Fittingly, Goya is triumphantly represented; and, equally fittingly, his work is housed separately from the main gallery in its own sumptuous architectural island. The visitor walks through the spectacular gallery rooms of the main part of the museum and then finally comes upon a small rotunda, swollen with Goya’s paintings. But, interestingly, even here Goya could not be penned in; and gradually the accumulation of his works spread from the rotunda downstairs, to an anthill of darker rooms on lower floors.

Scurrying down the back stairs, Leon relished the quiet of the closed gallery, the crowds of tourists all tipped out into the street, the lights dimmed, only the necessary illumination marking out his pathway. Preoccupied, he hurried on, then paused in front of the painting of The Family of Charles IV. By the time Goya was painting the royal family he had become well known, respected and acid in his judgement. Yet although highly sexed himself, the artist was – like most of his contemporaries – outraged that the plain, vain and mendacious Queen Maria Luisa had given power over to her young bedfellow, the despised Manuel Godoy. Duped, and glad to be relieved of the burden of kingship, Charles IV restricted his royal duties to asking Godoy nightly ‘whether affairs were going well or badly’.

Leon studied the familiar figures, as always in awe of Goya’s acerbic daring. The painter had held nothing in reserve. The Queen had been made plain and ridiculous, the King an idle buffoon. A noise from below jolted Leon out of his reverie. He had work to do, and he had to hurry. Admittedly the gallery was open to him at any time during the day, but at night he could only stay by special dispensation and had to leave by twelve.

Past the royal portraits Leon hurried, clutching his books, his shadow crossing the faces of The Naked Maja and The Colossus. He wasn’t interested in the earlier images, just the ones which hobgoblined their way through his dreams and trick-or-treated into his studies. Leon knew that he was taking a chance, that by cutting down on his medication he wasn’t just trying to please Gina or improve his condition. He knew exactly what his medicinal Russian roulette might mean, but cocked the gun anyway. By his reckoning he had a week, maybe two, before he would collapse and be forced back on to the drugs. He had to make sure that he found the answer in time.

Letting out a sigh of nervous excitement, Leon entered one of the rooms exhibiting Goya’s Black Paintings and then paused. In front of him hung Deaf Man, originally painted on one of the walls of the Quinta del Sordo before being transferred to the Prado. He studied the work intently: the weird blacks, ochres and malevolent whites, the scurry of paint, hurried, as though the artist’s hand was being guided. The left-hand figure in the picture was benign – an old man like a sage or a Biblical scholar – but leaning on his shoulder was a beast, half-man, half-skeleton, bald, blank-eyed, whispering into the old man’s ear. But whispering what?

‘Leon Golding?’

He spun round, almost losing his footing as a corpulent man called out from behind a pillar. Jimmy Shaw was limping slightly, his suit stained and crumpled, holding his hand across his chest, half-tucked into his jacket. His face was puffy, his eyes small in the swelling folds of flesh. He looked decayed, sick, like someone who had just stepped out of one of Goya’s pictures.

‘What …?’ Leon stared at the vision, then realised it was only a man. A sick, fat man. ‘What are you doing here? The gallery’s closed to the public.’

‘I had to talk to you,’ Shaw said, keeping to the shadows and breathing heavily with the effort. ‘I’ve been trying to talk to you for days. I called you on the phone, then lost my nerve.’ He paused, running his tongue over his bulging lips. ‘I thought I should talk to you in person about the skull …’

Immediately, Leon glanced around him.

‘There are no guards here, Mr Golding. Only the ones on night duty at Reception. I hid here when the gallery closed—’

‘What?’

‘I hid here,’ Shaw repeated. ‘I stood over there, stock still, for nearly a fucking hour. You’re late tonight. I didn’t think I could stand so still for so long.’

Nervous, Leon stepped back. ‘I don’t know what you want—’

‘The skull. The Goya skull.’

‘I don’t have it!’

‘Yes, you do.’

‘I don’t!’ Leon replied, his tone shrill. ‘I don’t know what you’re talking about.’

‘I’m not going to hurt you,’ Shaw went on, his speech muffled until he cleared his throat. He was dying – anyone could see that. He could see that. He just had to get the skull, get it to Dwappa, and he’d be all right. ‘Give me the skull. I’ll buy it from you.’

‘I told you, I don’t have it—’

‘I’ll give you a good price,’ Shaw said, stepping out of the shadows into the full overhead light. His face was bloated, red weals under the eyes, his hand double-bandaged, although there was still a slight stench coming through the dressing.

Horrified, Leon stepped back again. ‘You’re ill—’

‘Yeah, and I won’t get better until I get the skull,’ Shaw said earnestly. ‘Listen to me, Mr Golding. You’re going to get into deep trouble. Real trouble. There are people worse than me after that skull. One man in particular, he wants it. He’s got a buyer for the skull. He hired me to get it for him and he won’t rest until it’s in his hands. You’ve got to listen to me—’

He reached out and Leon stepped further back.

‘I’m trying to help you! Keep that fucking skull and you’ll end up like me. Worse.’ He sighed raggedly. ‘What d’you want for it?’

Leon stood mute. He was terrified of the man in front of him, but he wasn’t about to give up the skull. Around them the Black Paintings hummed under the lights and the fat man leaned against the pillar again.

‘I could kill you—’

‘What?’

‘But what would be the point? You don’t have the skull on you.’ Shaw laughed shortly. ‘So name your price.’

‘The skull I had turned out to be a fake.’

‘Oh, I heard you tell Gabino Ortega that. He didn’t believe it either. I suppose Ortega wants it for his brother.’ Shaw sighed again. ‘Yes, I’ve been watching you, Mr Golding. I’ve seen who you’ve talked to. We’re all watching each other.’ He smiled, the oily skin of his cheeks creasing. ‘Give up the skull, otherwise you’ll regret it.’

‘I tell you, I don’t have it!’

Shaw bowed his head for a moment. ‘I’m not used to all this, you know. Usually I have minions doing the dirty work … And that’s what it is.’

Leon frowned. ‘What?’

‘Dirty work,’ Shaw explained. ‘It’s very dirty work. And I’m stuck in it. Stuck tight. I can’t get out of it, but you can. Just give up the skull and you’ll be safe. I don’t want it for the Ortega brothers. I’ve told you, I want it for someone else entirely. Someone much, much more dangerous. The Ortegas have money, but the African …’ He passed Leon a piece of paper. ‘That’s my number. Call me when you want to meet up. Bring the skull and I’ll pay you what you want.’

‘But—’

‘I’m dying, Mr Golding,’ Shaw said helplessly. ‘If I don’t get that skull I’ll be dead soon. You want that on your conscience?’ He stared at Leon. ‘I don’t think you could manage that – you’re not that kind of man.’ He tried to shrug, but winced. ‘It’s just a skull. It’s just the head of a dead man … I’m asking you, begging you. What’s it worth? I want a dead man’s head to save a life. My life. And I don’t expect any favours, I’ll pay you—’

‘It’s not the money.’

Shaw shook his head incredulously. ‘It’s always the money, Mr Golding.’

‘You don’t understand—’

‘No, you don’t understand. Because if you did – if you realised what that skull means – you’d have got rid of it already. And if you knew what’s coming to you, you’d sell it to me now. You’d get the fucking thing off your hands and keep yourself safe.’ He stared at Leon intently. ‘Call me. I’ll come and meet you whenever, wherever, you want. Just make it quick, for all our sakes. I’m trying to save you, Mr Golding. Please, save me.’