Eight ninety-one Riverside Drive. The home-one of the homes- of Special Agent Aloysius Pendergast, D'Agosta's friend and partner in several unusual cases. The mysterious FBI agent whom D'Agosta knew, and yet did not know, who seemed to have as many lives as a cat…

Until that day not two months ago, when he'd seen Pendergast for the last time.

It had been on the steep flank of a hill south of Florence, Italy. The special agent had been below him, surrounded by a ravening pack of boar-hunting dogs, backed up by a dozen armed men. Pendergast had sacrificed himself so D'Agosta could get away.

And D'Agosta had let him do it.

D'Agosta stirred restlessly at the memory. Someone who has requested your presence, Proctor had said. Was it possible that, despite everything, Pendergast had somehow managed to escape? It wouldn't be the first time. He suppressed a surge of hope…

But no, it was not possible. He knew in his heart that Pendergast was dead.

Now the Rolls was cruising up Riverside Drive. D'Agosta shifted again, glancing out at the passing street signs: 125th Street, 130th. Very quickly, the well-tended neighborhood surrounding Columbia University gave way to dilapidated brownstones and decaying hulks. The usual loiterers had been chased indoors by the January chill, and in the dim light of evening the street looked deserted.

Up ahead now, just past 137th Street, D'Agosta could make out the boarded-up facade and widow's walk of Pendergast's mansion. The dark lines of the vast structure sent a chill through him.

The Rolls pulled past the gates of the spiked iron fence and stopped beneath the porte-cochere. Without waiting for Proctor, D'Agosta let himself out and stared up at the familiar lines of the rambling mansion, windows covered with tin, looking for all the world like the other abandoned mansions along the drive. Inside, it was home to wonders and secrets almost beyond belief. He felt his heart begin to race. Maybe Pendergast was inside, after all, in his usual black suit, sitting in the library before a blazing fire, the dancing flames casting strange shadows over his pale face. "My dear Vincent," he would say, "thank you for coming. May I interest you in a glass of Armagnac?"

D'Agosta waited as Proctor unlocked, then opened, the heavy door. Pale yellow light streamed out onto the worn brickwork. He stepped forward while Proctor carefully relocked the door behind him. He felt his heart beat still faster. Just being back inside the mansion sent a strange mix of emotions coursing through him: excitement, anxiety, regret.

Proctor turned toward him. "This way, sir, if you please."

The chauffeur led the way down the length of the gallery and into the blue-domed reception hall. Here, dozens of rippled-glass cabinets displayed an array of fabulous specimens: meteorites, gems, fossils, butterflies. D'Agosta's eyes stole across the parquet floor to the far side, where the double doors of the library lay open. If Pendergastwas waiting for him, that's where he'd be: sitting in a wing chair, a half-smile playing across his lips, enjoying the effect of this little drama on his friend.

Proctor ushered D'Agosta toward the library. Heart pounding, he stepped through the doors and into the sumptuous room.

The smell of the place was as he remembered it: leather, buckram, a faint hint of woodsmoke. But today there was no fire crackling merrily on the hearth. The room was cold. The inlaid bookshelves, full of leather-bound volumes tooled in gold, were dim and indistinct. Only a single lamp glowed-a Tiffany piece standing on a side table-casting a small pool of light in a vast lake of darkness.

After a moment, D'Agosta made out a form standing beside the table, just outside the circle of light. As he watched, the form advanced toward him across the carpeting. He recognized immediately the young girl as Constance Greene, Pendergast's ward and assistant. She was perhaps twenty, wearing a long, old-fashioned velvet dress that snugged her slender waist and fell in lines almost to the floor. Despite her obvious youth, her bearing had the poise of a much older woman. And her eyes, too-D'Agosta remembered her strange eyes, full of experience and learning, her speech old-fashioned, even quaint. And then there was that something else, something just the other side of normal, that seemed to cling to her like the antique air that exhaled from her dresses.

Those eyes seemed different today. They looked haunted, dark, heavy with loss… and fear?

Constance held out her right hand. "Lieutenant D'Agosta," she said in a measured tone.

D'Agosta took the hand, uncertain as always whether to shake it or kiss it. He did neither, and after a moment the hand was withdrawn.

Normally, Constance was polite to a fault. But today she simply stood before D'Agosta, without offering him a chair or inquiring after his health. She seemed uncertain. And D'Agosta could guess why. The hope that had been stirring within him began to fade.

"Have you heard anything?" she asked, her voice almost too low to make out. "Anything at all?"

D'Agosta shook his head, the flame of hope dashed out.

Constance held his glance a moment longer. Then she nodded her understanding, her gaze dropping to the floor, her hands fluttering at her sides like confused white moths.

They stood there together in silence for a minute, perhaps two.

Constance raised her eyes again. "It's foolish for me to continue to hope. More than six weeks have passed without a word."

"I know."

"He is dead," she said, voice even lower.

D'Agosta said nothing.

She roused herself. "That means it is time for me to give you this." She went to the mantelpiece, took down a small sandalwood box inlaid with mother-of-pearl. A tiny key already in her hand, she unlocked it and, without opening it, held it out toward D'Agosta.

"I have delayed this moment too long already. I felt that there was still a chance he might appear."

D'Agosta stared at the box. It looked familiar, but for a moment he could not place where he'd seen it before. Then it came to him: it had been in this house, this very room, the previous October. He'd entered the library and disturbed Pendergast in the act of writing a note. The agent had slipped it into this same box. That had been the night before they left on their fateful trip to Italy-the night Pendergast told him about his brother, Diogenes.

"Take it, Lieutenant," Constance said, her voice breaking. "Please don't draw this out."

"Sorry." D'Agosta gently took the box, opened it. Inside lay a single sheet of heavy cream-colored paper, folded once.

Suddenly, the very last thing D'Agosta wanted to do was to take out that piece of paper. With deep misgivings, he reached for it, opened it, and began to read.

My dear Vincent,

If you are reading this letter, it means that I am dead. It also means I died before I could accomplish a task that, rightfully, belongs to me and no other. That task is preventing my brother, Diogenes, from committing what he once boasted would be the "perfect" crime.

I wish I could tell you more about this crime, but all I know of it is that he has been planning it for many years and that he intends it to be his apotheosis. Whatever this "perfect" crime is, it will be infamous. It will make the world a darker place. Diogenes is a man with exceptional standards. He would not settle for less.

I'm afraid, Vincent, that the task of stopping Diogenes must now fall to you. I cannot tell you how much I regret this. It is something I would not wish on my worst enemy, and especially not on somebody I've come to regard as a trusted friend. But it is something I believe you are best equipped to handle. Diogenes's threat is too amorphous for me to take to the FBI or other law enforcement agency, since he contrived his own false death some years ago. A single, dedicated individual has the best chance of preventing my brother from carrying out this crime. That individual is you.