Had Eoin changed his mind about living here partway through the renovation? Or had his untimely death cut the work short?

     After a more thorough inspection downstairs, Seb noticed a table and two chairs. On the table was a Staunton chess set laid out, ready for a game. The pieces looked pristine and shiny as if they'd been coated with too much varnish.

     Seb sat down and played a long sequence of moves almost without thinking. White opening with e4, black responding with the Sicilian Defence. From there, he made the exact sequence of moves that had comprised his first tournament game against Eoin, playing both sides, feeling his way through the memory. To his surprise, he recalled the whole game easily.

     For a while, Seb had been content to linger in Eoin's shadow until he realised his moment in the sunshine might never arrive. Eoin glided from victory to stunning victory, and while Seb achieved some notable wins of his own, his jealousy increased faster than his Elo rating. He studied through the long nights and arrived at every tournament better prepared than ever before, but Eoin played with a fluidity and focus that couldn't be matched. He was unpredictable, constantly finding new lines. The best-laid plans crumbled in the face of his chameleon talent.

     Seb cultivated a fierce hatred of Eoin; Eoin sometimes forgot his rival's name.

     Like almost everyone else, Seb sometimes used a chess engine to win matches online. But deploying such tactics over-the-board was more of a challenge, logistically speaking.

     It was after the Sinquefield Cup in St Louis that Seb decided Eoin had to be brought down by any means. In the final of that tournament, Seb played the Kings Indian, and the early game was blitzed out in predictable fashion until Eoin played a line that Seb had never seen before and which at first looked insane. Taken out of his preparation, Seb sweated for eighteen minutes before responding. Minutes later, he'd blundered a pawn and was soon crushed in a vice of two marauding rooks. He played for a draw long after any real hope had drained away and brought his hand down too hard on the top of the game clock, cracking the glass. Eoin stared on impassively with his bright blue eyes.

     After that, the gloves came off, and Seb rocketed through the rankings. Some speculated that he was receiving secret instructions from a chess engine. Theories ranged from buzzing implants to a blinking accomplice. He flatly denied the accusations, but they continued to swirl around him.

     He beat Eoin in Paris, Moscow, Mexico City. Prize money poured in, but the rumours about foul play intensified. Seb had expected that and prepared for it. Rather than let the embers cool, he threw petrol on the fire. Using fabricated online identities, he posted detailed analyses comparing certain passages of Eoin's play to moves suggested by popular chess engines, implying that Eoin might not be the paragon he presented himself as. It was hardly conclusive — neither was the evidence against Seb — but Seb's goal was not to prove that his rival had cheated but to muddy the water and frustrate his own critics. After all, if everyone was cheating, no one was.

     The ploy worked. Eoin was dogged by accusations and bore them more heavily than Seb had. He withdrew from tournament play. In his heart, Seb did not think he'd done anything wrong. Sometimes the path to victory was not a royal road but a winding and filth-encrusted track.

     Eoin drove his car into the side of a church in rural Devon at eighty miles an hour. The death was ruled accidental.

     Seb wondered if he'd emerged from his great rivalry as the victor or merely the sole survivor. He was a grandmaster, but his reputation was in tatters, and he struggled to remember why he'd been so desperate to climb the rankings in the first place.

     Then came the baffling coda — the will in which Eoin bequeathed Seb the White Castle. What was he to make of that? What did it mean?

     Seb realised he'd been staring at a white rook for a long time. He shivered and reset the board. The storm outside was intensifying, the rain lashing down in sheets. It would be futile and possibly dangerous to go looking for the Audi now. He'd have to wait until morning or at least until the weather cleared.

     As he stood up, he saw in his mind's eye, with almost preternatural clarity, Eoin's face. The shining blue eyes. The tufts of straight black hair. The flat bridge of his nose that he sometimes held between forefinger and thumb when contemplating a move.

     Seb had always thought he hated Eoin, but now he wondered if that was true. Those games! He looked back on them fondly now, even the crushing defeats.

     He turned back to the board and was surprised to see a white pawn on e4. He'd meant to return all the pieces to their starting squares, but clearly, these memories had fogged his mind, and he'd begun a new game without realising it. His hand hovered over the pawn, then he changed his mind and left it where it was. He went upstairs, but the bed was too wet and dirty to offer any comfort, so instead, he made a nest of his cashmere duffle coat far away from the broken window.

     He woke a while later on the other side of the room, prone, his nose pressed hard against the cold stone floor. It was very dark, so he activated the torch on his iPhone before standing up. His breath fogged the air, and his feet felt horribly itchy in his squelching socks. He took the socks off and stuffed them inside the loafers, which he held in his free hand. The stairs felt icy against his bare feet. Halfway down, he stepped on something gelatinous and sticky. In a surge of panic and disgust, he dropped the loafers and reached for the bannister, but his hand closed around air. He careened down the stairs, his pelvis slamming hard on the steps as he fell.

     He lay on the floor moaning softly, winded, in agony, disorientated. The iPhone lay a few metres away, the torch still switched on, an upturned spotlight in the dark room.

     Something small and sharp was stabbing into the small of his back. He reached behind and retrieved it. It was a white rook from the chess set. He threw it away, stood up, grabbed the iPhone, and pointed the torch at the nearby table. The chess pieces were scattered on the floor around it. A huge black slug had slithered onto the board and was moving ponderously across the squares.

     Seb felt a twist of anger in his gut. A slug had caused him to fall down the stairs, and now another one was ruining a beautiful chessboard. He picked up the board and shook it violently. The slug remained where it was but turned its questing antenna in his direction with apparent curiosity. Anger and disgust bubbled inside Seb — he picked up the slug and flung it away.

     The board, he now noticed, was marred by a stripe of dark red slime. Why was it red? He looked at his hand and saw that it was coated in the same glistening red substance. He tried to wipe it on his trousers, but the slime stuck like glue. Turning the iPhone torch toward the centre of the room, he saw another dozen or so large black slugs slinking across the floor, leaving trails of crimson slime in their wake.

     He moved through the slugs like a man negotiating a minefield. Despite his caution, one of them caressed his heel with its wet, engorged body. When he reached the door, he found it was locked, and the key was missing from his coat pocket. He gripped the unyielding handle tightly and tried to control his breathing.

     Back upstairs, he climbed into the recess in the wall that contained the broken window. The storm had settled down, and the breeze wafting through the broken glass was almost pleasant. Was climbing out of the window a good idea? Probably not, but it appeared to be the only escape route. No other moves were possible, and he couldn't bear to stay in this place a minute more.

     Eoin, he thought bitterly. Eoin must have known about the slug infestation and had given him this place as a prank or act of petty revenge.

     He inched through the jagged shards that stuck out of the window frame and dangled his legs over the drop. He angled the iPhone down and peered along its beam. All he could see was darkness, but he reminded himself that the White Castle wasn't very tall and the ground would be spongy after the rain.

     He began lowering himself.

     Then he saw, not so far away — the sea. Dark but shimmering slightly in the first light of dawn.

     He realised, with a cold rush of terror, that this side of the tower was right on the edge of the cliff. He reached back to grip the window frame but instead touched a sharp triangle of glass. Blood sluiced hotly down his arm. The pain didn't register. He held on.

     The piece of glass silently detached itself from the window frame. Seb slipped into the abyss, tumbling through the air, a hoarse but barely audible scream on his lips, the iPhone spinning alongside him like a tiny lighthouse.

     Five hours later, a dog walker found his body on the rocks below. The death was ruled accidental.