It was a silent call of distress.

The U. S. Embassy in Paris is a compact complex on Avenue Gabriel, just north of the Champs-Elysées. The three-acre compound is considered U. S. soil, meaning all those who stand on it are subject to the same laws and protections as they would encounter standing in the United States.

The embassy’s night operator was reading Time magazine’s International Edition when the sound of her phone interrupted.

«U. S. Embassy,» she answered.

«Good evening.» The caller spoke English accented with French. «I need some assistance.» Despite the politeness of the man’s words, his tone sounded gruff and official. «I was told you had a phone message for me on your automated system. The name is Langdon. Unfortunately, I have forgotten my three-digit access code. If you could help me, I would be most grateful.»

The operator paused, confused. «I’m sorry, sir. Your message must be quite old. That system was removed two years ago for security precautions. Moreover, all the access codes were five-digit. Who told you we had a message for you?» «You have no automated phone system?» «No, sir. Any message for you would be handwritten in our services department. What was your name again?»

But the man had hung up.

Bezu Fache felt dumbstruck as he paced the banks of the Seine. He was certain he had seen Langdon dial a local number, enter a three-digit code, and then listen to a recording. But if Langdon didn’t phone the embassy, then who the hell did he call?

It was at that moment, eyeing his cellular phone, that Fache realized the answers were in the palm of his hand. Langdon used my phone to place that call.

Keying into the cell phone’s menu, Fache pulled up the list of recently dialed numbers and found the call Langdon had placed.

A Paris exchange, followed by the three-digit code 454.

Redialing the phone number, Fache waited as the line began ringing.

Finally a woman’s voice answered. «Bonjour, vous êtes bien chez Sophie Neveu,» the recording announced. «Je suis absente pour le moment, mais…»

Fache’s blood was boiling as he typed the numbers 4… 5… 4.

Despite her monumental reputation, the Mona Lisa was a mere thirty-one inches by twenty-one inches – smaller even than the posters of her sold in the Louvre gift shop. She hung on the northwest wall of the Salle des Etats behind a two-inch-thick pane of protective Plexiglas. Painted on a poplar wood panel, her ethereal, mist-filled atmosphere was attributed to Da Vinci’s mastery of the sfumato style, in which forms appear to evaporate into one another.

Since taking up residence in the Louvre, the Mona Lisa – or La Jaconde as they call her in France – had been stolen twice, most recently in 1911, when she disappeared from the Louvre’s» satte impénétrable» – Le Salon Carre. Parisians wept in the streets and wrote newspaper articles begging the thieves for the painting’s return. Two years later, the Mona Lisa was discovered hidden in the false bottom of a trunk in a Florence hotel room.

Langdon, now having made it clear to Sophie that he had no intention of leaving, moved with her across the Salle des Etats. The Mona Lisa was still twenty yards ahead when Sophie turned on the black light, and the bluish crescent of penlight fanned out on the floor in front of them. She swung the beam back and forth across the floor like a minesweeper, searching for any hint of luminescent ink.

Walking beside her, Langdon was already feeling the tingle of anticipation that accompanied his face-to-face reunions with great works of art. He strained to see beyond the cocoon of purplish light emanating from the black light in Sophie’s hand. To the left, the room’s octagonal viewing divan emerged, looking like a dark island on the empty sea of parquet.

Langdon could now begin to see the panel of dark glass on the wall. Behind it, he knew, in the confines of her own private cell, hung the most celebrated painting in the world.

The Mona Lisa’s status as the most famous piece of art in the world, Langdon knew, had nothing to do with her enigmatic smile. Nor was it due to the mysterious interpretations attributed her by many art historians and conspiracy buffs. Quite simply, the Mona Lisa was famous because Leonardo Da Vinci claimed she was his finest accomplishment. He carried the painting with him whenever he traveled and, if asked why, would reply that he found it hard to part with his most sublime expression of female beauty.

Even so, many art historians suspected Da Vinci’s reverence for the Mona Lisa had nothing to do with its artistic mastery. In actuality, the painting was a surprisingly ordinary sfumato portrait. Da Vinci’s veneration for this work, many claimed, stemmed from something far deeper: a hidden message in the layers of paint. The Mona Lisa was, in fact, one of the world’s most documented inside jokes. The painting’s well-documented collage of double entendres and playful allusions had been revealed in most art history tomes, and yet, incredibly, the public at large still considered her smile a great mystery.

No mystery at all, Langdon thought, moving forward and watching as the faint outline of the painting began to take shape. No mystery at all.

Most recently Langdon had shared the Mona Lisa’s secret with a rather unlikely group – a dozen inmates at the Essex County Penitentiary. Langdon’s jail seminar was part of a Harvard outreach program attempting to bring education into the prison system – Culture for Convicts, as Langdon’s colleagues liked to call it.

Standing at an overhead projector in a darkened penitentiary library, Langdon had shared the MonaLisa’s secret with the prisoners attending class, men whom he found surprisingly engaged – rough, but sharp. «You may notice,» Langdon told them, walking up to the projected image of the MonaLisa on the library wall,» that the background behind her face is uneven.» Langdon motioned to the glaring discrepancy. «Da Vinci painted the horizon line on the left significantly lower than the right.»

«He screwed it up?» one of the inmates asked.

Langdon chuckled. «No. Da Vinci didn’t do that too often. Actually, this is a little trick Da Vinci played. By lowering the countryside on the left, Da Vinci made Mona Lisa look much larger from the left side than from the right side. A little Da Vinci inside joke. 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.»

«I heard he was a fag,» said a small man with a goatee.

Langdon winced. «Historians don’t generally put it quite that way, but yes, Da Vinci was a homosexual.»

«Is that why he was into that whole feminine thing?»

«Actually, Da Vinci was in tune with the balance between male and female. He believed that a human soul could not be enlightened unless it had both male and female elements.»

«You mean like chicks with dicks?» someone called.

This elicited a hearty round of laughs. Langdon considered offering an etymological sidebar about the word hermaphrodite and its ties to Hermes and Aphrodite, but something told him it would be lost on this crowd.

«Hey, Mr. Langford,» a muscle-bound man said. «Is it true that the Mona Lisa is a picture of Da Vinci in drag? I heard that was true.»