Most people doing house clearances are lucky if they uncover some old crockery and perhaps a porcelain statuette or two. So when French diplomat Jean-Yves Berthault was helping a friend declutter a dusty cellar in Paris, the last thing he expected to stumble on was a treasure trove of eye-wateringly graphic love letters from the Twenties that make the exploits detailed in Fifty Shades of Grey look positively pedestrian. Now, the correspondence he uncovered has been turned into a book that critics are calling the "steamiest erotic text ever written".
Mild-mannered and bespectacled, Jean-Yves is slightly embarrassed by his accidental segue from ambassador to global purveyor of filth. Before he produced The Passions of Mademoiselle S, his written work centred on essays about geopolitics.
"It is not the sort of thing a retiring diplomat would usually spend his time doing," he concedes.
"I have written a lot, mostly about the status of the Christian religion in the Middle East. That is more my cup of tea."
Instead, the 66-year-old now finds himself the author of a rather racy text that details bondage, role play, gender reversal, sodomy and group sex, and is being hailed as a must-buy book for lovers this Valentine's Day.
Jean-Yves recalls how he found the letters in the cellar. "There was stuff everywhere. One corner was so messy it was as if an effort had been made to conceal something. There were three stacked crates. In the bottom one I found layers of newspapers and empty jars, underneath which there was a leather satchel full of letters.
"They were all written by the same woman. There were postcards and photos of her when she was a young girl. It was her past, she kept it. She could not divulge them so she hid them away."
As he leafed through the pages, Jean-Yves realised that he had found something unique and offered to buy them from his friend. He then spent a year editing the correspondence in his spare time, while on a posting as ambassador to Brunei, carefully piecing together the astonishing story it told: that of a forbidden affair between a young girl and a married man.
The letters, dating back to the Twenties and Thirties, were from the pen of a woman identified only as Simone to her lover Charles. They were carefully concealed in a leather case and it is little wonder why. Spanning a two-year period from 1929, they become increasingly depraved, with talk of "fervent kisses", "perverted coupling" and "shivers of pleasure". In one relatively tame note, Simone writes: "Give me your wonderful body, I want to hold it in my arms, hold it tight until I am imbued with its intoxicating smell." Others refer to sadomasochism, using phrases such as "my battered rump".
As he read, Jean-Yves began to piece together further clues as to the writer's identity. He believes she was a society girl from a well-to-do family, living in a wealthy district of central Paris, but he has protected her full name in case she has surviving relatives.
"I did some research and discovered that she was not married and that she didn't get married after the affair was over. I am not sure she would have wanted to be named because if she took the precaution to hide the letters, even if she is long dead now, it is important to respect that discretion. I became attached to her and am protective of her."
Although Simone was from a privileged background, her vocabulary still has the power to shock today - littered as it is with obscenities, something that was fashionable in high society at that time.
"Lower-class people would have been much more inhibited than she was," explains Jean-Yves. "Her style was that of a cultured person. From the letters, it is clear she worked, as did a lot of society women in the Twenties, as they wanted financial and social independence. Simone was a very modern person. She was swept up in the moral revolution that was happening at the time. Britain was still Victorian, while France and Germany were throwing old traditions out of the window. She was part of that phenomenon."
He does, however, concede that Simone's was ultimately a tragic one-sided love affair, and her insecurity pours forth from the letters. We cannot know what Charles wrote in response, as it was then customary for men and women to return each other's correspondence as a show of trust and respect. His replies have been lost to time.
"I have a lot of affection for her but also see her as a desperate soul, a poor girl who wanted real, reciprocal love. She is very dramatic and romantic in spite of her extreme search for physical pleasures. She is neurotic, out of her mind and not realistic. Although we don't have any letters from Charles, there is little doubt that he did not love her; he took advantage of her and never cared for her. She was totally blind. The blinder she got, the more she fell in love and lost herself in a passion that he probably did not deserve."
The letters offer a fascinating insight into continental attitudes to sex in the Twenties and Thirties, something in which Jean-Yves is now an expert, much to his amusement.
"We believe that we have never been as liberated as we are now, but I think that is wrong. Human nature has not evolved as much as we imagine. There have been periods in which social attitudes relating to sex have been as liberated, if not more so, then they are now. There may have been laws enacted to ban certain practices through history, but that doesn't mean people were not doing them. We tend to think that our grandmothers were like the Virgin Mary, which might not have been the case."
What is unusual, in Simone's case, is her extreme fantasies. "I don't think everyone would harbour those," he says. Indeed, it was at Simone's instigation that the couple introduced other men into their bed, a move that eventually led to the demise of their relationship.
Jean-Yves collated The Passions of Mademoiselle S as he neared retirement, having previously been posted with the French diplomatic corps in Syria, Afghanistan, Morocco, Pakistan, Nigeria, Azerbaijan and Eritrea. "I was mostly in the Arab Muslim world, not somewhere that nourishes your attachment to erotica," he admits.
While in Brunei he persuaded his secretary to help type up the risque content of Simone's letters, a job which it appears had some surprising benefits.
He was astonished when, one day, his secretary's partner came into the embassy to thank him profusely. The letters, he said, had transformed their love life. "I hear that other people who read it have also seen profound effects on their relationships," says Jean-Yves.
The book was snapped up just 48 hours after he submitted a manuscript to the publisher of Fifty Shades and is published in paperback in Britain next month. It has also been printed in Russia, Brazil and Japan. Some have doubted its authenticity, something Jean-Yves vociferously denies.
"There were articles in the French press - one said it was 'too beautiful to be true'. I was shocked."
He did, he admits, have to convince a few of his friends that the letters were real.
"People are amazed. I don't look like I could possibly have published this. I pointed out that if they thought the content was incredible then it would be even more incredible to think I had make it up."