Sophie guided Langdon toward the ticket window and said, "Buy us two tickets with your credit card."

Sophie raised her eyes to the enormous departure board overhead. The black and white tabs reshuffled, ruffling downward as the information refreshed. When the update was finished, Langdon eyed the offerings. The topmost listing read: LYON -- RAPIDE --3:06 "I wish it left sooner," Sophie said, "but Lyon will have to do."

The inside of Gare Saint-Lazare looked like every other train station in Europe, a gaping indoor-outdoor cavern dotted with the usual suspects -- homeless men holding cardboard signs, collections of bleary-eyed college kids sleeping on backpacks and zoning out to their portable MP3 players, and clusters of blue-clad baggage porters smoking cigarettes.

Sooner? Langdon checked his watch 2:59 A. M. The train left in seven minutes and they didn't even have tickets yet.

"I thought credit card usage could be traced by --"

Langdon decided to stop trying to keep ahead of Sophie Neveu. Using his Visa card, he purchased two coach tickets to Lyon and handed them to Sophie.

"Exactly."

As the taxi pulled away from station, Sophie took out their newly purchased train tickets and tore them up.

Sophie guided him out toward the tracks, where a familiar tone chimed overhead and a P. A. announcer gave the final boarding call for Lyon. Sixteen separate tracks spread out before them. In the distance to the right, at quay three, the train to Lyon was belching and wheezing in preparation for departure, but Sophie already had her arm through Langdon's and was guiding him in the exact opposite direction. They hurried through a side lobby, past an all-night cafe, and finally out a side door onto a quiet street on the west side of the station.

A lone taxi sat idling by the doorway.

The driver saw Sophie and flicked his lights.

Sophie jumped in the back seat. Langdon got in after her.

Langdon sighed. Seventy dollars well spent.

It was not until their taxi had settled into a monotonous northbound hum on Rue de Clichy that Langdon felt they'd actually escaped. Out the window to his right, he could see Montmartre and the beautiful dome of Sacré-Coeur. The image was interrupted by the flash of police lights sailing past them in the opposite direction.

Langdon and Sophie ducked down as the sirens faded.

Sophie had told the cab driver simply to head out of the city, and from her firmly set jaw, Langdon sensed she was trying to figure out their next move.

Langdon examined the cruciform key again, holding it to the window, bringing it close to his eyes in an effort to find any markings on it that might indicate where the key had been made. In the intermittent glow of passing streetlights, he saw no markings except the Priory seal.

"Which part?"

"It doesn't make sense," he finally said.

"I searched the whole area. This is all there was. This key, wedged behind the painting. I saw the Priory seal, stuck the key in my pocket, then we left." Langdon frowned, peering now at the blunt end of the triangular shaft. Nothing. Squinting, he brought the key close to his eyes and examined the rim of the head. Nothing there either. "I think this key was cleaned recently."

"That your grandfather would go to so much trouble to give you a key that you wouldn't know what to do with."

"I agree."

"Are you sure he didn't write anything else on the back of the painting?"

"It's stronger on the other side." He flipped it over. "Yes, it's alcohol-based, like it's been buffed with a cleaner or --" Langdon stopped.

She turned. "I'm sorry?"

He angled the key to the light and looked at the smooth surface on the broad arm of the cross. It seemed to shimmer in places…

"It smells like somebody polished it with a cleaner." Langdon held the key to his nose and sniffed.

"What?"

Langdon turned to her. "Do you still have the black light?"

"Well," Langdon said, smiling. "I guess we know what the alcohol smell was." Sophie stared in amazement at the purple writing on the back of the key.

like it was wet. "How well did you look at the back of this key before you put it in your pocket?"

"What? Not well. I was in a hurry."

Sophie reached in her pocket and produced the UV penlight. Langdon took it and switched it on, shining the beam on the back of the key.

"It smells like rubbing alcohol."

The back luminesced instantly. There was writing there. In penmanship that was hurried but legible.

"Why?"

24 Rue Haxo An address! My grandfather wrote down an address!

"Fastest route is through Bois de Boulogne," the driver told her in French. "Is that okay?"

"Where is this?" Langdon asked.

Sophie frowned. She could think of far less scandalous routes, but tonight she was not going to be picky.

Sophie had no idea. Facing front again, she leaned forward and excitedly asked the driver, "Connaissez-vous la Rue Haxo?"

"Oui." We can shock the visiting American.

Sophie looked back at the key and wondered what they would possibly find at 24 Rue Haxo. Achurch? Some kind of Priory headquarters?

Her mind filled again with images of the secret ritual she had witnessed in the basement grotto ten years ago, and she heaved a long sigh. "Robert, I have a lot of things to tell you." She paused, locking eyes with him as the taxi raced westward. "But first I want you to tell me everything you know about this Priory of Sion."

The driver thought a moment and then nodded. He told Sophie it was out near the tennis stadium on the western outskirts of Paris. She asked him to take them there immediately.