Munoz was pointing his pencil at square a2 now.

“It would be another case of imaginary check if the queen had been here, and so we can discount that square too.”

“Then it could obviously only have come from b2,” said Cesar.

“That’s possible.”

“What do you mean ‘possible’?” Cesar was confused and intrigued at the same time. “It looks obvious to me.”

“In chess,” replied Munoz, “very few things can be termed ‘obvious’. Look at the white pieces along line b. What would have happened if the queen had been on b2?”

Cesar stroked his chin thoughtfully.

“She would have been under threat by the white rook on b5. That’s probably why she moved to c2, to escape the rook.”

“Not bad,” conceded Munoz. “But that’s only a possibility. Anyway, the reason she moved isn’t important to us at the moment. Do you remember what I told you before? Once the impossible has been eliminated, what remains must, of necessity, be right. To sum up, (a) Black has just moved, (b) nine of the ten black pieces on the board could not have moved, (c) the only piece that could have moved was the queen, (d) three of the four hypothetical moves by the queen are impossible. Therefore, the black queen made the only possible move: it moved from b2 to c2, perhaps fleeing from the threat of the white rooks on b5 and b6. Is that clear?”

“Very,” said Julia, and Cesar agreed.

“That means,” Munoz went on, “that we’ve managed to take the first step in this reverse chess game that we’re playing. The subsequent Position, or rather, the previous position, since we’re working backwards, would be this.”

“Do you see? The black queen is still on b2, before its move to c2. So now we have to find out what move White made that obliged the queen to do that.”

“It must have been the white rook,” said Cesar. “The one on b5. The treacherous devil could have come from any square along row 5.”

“Possibly,” replied Munoz. “But that doesn’t entirely justify the queen’s flight.”

Cesar blinked, surprised.

“Why not?” His eyes went from the board to Munoz and back to the board again. “The queen was obviously fleeing the threat from the rook. You yourself said so a moment ago.”

“I said that perhaps she was fleeing from the white rooks, but at no point did I say that it was the white rook on b5 that caused the queen to flee.”

“I’m lost,” confessed Cesar.

“Look closely at the board. It doesn’t matter what the white rook now on b5 did, because the other white rook, the one on b6, would already have been holding the black queen in check. Do you see?”

Cesar studied the game again, this time for quite a long time.

“I still don’t get it,” he said at last, demoralised. He’d drained his gin and lemon to the last drop, while, at his side, Julia was smoking cigarette after cigarette. “If it wasn’t the white rook that moved to b5, then all your reasoning collapses. Wherever the piece was, that nasty queen had to move first, because she was in check before…”

“No,” said Munoz. “Not necessarily. The rook could, for example, have taken another black piece on b5.”

Encouraged by that possibility, Cesar and Julia studied the game with new heart. After a few more minutes, Cesar glanced up and gave Munoz a respectful glance.

“That’s right,” he said, astonished. “Don’t you see, Julia? A black piece on b5 was protecting the queen from the threat posed by the white rook on b6. When that black piece was captured by the white rook, the queen was under direct threat.” He looked back at Munoz for confirmation. “That must be it. There’s no other possibility.” He looked at the board again, doubtfully. “There isn’t, is there?”

“I don’t know,” Munoz replied honestly and, when she heard that, Julia uttered a desperate “Good God!”

“You’ve just formulated a hypothesis,” he continued, “and when you do that, you always run the risk of distorting the facts to fit the theory, instead of finding a theory that fits the facts.”

“What then?”

“Well, that’s just it. So far we can only consider as a hypothesis the idea that the white rook took a black piece on b5. We still have to ascertain if there are any other variants and, if so, discount all the impossible ones.” The gleam in his eyes grew dim. He seemed more tired and grey as he sketched an indefinable gesture in the air, which was part justification and part uncertainty. The confidence he’d displayed in explaining the moves had disappeared; now he seemed shy and awkward. “That’s what I meant,” he said, avoiding Julia’s eyes, “when I told you I’d run into problems.”

“What’s the next step then?” asked Julia.

Munoz regarded the pieces with a resigned air.

“A long, painstaking examination of the six black pieces that are off the board. I’ll try to find out how and where each one was taken.”

“That could take days,” said Julia.

“Or minutes, it depends. Sometimes, luck or intuition lend a hand.” He gave a long look at the board and then at the Van Huys. “But there’s one thing I’m sure of,” he said after a moment’s thought.

“Whoever painted this picture or thought up the problem, had a very peculiar way of playing chess.”

“How would you describe him?” asked Julia.

“Who?”

“The absent player. The one you just mentioned.”

Munoz looked first at the carpet and then at the painting. There was something like admiration in his eyes, Julia thought. Perhaps the instinctive respect a chess player always feels for a master.

“I don’t know,” he said in a low voice, as if unwilling to be pinned down. “Whoever it was, he was very devious. All the best players are, but this one had something else: a particular talent for laying false trails, for setting all kinds of traps. And he enjoyed doing it.”

“Is that possible?” asked Cesar. “Can you really judge the character of a person by the way he behaves when playing?”

“I think you can,” replied Munoz.

“In that case, what do you think of the person who thought up this game, bearing in mind that he did so in the fifteenth century?”

“I’d say” – Munoz was looking at the painting, absorbed – “I’d say there was something ‘diabolical’ about the way he played chess.”

Since they were double-parked, Menchu had moved over into the driver’s seat by the time Julia got back to the car. Julia opened the door of the small Fiat and dropped into the seat.

“What did they say?” Menchu asked.

Julia didn’t reply at first; she still had too many things to think about. Staring into the traffic flowing down the street, she took a cigarette out of her handbag, put it to her lips and pressed the automatic lighter in the dashboard.

“There were two policemen here yesterday,” she said at last, “asking the same questions as me.” When the lighter clicked out, she held it to her cigarette. “According to the man in charge, the envelope was delivered to them that Thursday, first thing in the afternoon.”

Menchu’s hands were gripping the wheel hard, her knuckles white amongst the glittering rings.

“Who delivered it?”

Julia slowly exhaled.

“According to him, it was a woman.”

“A woman?”

“That’s what he said.”

“What woman?”

“Middle-aged, well-dressed, blonde. Wearing a raincoat and sunglasses.” She turned to her friend. “It could have been you.”

“That’s not funny.”

“No, I know it isn’t.” Julia let out a long sigh. “But it could have been anyone. She didn’t give her name or her address, she just gave Alvaro’s details as sender. She asked for the fast delivery service. And that was it.”