loyal friends of both sexes. Louis singled her out, again and again, for special attention, and his support to some extent protected her from the attacks of Monsieur and his favourites. Her religious faith grew steadily more important to her. So did her desire to bring her Protestant brother, the Supreme Head of the Church of England, to the True Faith of Rome.

In the seventeenth century, European politics were essentially a family business. Both Louis and Charles trusted Madame implicitly. When the two kings began privately to discuss a possible alliance between them, she was ideally placed to facilitate the negotiations through conversations with Louis and private letters to her brother. She represented the views of the one to the other, and helped to smooth over their differences. In her way, she did her utmost to be loyal to them both.

To Madame’s delight, one of the secret clauses was her brother’s commitment to announce his conversion to the Roman Catholic faith. This was the ‘Great Secret’, and only a handful of Charles’s most trusted advisers were aware of it.

In April 1670, Madame and small suite (a mere 237 persons) crossed the Channel to Dover, where Charles II and his court were waiting to welcome them. It was here that the treaty was signed. When, after a joyful and rather longer visit than intended, Madame returned to France, Charles sailed part of the way with her. He came back for a last embrace three times. Colbert de Croissy, Louis’s ambassador in England, wrote to his master that he:

“…had never known so sorrowful a leave-taking, or known before how much royal personages could love one another. It appeared during her stay at Dover that she had much more power over the King her brother than any other person in the world…”

Perhaps their grief was accentuated by the bitter knowledge that Monsieur was waiting in France for his wife’s return.  Louis wanted her to come to Versailles, but Monsieur refused. The unhappy couple went instead to their country house at St Cloud.

Because of its controversial nature, there are a number of eye-witness and other contemporary accounts of the events of Sunday, 29 June 1670. In the morning, Madame dined early and made a good meal, though usually she had little appetite. Afterwards she fell asleep. When she woke, she complained of a pain in the side, which was assumed to be indigestion. She asked for a glass of chicory water. As soon as she had drunk it, she collapsed in acute pain, crying out that she had been poisoned.

Monsieur’s doctor, however, assured her that it was no more than an attack of colic. But Madame was convinced that she was dying. In the next few hours, Louis and his queen, along with assorted courtiers, physicians and priests, flocked to her bedside. Even Monsieur appeared greatly upset – indeed he was so histrionically overcome with emotion that Madame begged him to leave her in peace.

At three o’clock the following morning she died with her favourite confessor beside her and a crucifix in her hand. Afterwards, some of the chicory water was given to Madame’s dog. Her maid and several friends drank some too. Even Monsieur is said to have tried it. None of them suffered ill effects. But when they finally traced the cup from which Madame had drunk, they discovered that it had already been cleaned by – rather oddly, surely – putting in the fire.

There was, of course, a post-mortem. Louis’s grief for his sister-in-law’s death was entirely sincere. But he was alive to its political ramifications, and particularly to its potential effect on the fragile, recently concluded alliance. The rumour that Madame had been murdered spread rapidly across Europe. Suspicions centred on the Chevalier de Lorraine: it was alleged that he had orchestrated the poisoning from his exile in Rome.

The autopsy was performed by two French doctors and watched by two English ones. The Frenchmen found no evidence of poisoning and concluded that Madame had died of cholera. The English doctors were not entirely convinced. But both Charles and Louis accepted the verdict, and the majority of their contemporaries followed suit.

Most modern historians have agreed, accepting an early twentieth-century diagnosis that Madame died from acute peritonitis, following the perforation of a duodenal ulcer. The fairy tale appears to have ended for her with a painful death from natural causes. Probably. But there will always be a doubt.

Andrew Taylor is a bestselling novelist and of The Royal Secret.