THEY COMPLETED THEIR first circuit of the yard and started a second. Isherwood’s pace slowed as he began to talk of his father.

“He was always a Berliner at heart. He would have stayed there forever. That wasn’t possible, of course. My father saw the storm clouds coming and didn’t waste any time getting out of town. By the end of 1936, we’d left Berlin and moved to Paris.” He looked at Gabriel. “Too bad your grandfather didn’t do the same thing. He was a great painter, your grandfather. You come from a good bloodline, my boy.”

Gabriel quickly changed the subject. “Your father’s gallery was on the rue de la Boétie, wasn’t it?”

“Of course,” Isherwood replied. “The rue de la Boétie was the center of the art world at that time. Paul Rosenberg had his gallery at Number Twenty-one. Picasso and Olga lived on the other side of the courtyard at Number Twenty-three. Georges Wildenstein, Paul Guillaume, Josse Hessel, Étienne Bignou-everyone was there. Isakowitz Fine Arts was next door to Paul Rosenberg’s. We lived in an apartment above the exposition rooms. Picasso was my ‘Uncle Pablo.’ He used to let me watch him paint, and Olga would give me chocolates until I was sick.”

Isherwood permitted himself a brief smile, which faded quickly as he resumed the story of his father in Paris.

“The Germans came in May 1940 and started looting the place. My father rented a chateau in Bordeaux on the Vichy side of the line and moved most of his important pieces there. We followed him soon after. The Germans crossed over into the Unoccupied Zone in 1942, and the roundups and deportations began. We were trapped. My father paid a pair of Basque shepherds to take me over the mountains to Spain. He gave me some documents to carry with me, a professional inventory and a couple of diaries. It was the last time I ever saw him.”

A horn sounded loudly in Duke Street; a squadron of pigeons burst into flight over the shadowed yard.

“It was years before I got around to reading the diaries. In one of them I found an entry about a painting my father had seen in Paris one night at the home of man named Isaac Weinberg.”

“Marguerite Gachet at Her Dressing Table.”

“Weinberg told my father he’d bought the painting from Johanna not long after Vincent’s death and had given it to his wife as a birthday gift. Apparently Mrs. Weinberg bore a resemblance to Marguerite. My father asked Isaac whether he would be willing to sell, and Isaac said he wasn’t. He asked my father not to mention the painting to anyone, and my father was all too happy to oblige him.”

Isherwood’s mobile phone chirped. He ignored it.

“In the early seventies, right before I met you, I was in Paris on business. I had a few hours to kill between appointments and decided to look up Isaac Weinberg. I went to the address in the Marais that was listed in my father’s notebooks, but Weinberg wasn’t there. He hadn’t survived the war. But I met his son, Marc, and told him about the entry in my father’s notes. He denied the story at first, but finally relented and allowed me to see the painting after swearing me to eternal secrecy. It was hanging in his daughter’s bedroom. I asked whether he might be interested in parting with it. He refused, of course.”

“You’re certain it’s Vincent?”

“Without a doubt.”

“And you haven’t been back since?”

“Monsieur Weinberg made it quite clear the painting would never be for sale. I didn’t see the point.” Isherwood stopped walking and turned to face Gabriel. “All right, petal. I’ve told you the story. Now suppose you tell me what this is all about.”

“I need that van Gogh, Julian.”

“Whatever for?”

Gabriel took Isherwood’s sleeve and led him toward the door of the gallery.

THERE WAS an intercom panel next to the glass door, with four buttons and four corresponding nameplates. One read: ISHER OO FINE AR S: BY APPOINTMENT ONLY. Isherwood opened the door with a key and led Gabriel up a flight of stairs covered in a threadbare brown carpet. On the landing were two more doors. To the left was a melancholy little travel agency. The owner, a spinster named Miss Archer, was seated at her desk beneath a poster of a happy couple splashing in azure water. Isherwood’s door was on the right. His latest secretary, an apologetic-looking creature named Tanya, glanced up furtively as Isherwood and Gabriel came inside. “This is Mr. Klein,” said Isherwood. “He’d like to have a look at something upstairs. No interruptions, please. That’s a good girl, Tanya darling.”

They entered a lift the size of a phone booth and rode it upward, standing so close to one another that Gabriel could smell last night’s claret on Isherwood’s breath. A few seconds later the lift shuddered to a stop and the door opened with a groan. Isherwood’s exposition room was in semidarkness, illuminated only by the mid-morning sun filtering through the skylight. Isherwood settled himself on the velvet-covered divan in the center of the room while Gabriel led himself on a slow tour. The paintings were nearly invisible in the deep shadows, but he knew them well: a Venus by Luini, a nativity by Perino del Vaga, a baptism of Christ by Bordone, a luminous landscape by Claude.

Isherwood opened his mouth to speak, but Gabriel raised a finger to his lips and from his coat pocket removed what appeared to be an ordinary Nokia cellular telephone. It was indeed a Nokia, but it contained several additional features not available to ordinary customers, such as a GPS beacon and a device that could detect the presence of hidden transmitters. Gabriel toured the room again, this time with his eyes on the display panel of the phone. Then he sat down next to Isherwood and, in a low voice, told him why he needed the van Gogh.

“Zizi al-Bakari?” asked Isherwood incredulously. “A bloody terrorist? Are you sure?”

“He’s not planting the bombs, Julian. He’s not even making the bombs. But he’s footing the bill, and he’s using his business empire to facilitate the movement of the men and matériel around the globe. In today’s world that’s just as bad. Worse.”

“I met him once, but not so he’d remember. Went to a party at his estate out in Gloucestershire. Huge party. Sea of people. Zizi was nowhere to be found. Came down at the end like bloody Gatsby. Surrounded by bodyguards, even inside his own home. Strange chap. Voracious collector, though, isn’t he? Art. Women. Anything money can buy. Predatory, from what I hear. Never had any dealings with him, of course. Zizi’s tastes don’t run to the Old Masters. Zizi goes for the Impressionists and a bit of other Modern stuff. All the Saudis are like that. They don’t hold with the Christian imagery of the Old Masters.”

Gabriel sat down next to Isherwood. “He doesn’t have a van Gogh, Julian. He’s dropped hints from time to time that he’s looking for one. And not just any van Gogh. He wants something special.”

“From what I hear, he buys very carefully. He spends buckets of money, but he does it wisely. He’s got a museum-quality collection, but I didn’t realize it was sans van Gogh.”

“His art adviser is an Englishman named Andrew Malone. Know him?”

“Unfortunately, Andrew and I are well acquainted. He’s burrowed his way deeply into Zizi’s pockets. Spends holidays on Zizi’s yacht. Big as the bloody Titanic, from what I hear. Andrew is as slippery as they come. Dirty, too.”

“In what way?”

“He’s taking it on both ends, petal.”

“What do you mean, Julian?”

“Andrew has an exclusive agreement with Zizi, which means he’s not supposed to take money from any other dealer or collector. It’s the way big boys like Zizi ensure that the advice they’re being given is untainted by any conflicts of interest.”

“What’s Malone up to?”

“Shakedowns, double commissions, you name it.”