The stately Beaux-Arts mansion on Riverside Drive between 137th and 138th Streets, while carefully tended and impeccably preserved, appeared to be untenanted. On this stormy June evening, no figures paced the widow’s walk overlooking the Hudson River. No yellow glow from within flowed through the decorative oriel windows. The only visible light, in fact, came from the front entrance, illuminating the drive beneath the building’s porte cochere.

Appearances can be deceiving, however — sometimes intentionally. Because 891 Riverside was the residence of FBI Special Agent Aloysius Pendergast — and Pendergast was a man who valued, above all, his privacy.

In the mansion’s elegant library, Pendergast sat in a leather wing chair. Although it was early summer, the night was blustery and chill, and a low fire flickered on the grate. He was leafing through a copy of the Manyōshū, an old and celebrated anthology of Japanese poetry, dating to AD 750. A small tetsubin, or cast-iron teapot, sat on a table beside him, along with a china cup half-full of green tea. Nothing disturbed his concentration. The only sounds were the occasional crackle of settling embers and rumble of thunder from beyond the closed shutters.

Now there was a faint sound of footsteps from the reception hall beyond and Constance Greene appeared, framed in the library doorway. She was wearing a simple evening dress. Her violet eyes and dark hair, cut in an old-fashioned bob, offset the paleness of her skin. In one hand she held a bundle of letters.

“The mail,” she said.

Pendergast inclined his head, set the book aside.

Constance took a seat beside Pendergast, noting that, since returning from what he called his “Colorado adventure,” he was at last looking like his old self. His state of mind had been a cause of uneasiness in her since the dreadful events of the prior year.

She began sorting through the small stack of mail, putting aside the things that would not interest him. Pendergast did not like to concern himself with quotidian details. He had an old and discreet New Orleans law firm, long in the employ of the family, to pay bills and manage part of his unusually extensive income. He had an equally hoary New York banking firm to manage other investments, trusts, and real estate. And he had all mail delivered to a post office box, which Proctor, his chauffeur, bodyguard, and general factotum, collected on a regular basis. At present, Proctor was preparing to leave for a visit to relatives in Alsace, so Constance had agreed to take over the epistolary matters.

“Here’s a note from Corrie Swanson.”

“Open it, if you please.”

“She’s attached a photocopy of a letter from John Jay. Her thesis won the Rosewell Prize.”

“Indeed. I attended the ceremony.”

“I’m sure Corrie appreciated it.”

“It is rare that a graduation ceremony offers more than an anesthetizing parade of platitudes and mendacity, set to the tiresome refrain of ‘Pomp and Circumstance.’ ” Pendergast took a sip of tea at the recollection. “This one did.”

Constance sorted through more mail. “And here’s a letter from Vincent D’Agosta and Laura Hayward.”

He nodded for her to scan it. “It’s a thank-you note for the wedding gift and once again for the dinner party.”

Pendergast inclined his head as she put the letter aside. The month before, on the eve of D’Agosta’s wedding, Pendergast had hosted a private dinner for the couple, consisting of several courses he had prepared himself, paired with rare wines from his cellar. It was this gesture, more than anything, that had convinced Constance that Pendergast had recovered from his recent emotional trauma.

She read over a few other letters, then put aside those of interest and tossed the rest on the fire.

“How is the project coming, Constance?” Pendergast asked as he poured himself a fresh cup of tea.

“Very well. Just yesterday I received a packet from France, the Bureau Ancestre du Dijon, which I’m now trying to integrate with what I’ve already collected from Venice and Louisiana. When you have the time, I do have a couple of questions I’d like to ask about Augustus Robespierre St. Cyr Pendergast.”

“Most of what I know consists of oral family history — tall tales, legends, and some whispered horror stories. I’d be glad to share most of them with you.”

“Most? I was hoping you’d share them all.”

“I fear there are skeletons in the Pendergast family closet, figurative and literal, that I must keep even from you.”

Constance sighed and rose. As Pendergast returned to his book of poetry, she walked out of the library, across the reception hall lined with museum cabinets full of curious objects, and through a doorway into a long, dim space paneled in time-darkened oak. The main feature of the room was a wooden refectory table, almost as long as the room itself. The near end of the table was covered with journals, old letters, census pages, yellowed photographs and engravings, court transcripts, memoirs, reprints from newspaper microfiche, and other documents, all arranged in neat stacks. Beside them sat a laptop computer, its screen glowing incongruously in the dim room. Several months before, Constance had taken it upon herself to prepare a genealogy of the Pendergast family. She wanted both to satisfy her own curiosity and to help draw Pendergast out of himself. It was a fantastically complex, infuriating, and yet endlessly fascinating undertaking.

At the far end of the long room, beyond an arched door, was the foyer leading to the mansion’s front door. Just as Constance was about to take a seat at the table, a loud knock sounded.

Constance paused, frowning. They rarely entertained visitors at 891 Riverside Drive — and never did one arrive unannounced.

Knock. Another rap resounded from the entryway, accompanied by a low grumble of thunder.

Smoothing down her dress, Constance walked down the length of the room, through the archway, and into the foyer. The heavy front door was solid, with no fish-eye lens, and she hesitated a moment. When no third rap came, she undid the upper lock, then the lower, and slowly opened the door.

There, silhouetted in the light of the porte cochere, stood a young man. His blond hair was wet and plastered to his head. His rain-spattered features were fine and quintessentially Nordic, with a high-domed forehead and chiseled lips. He was dressed in a linen suit, sopping wet, which clung to his frame.

He was bound with heavy ropes.

Constance gasped, began to reach out to him. But the bulging eyes took no notice of the gesture. They stared straight ahead, unblinking.

For a moment, the figure remained standing, swaying ever so slightly, fitfully illuminated by flashes of lightning — and then it began to fall, like a tree toppling, slowly at first and then faster, before crashing facedown across the threshold.

Constance backed up with a cry. Pendergast arrived at a run, followed by Proctor. Pendergast grasped her, pulled her aside, and quickly knelt over the young man. He gripped the figure by the shoulder and turned him over, brushing the hair from his eyes, and feeling for the pulse that was so obviously absent beneath the cold flesh of the neck.

“Dead,” he said, his voice low and unnaturally composed.

“My God,” Constance said, her own voice breaking. “It’s your son Tristram.”

“No,” Pendergast said. “It’s Alban. His twin.”

For just a moment longer he knelt by the body. And then he leapt to his feet and, in a flash of feline motion, disappeared into the howling storm.