The key that breaks the Da Vinci Code lies in a hilltop hamlet half-an-hour's drive from Carcassonne in southern France. But the village, little more than a hamlet, warrants not a single mention in the 21st century's biggest-selling novel and has benefited little from the surge of Da Vinci Code tourism.
Those who do visit are struck by the place's extreme beauty, by a stunning panorama over the peaks of the Pyrenees and by names and events that have been familiar here for generations but find new echoes throughout Dan Brown's blockbuster.
The book tells of the murder of Jacques Saunière, curator of the Louvre and leader of an occult group known as the Priory of Sion. The detective in charge of the case is an irascible man called Bézu Fache. The American hero of the book, Robert Langdon, seeks the assistance of a British expert called Sir Leigh Teabing.
Mysterious cylinders containing coded messages on parchment lead the book's protagonists on a trail from France to England and finally to Scotland. The story emerges that Christ and Mary Magdalene made their way to Europe and that their children became the Merovingian Dynasty of French Kings.
Jacques Saunière's surname, Bézu Fache's first name, the first and second names of Sir Leigh Teabing, the mysterious cylinders and the Magdalene legend all point conclusively to the little village of Rennes-le-Chateau as Brown's inspiration.
At the turn of the 19th and 20th centuries the parish priest of Rennes, the Abbé François Bérenger Saunière, became the centre of mysterious events which led to his becoming far richer than the parish priest of such a remote settlement could hope to be.
Saunière built himself a new house called Bethany and a tower called the Magdalene Tower was erected to look across the magnificent mountain panorama which, incidentally, includes a peak called Le Bézu.
He refurbished his little church in a bizarre style. The keystone over the church is inscribed "Terribilis Est Locus Iste" ("This is a Terrible Place"). A statue of the devil holds up the holy-water font. The Virgin Mary's statue holds a baby, but so too does the nearby statue of Saint Joseph.
Where did Saunière's money come from? The story goes that he found a parchment in a cylinder which led him to discover a secret. He was seen digging around the graveyard in the dead of night. Some claimed he had found a golden treasure buried by the Cathars, a medieval religious sect which thrived in the region. There was a theory too that Saunière had discovered a secret that undermined the entire basis of Christian belief and was being paid by the Vatican to keep quiet.
All this is contained in a book called Holy Blood, Holy Grail by Michael Baigent, Richard Leigh, and Henry Lincoln which was a bestseller in the 1970s. The name Sir Leigh Teabing stems from the surname of Richard Leigh and an anagram of the surname of Michael Baigent. Perhaps not surprisingly, Baigent and Leigh are suing Dan Brown for plagiarism.
In the meantime American readers of the Da Vinci Code are thronging Europe to visit places mentioned in the book. Rosslyn Chapel in Scotland and the Temple Church in London have seen huge increases in visitor numbers. The authorities at Saint Sulpice in Paris where, in the book, a giant albino Opus Dei monk - in reality there are no monks in Opus Dei - brutally murders a nun, have posted the following notice: "Contrary to fanciful allegations in a recent best-selling novel this is not the vestige of a pagan temple. Please also note that the letters 'P' and 'S' in the small round windows at both ends of the transept refer to Peter and Sulpice, the patron saints of the church, not an imaginary 'Priory of Sion'."
It is ironic that while Saint Suplice is being overwhelmed by tourists Rennes-le-Chateau is being ignored, for there are strong reasons to believe that tourism was the main inspiration for the mystery of the Abbé Saunière.
Little was heard of the stories until 1956 when Noël Corbu, who had bought the Saunière estate, wanted to promote a restaurant he had opened there and contacted some friends in the local press who published Corbu's version of the tale. Later he met a man called Pierre Plantard who claimed to be descended from the Merovingian dynasty and presumably, therefore, from Christ and Mary Magdalen.
Plantard added his own myths and forged a number of parchments which he claimed Saunière had discovered while renovating the church.
He also invented the Priory of Sion in the mid 20th century - which certainly debunks claims that Leonardo Da Vinci had been a member.
Other, more plausible reasons for Saunière's wealth have emerged. To start with, he may not have been all that rich. The Bethany house is a fairly modest construction and the Magdalen Tower is unlikely to have cost a fortune. The parish priest, it would appear, made a lot of money from trafficking in Masses.
He advertised in Catholic magazines in France and in other countries offering to celebrate so many Masses for the intentions of the faithful that it would have been impossible for him to fulfil the commitments for which he was paid. The local bishop removed him from office and Saunière retaliated by handing over his estate to his housekeeper, Marie Denarnaud, to stop the diocese from gaining possession.
So, as in most conspiracy theories, a more sober and rational explanation lies in the background - and that brings us to another name that crops up in the Da Vinci Code. The Opus Dei bishop who controls the giant albino monk is called Manuel Aringarosa, a surname that bears as close a resemblance as makes little difference to the literal Italian translation of the English words "red herring".