The hammering on the door shot him into wakefulness like a handgun going off in his face. The young man scrambled for the dagger under his pillow, stumbling to his bare feet on the icy floor of the stone cell. He had been dreaming of his parents, of his old home, and he gritted his teeth against the usual wrench of longing for everything he had lost: the farmhouse, his mother, the old life.

The thunderous banging sounded again, and he held the dagger behind his back as he unbolted the door and cautiously opened it a crack. A dark-hooded figure stood outside, flanked by two heavy-set men, each carrying a burning torch. One of them raised his torch so the light fell on the slight dark-haired youth, naked to the waist, wearing only breeches, his hazel eyes blinking under a fringe of dark hair. He was about seventeen, with a face as sweet as a boy, but with the body of a young man forged by hard work.

‘Luca Vero?’

‘Yes.’

‘You are to come with me.’

They saw him hesitate. ‘Don’t be a fool. There are three of us and only one of you and the dagger you’re hiding behind your back won’t stop us.’

‘It’s an order,’ the other man said roughly. ‘Not a request. And you are sworn to obedience.’

Luca had sworn obedience to his monastery, not to these strangers, but he had been expelled from there and now it seemed he must obey anyone who shouted a command. He turned to the bed, sat to pull on his boots, slipping the dagger into a scabbard hidden inside the soft leather, pulled on a linen shirt, and then threw his ragged woollen cape around his shoulders.

‘Who are you?’ he asked, coming unwillingly to the door.

The man made no answer, but simply turned and led the way, as the two guards waited in the corridor for Luca to come out of his cell and follow.

‘Where are you taking me?’

The two guards fell in behind him without answering. Luca wanted to ask if he was under arrest, if he was being marched to a summary execution, but he did not dare. He was fearful of the very question, he acknowledged to himself that he was terrified of the answer. He could feel himself sweating with fear under his woollen cape, though the air was icy and the stone walls were cold and damp.

He knew that he was in the most serious trouble of his young life. Only yesterday four dark-hooded men had taken him from his monastery and brought him here, to this prison, without a word of explanation. He did not know where he was, or who was holding him. He did not know what charge he might face. He did not know what the punishment might be. He did not know if he was going to be beaten, tortured or killed.

‘I insist on seeing a priest, I wish to confess . . .’ he said.

They paid no attention to him at all, but pressed him on, down the narrow stone-flagged gallery. It was silent, with the closed doors of cells on either side. He could not tell if it was a prison or a monastery, it was so cold and quiet. It was just after midnight and the place was in darkness and utterly still. Luca’s guides made no noise as they walked along the gallery, down the stone steps, through a great hall, and then down a little spiral staircase, into a darkness that grew more and more black as the air grew more and more cold.

‘I demand to know where you are taking me,’ Luca insisted, but his voice shook with fear.

No-one answered him; but the guard behind him closed up a little.

At the bottom of the steps, Luca could just see a small arched doorway and a heavy wooden door. The leading man opened it with a key from his pocket and gestured that Luca should go through. When he hesitated, the guard behind him simply moved closer until the menacing bulk of his body pressed Luca onwards.

‘I insist . . .’ Luca breathed.

A hard shove thrust him through the doorway and he gasped as he found himself flung to the very edge of a high narrow quay, a boat rocking in the river a long way below, the far bank a dark blur in the distance. Luca flinched back from the brink. He had a sudden dizzying sense that they would be as willing to throw him over, onto the rocks below, as to take him down the steep stairs to the boat.

The first man went light-footed down the wet steps, stepped into the boat and said one word to the boatman who stood in the stern, holding the vessel against the current with the deft movements of a single oar. Then he looked back up to the handsome white-faced young man.

‘Come,’ he ordered.

Luca could do nothing else. He followed the man down the greasy steps, clambered into the boat and seated himself in the prow. The boatman did not wait for the guards but turned his craft into the middle of the river and let the current sweep them around the city wall. Luca glanced down into the dark water. If he were to fling himself over the side of the boat he would be swept downstream – he might be able to swim with the current and make it to the other side and get away. But the water was flowing so fast he thought he was more likely to drown, if they did not come after him in the boat and knock him senseless with the oar.

‘My lord,’ he said, trying for dignity. ‘May I ask you now where we are going?’

‘You’ll know soon enough,’ came the terse reply. The river ran like a wide moat around the tall walls of the city of Rome. The boatman kept the little craft close to the lee of the walls, hidden from the sentries above, then Luca saw ahead of them the looming shape of a stone bridge and, just before it, a grille set in an arched stone doorway of the wall. As the boat nosed inwards, the grille slipped noiselessly up and, with one practised push of the oar, they shot inside, into a torch-lit cellar.

With a deep lurch of fear Luca wished that he had taken his chance with the river. There were half a dozen grim-faced men waiting for him and, as the boatman held a well-worn ring on the wall to steady the craft, they reached down and hauled Luca out of the boat, to push him down a narrow corridor. Luca felt, rather than saw, thick stone walls on either side, smooth wooden floorboards underfoot, heard his own breathing, ragged with fear, then they paused before a heavy wooden door, struck it with a single knock and waited.

A voice from inside the room said ‘Come!’ and the guard swung the door open and thrust Luca inside. Luca stood, heart pounding, blinking at the sudden brightness of dozens of wax candles, and heard the door close silently behind him.

A solitary man was sitting at a table, papers before him. He wore a robe of rich velvet in so dark a blue that it appeared almost black, the hood completely concealing his face from Luca, who stood before the table and swallowed down his fear. Whatever happened, he decided, he was not going to beg for his life. Somehow, he would find the courage to face whatever was coming. He would not shame himself, nor his tough stoical father, by whimpering like a girl.

‘You will be wondering why you are here, where you are, and who I am,’ the man said. ‘I will tell you these things. But, first, you must answer me everything that I ask. Do you understand?’

Luca nodded.

‘You must not lie to me. Your life hangs in the balance here, and you cannot guess what answers I would prefer. Be sure to tell the truth: you would be a fool to die for a lie.’

Luca tried to nod but found he was shaking.

‘You are Luca Vero, a novice priest at the monastery of St Xavier, having joined the monastery when you were a boy of eleven? You have been an orphan for the last three years, since your parents died when you were fourteen?’

‘My parents disappeared,’ Luca said. He cleared his tight throat. ‘They may not be dead. They were captured by an Ottoman raid but nobody saw them killed. Nobody knows where they are now; but they may very well be alive.’

The Inquisitor made a minute note on a piece of paper before him. Luca watched the tip of the black feather as the quill moved across the page. ‘You hope,’ the man said briefly. ‘You hope that they are alive and will come back to you.’ He spoke as if hope was the greatest folly.

‘I do.’

‘Raised by the brothers, sworn to join their holy order, yet you went to your confessor, and then to the abbot, and told them that the relic that they keep at the monastery, a nail from the true cross, was a fake.’

The monotone voice was accusation enough. Luca knew this was a citation of his heresy. He knew also, that the only punishment for heresy was death.

‘I didn’t mean . . .’

‘Why did you say the relic was a fake?’

Luca looked down at his boots, at the dark wooden floor, at the heavy table, at the lime washed walls – anywhere but at the shadowy face of the softly spoken questioner. ‘I will beg the abbot’s pardon and do penance,’ he said. ‘I didn’t mean heresy. Before God, I am no heretic. I meant no wrong.’

‘I shall be the judge if you are a heretic, and I have seen younger men than you, who have done and said less than you, crying on the rack for mercy, as their joints pop from their sockets. I have heard better men than you begging for the stake, longing for death as their only release from pain.’

Luca shook his head at the thought of the Inquisition, which could order this fate for him and see it done, and think it to the glory of God. He dared to say nothing more.

‘Why did you say the relic was a fake?’

‘I did not mean . . .’

‘Why?’

‘It is a piece of a nail about three inches long, and a quarter of an inch wide,’ Luca said unwillingly. ‘You can see it, though it is now mounted in gold and covered with jewels. But you can still see the size of it.’

The Inquisitor nodded. ‘So?’

‘The abbey of St Peter has a nail from the true cross. So does the abbey of St Joseph. I looked in the monastery library to see if there were any others, and there are about four hundred nails in Italy alone, more in France, more in Spain, more in England.’