He now flashed on an old adage from early Grecian free divers who hunted lobsters in the coral caves of the Aegean Islands. When swimming into a dark tunnel, there arrives a point of no return when you no longer have enough breath to double back. Your only choice is to swim forward into the unknown … and pray for an exit.

Langdon wondered if they had reached that point.

He eyed the maze of garden pathways before them. If he and Sienna could reach the Pitti Palace and exit the gardens, then the old city was just a short walk across the most famous footbridge in the world — the Ponte Vecchio. It was always crowded and would provide good cover. From there, the Palazzo Vecchio was only a few blocks away.

The drone hummed closer now, and Langdon felt momentarily overwhelmed by exhaustion. The realization that he had not been saying “very sorry” left him feeling conflicted about running from the police.

“Eventually, they’re going to catch me, Sienna,” Langdon said. “It might be better for me to stop running.”

Sienna looked at him with alarm. “Robert, every time you stop, someone starts shooting at you! You need to figure out what you’re involved in. You need to look at that Vasari mural and hope it jars your memory. Maybe it will help you learn where this projector came from and why you’re carrying it.”

Langdon pictured the spike-haired woman coldly killing Dr. Marconi … the soldiers firing on them … the Italian military police gathering in the Porta Romana … and now a surveillance drone tracking them through the Boboli Gardens. He fell silent, rubbing his tired eyes as he considered his options.

“Robert?” Sienna’s voice rose. “There’s one other thing … something that didn’t seem important, but now seems like it might be.”

Langdon raised his eyes, reacting to the gravity in her tone.

“I intended to tell you at the apartment,” she said, “but …”

“What is it?”

Sienna pursed her lips, looking uncomfortable. “When you arrived at the hospital, you were delirious and trying to communicate.”

“Yes,” Langdon said, “mumbling ‘Vasari, Vasari.’ ”

“Yes, but before that … before we got out the recorder, in the first moments after you arrived, you said one other thing I remember. You only said it once, but I’m positive I understood.”

“What did I say?”

Sienna glanced up toward the drone and then back at Langdon. “You said, ‘I hold the key to finding it … if I fail, then all is death.’ ”

Langdon could only stare.

Sienna continued. “I thought you were referring to the object in your jacket pocket, but now I’m not so sure.”

If I fail, then all is death? The words hit Langdon hard. The haunting images of death flickered before him … Dante’s inferno, the biohazard symbol, the plague doctor. Yet again, the face of the beautiful silver-haired woman pleaded with him across the bloodred river. Seek and find! Time is running out!

Sienna’s voice pulled him back. “Whatever this projector ultimately points to … or whatever you’re trying to find, it must be something extremely dangerous. The fact that people are trying to kill us …” Her voice cracked slightly, and she took a moment to regroup. “Think about it. They just shot at you in broad daylight … shot at me—an innocent bystander. Nobody seems to be looking to negotiate. Your own government turned on you … you called them for help, and they sent someone to kill you.”

Langdon stared vacantly at the ground. Whether the U.S. Consulate had shared Langdon’s location with the assassin, or whether the consulate itself had sent the assassin, was irrelevant. The upshot was the same. My own government is not on my side.

Langdon looked into Sienna’s brown eyes and saw bravery there. What have I gotten her involved in? “I wish I knew what we were looking for. That would help put all of this into perspective.”

Sienna nodded. “Whatever it is, I think we need to find it. At least it would give us leverage.”

Her logic was hard to refute. Still Langdon felt something nagging at him. If I fail, then all is death. All morning he’d been running up against macabre symbols of biohazards, plagues, and Dante’s hell. Admittedly, he had no clear proof of what he was looking for, but he would be naive not to consider at least the possibility that this situation involved a deadly disease or large-scale biological threat. But if this were true, why would his own government be trying to eliminate him?

Do they think I’m somehow involved in a potential attack?

It made no sense at all. There was something else going on here.

Langdon thought again of the silver-haired woman. “There’s also the woman from my visions. I feel I need to find her.”

“Then trust your feelings,” Sienna said. “In your condition, the best compass you have is your subconscious mind. It’s basic psychology — if your gut is telling you to trust that woman, then I think you should do exactly what she keeps telling you to do.”

“Seek and find,” they said in unison.

Langdon exhaled, knowing his path was clear.

All I can do is keep swimming down this tunnel.

With hardening resolve, he turned and began taking in his surroundings, trying to get his bearings. Which way out of the gardens?

They were standing beneath the trees at the edge of a wide-open plaza where several paths intersected. In the distance to their left, Langdon spied an elliptical-shaped lagoon with a small island adorned with lemon trees and statuary. The Isolotto, he thought, recognizing the famous sculpture of Perseus on a half-submerged horse bounding through the water.

“The Pitti Palace is that way,” Langdon said, pointing east, away from the Isolotto, toward the garden’s main thoroughfare — the Viottolone, which ran east — west along the entire length of the grounds. The Viottolone was as wide as a two-lane road and lined by a row of slender, four-hundred-year-old cypress trees.

“There’s no cover,” Sienna said, eyeing the uncamouflaged avenue and motioning up at the circling drone.

“You’re right,” Langdon said with a lopsided grin. “Which is why we’re taking the tunnel beside it.”

He pointed again, this time to a dense hedgerow adjacent to the mouth of the Viottolone. The wall of dense greenery had a small arched opening cut into it. Beyond the opening, a slender footpath stretched out into the distance — a tunnel running parallel with the Viottolone. It was enclosed on either side by a phalanx of pruned holm oaks, which had been carefully trained since the 1600s to arch inward over the path, intertwining overhead and providing an awning of foliage. The pathway’s name, La Cerchiata — literally “circular” or “hooped”—derived from its canopy of curved trees resembling barrel stays or cerchi.

Sienna hurried over to the opening and peered into the shaded channel. Immediately she turned back to him with a smile. “Better.”

Wasting no time, she slipped through the opening and hurried off among the trees.

Langdon had always considered La Cerchiata one of Florence’s most peaceful spots. Today, however, as he watched Sienna disappear down the darkened allée, he thought again of the Grecian free divers swimming into corral tunnels and praying they’d reach an exit.

Langdon quickly said his own little prayer and hurried after her.

A half mile behind them, outside the Art Institute, Agent Brüder strode through a bustle of police and students, his icy gaze parting the crowds before him. He made his way to the makeshift command post that his surveillance specialist had set up on the hood of his black van.

“From the aerial drone,” the specialist said, handing Brüder a tablet screen. “Taken a few minutes ago.”

Brüder examined the video stills, pausing on a blurry enlargement of two faces — a dark-haired man and a blond ponytailed woman — both huddled in the shadows and peering skyward through a canopy of trees.