Sophie looked relieved as she jumped behind the wheel. Within seconds she had the car humming smoothly westward along Allée de Longchamp, leaving the Garden of Earthly Delights behind.

When Langdon reached the exit of the park, he swallowed his machismo and jammed on the brakes. "You'd better drive."

Struggling with the gear shift, Langdon managed to maneuver the hijacked taxi to the far side of the Bois de Boulogne while stalling only twice. Unfortunately, the inherent humor in the situation was overshadowed by the taxi dispatcher repeatedly hailing their cab over the radio.

"Which way is Rue Haxo?" Langdon asked, watching Sophie edge the speedometer over a hundred kilometers an hour.

Sophie's eyes remained focused on the road. "The cab driver said it's adjacent to the Roland Garrostennis stadium. I know that area."

"Voiture cinq-six-trois. Où êtes-vous? Répondez!"

Langdon pulled the heavy key from his pocket again, feeling the weight in his palm. He sensed it was an object of enormous consequence. Quite possibly the key to his own freedom.

The equal-armed cruciform was symbolic of balance and harmony but also of the Knights Templar.

Earlier, while telling Sophie about the Knights Templar, Langdon had realized that this key, in addition to having the Priory seal embossed on it, possessed a more subtle tie to the Priory of Sion.

Everyone had seen the paintings of Knights Templar wearing white tunics emblazoned with red equal-armed crosses. Granted, the arms of the Templar cross were slightly flared at the ends, butthey were still of equal length.

Langdon felt his imagination starting to run wild as he fantasized about what they might find. The Holy Grail.

He almost laughed out loud at the absurdity of it. The Grail was believed to be somewhere in England, buried in a hidden chamber beneath one of the many Templar churches, where it had been hidden since at least 1500.

The era of Grand Master Da Vinci.

The Priory, in order to keep their powerful documents safe, had been forced to move them many times in the early centuries. Historians now suspected as many as six different Grail relocations since its arrival in Europe from Jerusalem. The last Grail "sighting" had been in 1447 when numerous eyewitnesses described a fire that had broken out and almost engulfed the documents before they were carried to safety in four huge chests that each required six men to carry. After that, nobody claimed to see the Grail ever again. All that remained were occasional whisperings that it was hidden in Great Britain, the land of King Arthur and the Knights of the Round Table.

A square cross. Just like the one on this key.

Wherever it was, two important facts remained: Leonardo knew where the Grail resided during his lifetime.

That hiding place had probably not changed to this day.

For this reason, Grail enthusiasts still pored over Da Vinci's art and diaries in hopes of unearthing a hidden clue as to the Grail's current location. Some claimed the mountainous backdrop in Madonna of the Rocks matched the topography of a series of cave-ridden hills in Scotland. Others insisted that the suspicious placement of disciples in The Last Supper was some kind of code. Still others claimed that X rays of the Mona Lisa revealed she originally had been painted wearing a lapis lazuli pendant of Isis -- a detail Da Vinci purportedly later decided to paint over. Langdon had never seen any evidence of the pendant, nor could he imagine how it could possibly reveal the Holy Grail, and yet Grail aficionados still discussed it ad nauseum on Internet bulletin boards and worldwide-web chat rooms.

Everyone loves a conspiracy.

And the conspiracies kept coming. Most recently, of course, had been the earthshaking discovery that Da Vinci's famed Adoration of the Magi was hiding a dark secret beneath its layers of paint.

Seracini had revealed beyond any doubt that while the Adoration's gray-green sketched underdrawing was indeed Da Vinci's work, the painting itself was not. The truth was that some anonymous painter had filled in Da Vinci's sketch like a paint-by-numbers years after Da Vinci'sdeath. Far more troubling, however, was what lay beneath the impostor's paint. Photographs taken with infrared reflectography and X ray suggested that this rogue painter, while filling in Da Vinci's sketched study, had made suspicious departures from the underdrawing… as if to subvert DaVinci's true intention. Whatever the true nature of the underdrawing, it had yet to be made public.

Even so, embarrassed officials at Florence's Uffizi Gallery immediately banished the painting to a warehouse across the street. Visitors at the gallery's Leonardo Room now found a misleading and unapologetic plaque where the Adoration once hung.

Italian art diagnostician Maurizio Seracini had unveiled the unsettling truth, which the New York Times Magazine carried prominently in a story titled "The Leonardo Cover-Up."

THIS WORK IS UNDERGOINGDIAGNOSTIC TESTS IN PREPARATIONFOR RESTORATION.

In the bizarre underworld of modern Grail seekers, Leonardo da Vinci remained the quest's great enigma. His artwork seemed bursting to tell a secret, and yet whatever it was remained hidden, perhaps beneath a layer of paint, perhaps enciphered in plain view, or perhaps nowhere at all.

Langdon's laugh sounded forced, even to him. "I really can't imagine. Besides, the Grail is believed to be hidden in the United Kingdom somewhere, not France." He gave her the quick history.

Maybe Da Vinci's plethora of tantalizing clues was nothing but an empty promise left behind to frustrate the curious and bring a smirk to the face of his knowing Mona Lisa.

"Is it possible," Sophie asked, drawing Langdon back, "that the key you're holding unlocks the hiding place of the Holy Grail?"

"But the Grail seems the only rational conclusion," she insisted. "We have an extremely secure key, stamped with the Priory of Sion seal, delivered to us by a member of the Priory of Sion -- a brotherhood which, you just told me, are guardians of the Holy Grail." Langdon knew her contention was logical, and yet intuitively he could not possibly accept it.

"Not only that. The location of the Holy Grail is one of the best kept secrets in history. Priory members wait decades proving themselves trustworthy before being elevated to the highest echelons of the fraternity and learning where the Grail is. That secret is protected by an intricate system of compartmentalized knowledge, and although the Priory brotherhood is very large, only four members at any given time know where the Grail is hidden -- the Grand Master and his three sénéchaux. The probability of your grandfather being one of those four top people is very slim." My grandfather was one of them, Sophie thought, pressing down on the accelerator. She had an image stamped in her memory that confirmed her grandfather's status within the brotherhood beyond any doubt.

Even if the Priory had managed to bring the Grail back to France, the address 24 Rue Haxo near a tennis stadium hardly sounded like a noble final resting place. "Sophie, I really don't see how this key could have anything to do with the Grail."

Rumors existed that the Priory had vowed someday to bring the Grail back to France to a final resting place, but certainly no historical evidence existed to suggest that this indeed had happened.

"Because the Grail is supposed to be in England?"

"And even if your grandfather were in the upper echelon, he would never be allowed to reveal anything to anyone outside the brotherhood. It is inconceivable that he would bring you into the inner circle."

"I've already been there," Sophie thought, picturing the ritual in the basement. She wondered if this were the moment to tell Langdon what she had witnessed that night in the Normandy chateau. For ten years now, simple shame had kept her from telling a soul. Just thinking about it, she shuddered.

Sirens howled somewhere in the distance, and she felt a thickening shroud of fatigue settling over her.

Sophie snaked her way toward the stadium. After several passes, they located the intersection of Rue Haxo and turned onto it, driving in the direction of the lower numbers. The road became more industrial, lined with businesses.

"There!" Langdon said, feeling excited to see the huge complex of the Roland Garros tennis stadium looming ahead.

We need number twenty-four, Langdon told himself, realizing he was secretly scanning the horizon for the spires of a church. Don't be ridiculous. A forgotten Templar church in this neighborhood?

The building was modern. A squat citadel with a giant, neon equal-armed cross emblazoned atopits facade. Beneath the cross were the words:

"There it is," Sophie exclaimed, pointing.

Langdon's eyes followed to the structure ahead.

DEPOSITORY BANK OF ZURICH Langdon was thankful not to have shared his Templar church hopes with Sophie. A career hazard of symbologists was a tendency to extract hidden meaning from situations that had none. In this case, Langdon had entirely forgotten that the peaceful, equal-armed cross had been adopted as the perfect symbol for the flag of neutral Switzerland.

What in the world?

Sophie and Langdon were holding the key to a Swiss bank deposit box.

At least the mystery was solved.