Mabillon metro station exit, Paris, 10.30am. A dozen warmly-dressed tourists are gathered around a young American woman with long brown hair under a black felt hat. Iris Spencer waves her arms and her mittens dance in the air like puppets as she welcomes her English-speaking clients to her walking tour in Saint-Germain-des-Prés.

The dozen tourists have braved a glacial wind this February day to visit sites in The Da Vinci Code, the blockbuster thriller by Dan Brown that has sold 40 million copies. The agency she works for, Paris Walks, is one of a half a dozen offering such tours in the French capital.

"What are the reasons for the success of this book?" Iris asks, then says, "A mystery! A quest!" Specifically, a quest for the Holy Grail, the legendary chalice used by Christ at the Last Supper. It leads to the revelation of an explosive secret. The tourists, who come from all over the world – Australia, England, US, China and Lebanon – hang on Iris's every word. All but two have read the novel. "I devoured it!" exclaims a woman bundled in a blue coat. "And I loved it because it's. . . it's all possible!"

The story that has stirred such passion is a work of fiction, says the author. But in his "Fact" page in the book, he claims: "All descriptions of artwork, architecture, documents, and secret rituals in this novel are accurate." Readers and nonreaders, sceptics or not, we will follow awhile in the footsteps of the characters and see for ourselves what is real.

"Two opposite forces are struggling in the book," Iris says, eyes mischievous under her black hat. "They are the Priory of Sion – the secret organisation that protects the Grail – and Opus Dei – the Catholic organisation that supposedly would destroy the Grail, according to Dan Brown. The heroes are Robert Langdon, professor of symbolism at Harvard University, and the sublime Sophie Neveu, cryptographer at the headquarters of the French Judiciary Police. "It all began with the assassination of the Curator of the Louvre Museum, Jacques Sauniere, the keeper of the hiding place of the Holy Grail. Before dying, Sauniere left clues that would enable Langdon and Sophie Neveu to decipher the enigma. Sauniere leads them to an invisible trail. It is this trail that I invite you to follow along with me."

Straightening her hat, which keeps sliding over her eyes, Iris leads the way to Saint Germain market, where fruit, vegetables, meat and pastries have been sold since the Middle Ages. She points to a 12-centimetre bronze disk embedded in the ground, engraved with the letters "N" and "S". There's no doubt for Dan Brown – it's a marker for the "Ancient Path of Sacred Significance", which leads to the Holy Grail.

"Ancient Path?" "Sacred Significance?" Iris tells us that, in fact, it marks the Paris meridian, created in 1667 by the cartographers of Louis XIV as the reference point by which world travellers calculated their position on the globe, at least until 1884, when the British imposed the Greenwich meridian. The medallion, which has initials indicating North and South, is one of 135 placed in the ground just 11 years ago.

Some in the group give each other a knowing look. We follow Iris down an alleyway to the imposing Saint-Sulpice church. In The Da Vinci Code, the author maintains that the Paris meridian leads straight to it. In fact, they are ten metres apart.

The Da Vinci discrepancies
We get a glimpse into how the Parisian faithful feel about the book when Iris cautions us, "This church requires our utmost respect. Not everyone appreciates the novel! Once inside, I will no longer say 'Da Vinci Code' rather 'the book' and 'DB' instead of Dan Brown."

Almost as vast as Notre Dame, Saint-Sulpice is elegant and well-illuminated by the white stained glass of its nave. Jean-Jacques Olier, the parish priest who began the building in the 17th century, wanted a building that would "shine light upon the mystery" of his religion. Dan Brown, on the other hand, opted to make it the scene of obscure practices and murder.

Inside, we gather around a square stone slab in the floor that is somewhat lighter in colour than the others. This is where the odious monk Silas, a fanatical member of Opus Dei, committed the novel's second murder.

Silas is convinced that he has located the Holy Grail. The focus of his existence is within reach, beneath the stone slab! He smashes it and discovers a cavity containing an engraved stone, which reads: "Book of Job, chapter 38, verse 11." On the massive altar nearby, the Bible on the golden lectern comes to his rescue. Verse 11 contains a single phrase: "You will come this far, but you will go no further." Enraged, Silas grabs a bronze candelabrum and smashes it over the skull of poor Sister Sandrine Bieil, guardian of the church.

Whatever the leaders of the actual Opus Dei think about the enormous publicity resulting from the success of The Da Vinci Code, they surely rue the novel's plot, which "demonises" the organisation. Opus Dei (literally, "Work of God" in Latin), with 85000 members, belongs to the Catholic Church and works directly with the Pope. Frequently controversial, sometimes described as the "Holy Mafia" by its detractors, it proclaims as its mission "the dissemination of a profound awareness of the universal call to sanctity". Quite unlike the portrait created by Dan Brown.

As for the Priory of Sion, the "proof" of its existence dating to the year 1099 is based on false documents, according to French journalists Marie-France Etchegoin and Frederic Lenoir. In Code da Vinci: the Investigation (2004, Robert Lafont editions), they traced the Priory to its creation in . . . 1956.

Intrigued by the book's references to Saint-Sulpice, I later returned to speak with Paul Roumanet, parish priest. A friendly man with a soft voice, Father Roumanet says he has become accustomed to answering the most incongruous questions. No, the letters "PS" adorning certain stained glass windows do not signify "Priory of Sion". Rather they mean "Saint Peter". No, the obelisk standing in the northern transept is not of Egyptian pagan origin. Rather, it is a gnomon, a scientific tool installed by astronomers in 1743 to perfect calculations of the earth's rotation and the date of the spring equinox.

"As for the candelabra utilised by Silas, it is so heavy that it requires three men to lift it," he said with a smile.

A sign he posted on the wall near the gnomon reads: "Contrary to the allegations of a recent blockbuster novel, Saint-Sulpice is not a former pagan temple dedicated to the goddess Isis. No such edifice ever existed on this site . . ." Nevertheless, since the book's publication, the number of visitors has grown by 100 000 to 700 000 annually.

As we follow Iris out of the church, one of our group, a middle-aged American in a baseball cap, shrugs his shoulders and says to her. "There is perhaps some truth in this book, but who cares?" he says. "It's just a novel, right? For me, the real treat is to be here in the heart of Paris, where walking is a pleasure."

Iris agrees. She devoured it too, but read it as a thriller and used the story as a perfect pretext to discover the town. Born in Indiana, she has led tour groups in Paris for five years, and started the Da Vinci tour in October 2004.

At the Quai Voltaire, our guide points to a long building on the opposite side of the Seine: the Louvre Museum, our next destination. On the second floor, the Grand Gallery presents its fabulous Italian paintings. Among them is the first female smile in the history of painting: La Joconde, or the Mona Lisa, as it is known in English. Leonardo da Vinci awaits his visitors.

"Oh, don't even mention that 'Da Vinci Code' to me!"
It is impossible to miss her. On every pillar, there are signs showing the way to the "Mona Lisa". In 2005, 7,3 million visited the museum, up 10%. The Dan Brown Effect? The person renting headsets at a counter under the clear glass of the vaulting Pyramid roof has only this exasperated comment, "Oh, don't even mention that 'Da Vinci Code' to me!"

"Jacques Sauniere", "Sophie Neveu", "Robert Langdon" and "Silas" are names now whispered in the Grand Gallery by many visitors. At the Gallery's main door, they glance up. In the book, Silas fires his pistol at Sauniere through "a thundering iron gate" that fell from this very ceiling. But there is no gate. Fans of Da Vinci are also disappointed to learn that there is no window in the public bathrooms at the far end of the Gallery – in the book, Robert and Sophie attempt to escape from the police by jumping from it down to the street.

"The Virgin on the Rocks", another of Leonardo's works, and until recently ignored by most visitors, now has non-stop crowds. It is here that the cryptographer discovers one of the clues left by the curator – a small golden key, hidden between the frame of the painting and the canvas.

But most of all, it is La Joconde that is scrutinised and scanned with renewed interest, especially the painted horizon behind Mona Lisa's face, which is lower on the left than it is on the right.

So what? Professor of symbolism that he is, book hero Robert Langdon knows the answer. "Historically, the concepts of male and female have assigned sides – left is female, and right is male. Because Da Vinci was a big fan of feminine principles, he made Mona Lisa look more majestic from the left than the right." Says Iris, "This revelation of Leonardo da Vinci as a 'great feminist' would lead Sophie and Robert to one of the most explosive secrets ever – the marriage of Jesus and Mary Magdelene, their offspring, and the Machiavellian obstinacy of the Catholic Church, intent on minimising the role of women in favour of a masculine-dominated religion."

This prompts the woman in the blue coat to say, "When I was little, I remember that my father wondered why Jesus wouldn't have had children. He was also a man, wasn't he?" It is all possible, she repeats.

On his official website (danbrown.com), the author is more cautious: "The 'Fact' page makes no statement whatsoever about any of the ancient theories discussed by fictional characters. Interpreting those ideas is left to the reader."

As for herself, Iris says she is not interested in presenting arguments against the theory. Rather, "I know that those who come to my tour are interested in searching. I just want to give them some keys to look further."

When all is said and done
Out in the courtyard, a 12-centimetre hole is further proof of Da Vinci Code mania – a golden medallion marking the Paris meridian has disappeared. In its place are a few leaves of grass and cigarette butts. "Stolen by an admirer," says Iris.

1.10pm. The tour comes to an end. Iris bids adieu with a slight dip of her chin. The tour members, still deeply absorbed in Dan Brown's imagination, are silent for a few seconds, as if listening to a voice "whispering up from the chasms of the earth". The moment passes, and they applaud.