The remorse that gripped her was powerful and sudden, a deep sadness laced with guilt. The man had reached out to her so many times over the past ten years, and yet Sophie had remained immovable -- leaving his letters and packages unopened in a bottom drawer and denying his efforts to see her. He lied to me! Kept appalling secrets! What was I supposed to do? And so she had blocked him out. Completely. Now her grandfather was dead, and he was talking to her from the grave. The Mona Lisa.

Sophie arrived breathless outside the large wooden doors of the Salle des Etats -- the room that housed the Mona Lisa. Before entering, she gazed reluctantly farther down the hall, twenty yards or so, to the spot where her grandfather's body still lay under the spotlight.

She reached for the huge wooden doors, and pushed. The entryway yawned open. Sophie stood on the threshold a moment, scanning the large rectangular chamber beyond. It too was bathed in a soft red light. The Salle des Etats was one of this museum's rare culs-de-sac -- a dead end and the only room off the middle of the Grand Gallery. This door, the chamber's sole point of entry, faced a dominating fifteen-foot Botticelli on the far wall. Beneath it, centered on the parquet floor, an immense octagonal viewing divan served as a welcome respite for thousands of visitors to rest their legs while they admired the Louvre's most valuable asset.

Even before Sophie entered, though, she knew she was missing something. A black light. She gazed down the hall at her grandfather under the lights in the distance, surrounded by electronic gear. If he had written anything in here, he almost certainly would have written it with the watermark stylus. Taking a deep breath, Sophie hurried down to the well-lit crime scene. Unable to look at her grandfather, she focused solely on the PTS tools. Finding a small ultraviolet penlight, she slipped it in the pocket of her sweater and hurried back up the hallway toward the open doors of the Salle des Etats.

Sophie turned the corner and stepped over the threshold. Her entrance, however, was met by an unexpected sound of muffled footsteps racing toward her from inside the chamber. There's someone in here! A ghostly figure emerged suddenly from out of the reddish haze. Sophie jumped back. "There you are!" Langdon's hoarse whisper cut the air as his silhouette slid to a stop in front of her. Her relief was only momentary. "Robert, I told you to get out of here! If Fache --"

"Sophie, listen." Langdon caught his breath as his blue eyes held her firmly. "The letters P. S… do they mean anything else to you? Anything at all?"

"Where were you?"

Afraid their voices might echo down the hall, Sophie pulled him into the Salle des Etats and closed the enormous twin doors silently, sealing them inside. "I told you, the initials mean Princess Sophie."

"I know, but did you ever see them anywhere else? Did your grandfather ever use P. S. in any other way? As a monogram, or maybe on stationery or a personal item?"

"I had to get the black light," she whispered, holding it up. "If my grandfather left me a message --"

The question startled her. How would Robert know that? Sophie had indeed seen the initials P. S. once before, in a kind of monogram. It was the day before her ninth birthday. She was secretly combing the house, searching for hidden birthday presents. Even then, she could not bear secrets kept from her. What did Grand-père get for me this year? She dug through cupboards and drawers. Did he get me the doll I wanted? Where would he hide it? Finding nothing in the entire house, Sophie mustered the courage to sneak into her grandfather's bed-room. The room was off-limits to her, but her grandfather was downstairs asleep on the couch.

I'll just take a fast peek! Tiptoeing across the creaky wood floor to his closet, Sophie peered on the shelves behind his clothing. Nothing. Next she looked under the bed. Still nothing. Moving to his bureau, she opened the drawers and one by one began pawing carefully through them. There must be something for me here! As she reached the bottom drawer, she still had not found any hint of a doll. Dejected, she opened the final drawer and pulled aside some black clothes she had never seen him wear. She was about to close the drawer when her eyes caught a glint of gold in the back of the drawer. It looked like a pocket watch chain, but she knew he didn't wear one. Her heart raced as she realized what it must be.

A necklace! Sophie carefully pulled the chain from the drawer. To her surprise, on the end was a brilliant gold key. Heavy and shimmering. Spellbound, she held it up. It looked like no key she had ever seen. Most keys were flat with jagged teeth, but this one had a triangular column with little pockmarks all over it. Its large golden head was in the shape of a cross, but not a normal cross. This was an even-armed one, like a plus sign. Embossed in the middle of the cross was a strange symbol -- two letters intertwined with some kind of flowery design.

Her grandfather walked in. "Sophie, you need to respect other people's privacy." Gently, he knelt down and took the key from her. "This key is very special. If you had lost it…" Her grandfather's quiet voice made Sophie feel even worse. "I'm sorry, Grand-père. I really am." She paused. "I thought it was a neck-lace for my birthday." He gazed at her for several seconds. "I'll say this once more, Sophie, because it's important. You need to learn to respect other people 's privacy."

"P. S.," she whispered, scowling as she read the letters. Whatever could this be? "Sophie?" her grandfather spoke from the doorway. Startled, she spun, dropping the key on the floor with a loud clang. She stared down at the key, afraid to look up at her grandfather's face. "I… was looking for my birthday present," she said, hanging her head, knowing she had betrayed his trust. For what seemed like an eternity, her grandfather stood silently in the doorway. Finally, he let out a long troubled breath. "Pick up the key, Sophie." Sophie retrieved the key.

A treasure hunt! Eagerly, she pored over the riddle until she solved it. The solution pointed her to another part of the house, where she found another card and another riddle. She solved this one too, racing on to the next card. Running wildly, she darted back and forth across the house, from clue to clue, until at last she found a clue that directed her back to her own bedroom. Sophie dashed up the stairs, rushed into her room, and stopped in her tracks. There in the middle of the room sat a shining red bicycle with a ribbon tied to the handlebars. Sophie shrieked with delight. "I know you asked for a doll," her grandfather said, smiling in the corner. "I thought you might like this even better."

The next morning, Sophie received no birthday present from her grand-father. She hadn't expected one, not after what she had done. But he didn't even wish her happy birthday all day. Sadly, she trudged up to bed that night. As she climbed in, though, she found a note card lying on her pillow. On the card was written a simple riddle. Even before she solved the riddle, she was smiling. I know what this is! Her grandfather had done this for her last Christmas morning.

"Yes, Grand-père."

"We'll talk about this some other time. Right now, the garden needs to be weeded." Sophie hurried outside to do her chores.

"I know, but these are important secrets. And someday, you'll learn to appreciate them as much as I do."

"What does it open? I never saw a key like that. It was very pretty."

The next day, her grandfather taught her to ride, running beside her down the walkway. When Sophie steered out over the thick lawn and lost her balance, they both went tumbling onto the grass, rolling and laughing. "Grand-père," Sophie said, hugging him. "I'm really sorry about the key."

Her grandfather was silent a long moment, and Sophie could see he was uncertain how to answer. Grand-père never lies. "It opens a box," he finally said. "Where I keep many secrets." Sophie pouted. "I hate secrets!"

"I saw letters on the key, and a flower."

"I know, sweetie. You're forgiven. I can't possibly stay mad at you. Grandfathers and grand-daughters always forgive each other." Sophie knew she shouldn't ask, but she couldn't help it.

"Yes, that's my favorite flower. It's called a fleur-de-lis. We have them in the garden. The white ones. In English we call that kind of flower a lily."

"Then I'll make a deal with you." Her grandfather's eyebrows raised the way they always did when he was about to give her a challenge. "If you can keep my key a secret, and never talk about it ever again, to me or anybody, then someday I will give it to you." Sophie couldn't believe her ears. "You will?"

"I know those! They're my favorite too!"

"I promise. When the time comes, the key will be yours. It has your name on it." Sophie scowled. "No it doesn't. It said P. S. My name isn't P. S.!"

"P. S.?" He tickled her. "Princesse Sophie." She giggled. "I'm not a princess!" He winked. "You are to me." From that day on, they never again spoke of the key. And she became his Princess Sophie. Inside the Salle des Etats, Sophie stood in silence and endured the sharp pang of loss.

"Of course. Granddaughters always have secret initials that only their grandfathers know."

Her grandfather lowered his voice and looked around as if to make sure no one was listening. "Okay, Sophie, if you must know, P. S. is a code. It's your secret initials." Her eyes went wide. "I have secret initials?"

Langdon locked eyes with her. "Sophie, this is crucial. Can you tell me if the initials appeared with a symbol? A fleur-de-lis?" Sophie felt herself staggering backward in amazement. "But… how could you possibly know that!" Langdon exhaled and lowered his voice. "I'm fairly certain your grandfather was a member of a secret society. A very old covert brotherhood." Sophie felt a knot tighten in her stomach. She was certain of it too. For ten years she had tried to forget the incident that had confirmed that horrifying fact for her. She had witnessed something unthinkable. Unforgivable.

"Where?" Sophie hesitated. "On something very important to him."

"The initials," Langdon whispered, eyeing her strangely. "Have you seen them?" Sophie sensed her grandfather's voice whispering in the corridors of the museum. Never speak of this key, Sophie. To me or to anyone. She knew she had failed him in forgiveness, and she wondered if she could break his trust again. P. S. Find Robert Langdon. Her grandfather wanted Langdon to help. Sophie nodded. "Yes, I saw the initials P. S. once. When I was very young."

"The fleur-de-lis," Langdon said, "combined with the initials P. S., that is the brotherhood's official device. Their coat of arms. Their logo."

Langdon was talking in rapid bursts now. "The Priory's membership has included some of history's most cultured individuals: men like Botticelli, Sir Isaac Newton, Victor Hugo." He paused, his voice brimming now with academic zeal. "And, Leonardo da Vinci." Sophie stared. "Da Vinci was in a secret society?"

"How do you know this?" Sophie was praying Langdon was not going to tell her that he himself was a member. "I've written about this group," he said, his voice tremulous with excitement. "Researching the symbols of secret societies is a specialty of mine. They call themselves the Prieuré de Sion -- the Priory of Sion. They're based here in France and attract powerful members from all over Europe. In fact, they are one of the oldest surviving secret societies on earth. "Sophie had never heard of them.

"Da Vinci presided over the Priory between 1510 and 1519 as the brotherhood's Grand Master, which might help explain your grandfather's passion for Leo-nardo's work. The two men share a historical fraternal bond. And it all fits perfectly with their fascination for goddess iconology, paganism, feminine deities, and contempt for the Church. The Priory has a well-documented history of reverence for the sacred feminine."

"You're telling me this group is a pagan goddess worship cult?"

Despite the total conviction in Langdon's eyes, Sophie's gut reaction was one of stark disbelief. A secret pagan cult? Once headed by Leonardo da Vinci? It all sounded utterly absurd. And yet, even as she dismissed it, she felt her mind reeling back ten years -- to the night she had mistakenly surprised her grandfather and witnessed what she still could not accept. Could that explain --?

"More like the pagan goddess worship cult. But more important, they are known as the guardians of an ancient secret. One that made them immeasurably powerful."

"The identities of living Priory members are kept extremely secret," Lang-don said, "but the P. S. and fleur-de-lis that you saw as a child are proof. It could only have been related to the Priory." Sophie realized now that Langdon knew far more about her grandfather than she had previously imagined. This American obviously had volumes to share with her, but this was not the place. "I can't afford to let them catch you, Robert. There's a lot we need to discuss. You need to go!" Langdon heard only the faint murmur of her voice. He wasn't going anywhere. He was lost in another place now. A place where ancient secrets rose to the surface. A place where forgotten histories emerged from the shadows.

A few miles away, on the riverbank beyond Les Invalides, the bewildered driver of a twin-bed Trailor truck stood at gunpoint and watched as the captain of the Judicial Police let out a guttural roar of rage and heaved a bar of soap out into the turgid waters of the Seine.

Slowly, as if moving underwater, Langdon turned his head and gazed through the reddish haze toward the Mona Lisa. The fleur-de-lis… the flower of Lisa… the Mona Lisa. It was all intertwined, a silent symphony echoing the deepest secrets of the Priory of Sion and Leonardo da Vinci.