‘We need a boat,’ he said, as he sketched his map.

Peter shook his head. ‘People built this?’ he said again. He found wonder in everything – the grafitti, the underground mosaics, the bronze fittings where no one could see them. ‘No one is this rich.’

‘The old Romans were this rich,’ Swan said.

‘Imagine fresh water in every house,’ Peter said.

‘We need a boat,’ Swan insisted.

‘I’ll just steal one on the waterfront and carry it through the streets, shall I?’ Peter asked.

Swan stopped drawing, the charcoal pinched in his fingers. ‘Mary and Joseph,’ he said. ‘There must be a water gate.’

Peter’s head came up. He grinned.

‘I know who can get us a boat,’ Swan said. ‘Let’s cast east.’

It took the rest of the day, but they found that the eastern branch of the sewer did indeed run down all the way to the sea. It ended at a grate like a portcullis, strong iron carefully wrought. The water ran out into the sea.

Swan’s legs hurt from climbing and crawling, but he looked at the sea with infinite satisfaction. ‘Thalatta, Thalatta,’ he said.

‘What’s that?’ Peter asked.

‘That’s our way out, my friend.’ Swan watched for a while, and began to search around the water gate for signs of use.

There were several.

Very cautiously indeed, he pushed against the great iron gate.

He found scratches on the floor that proved it had been opened. Repeatedly, and recently.

He climbed up the rough stone inside the gate, and near the top he found the simple bolt that held it fast. He released it, felt the heavy iron start to swing, and shoved it back with his shoulder, almost losing his grip. He put the bolt back and dropped to the walkway.

‘We can open it whenever we want from inside,’ he said. He pointed out the headland opposite. ‘We’re south of Galata. Look at the current.’

Peter nodded.

‘Our galley can drop down on the tide – and pass within a stone’s throw of right here.’ He nodded to himself. ‘Nightfall or daybreak would be best.’

Peter rubbed his beard. ‘It would be all or nothing,’ he said. ‘If the galley misses the boat—’

‘The boat is swept away on the current never to be seen again.’ Swan grinned.

Peter shook his head. ‘I pity the poor bastards in the boat.’

‘Save your pity,’ Swan said. ‘You’ll be with them.’

That evening, they climbed the ladder up the well-shaft into what they believed to be Cardinal Bessarion’s Constantinople house. Despite his meticulous scouting, Swan’s heart beat like an armourer’s hammer smoothing metal as he climbed the ladder as far as it would go. Then he threw the rope with a grapnel. It went up, and then it came down, and nearly hit him on the head.

‘Damn,’ Swan said.

Peter nodded. ‘I’ll just climb down and wait for you to do this on your own,’ he said.

Swan waited for the archer to climb down. Then he tossed the grapnel as high as he dared, and covered his head.

Nothing happened.

Head still covered, he tugged the rope.

It seemed to have caught.

Suddenly his whole plan for climbing out of the well seemed very, very foolish.

He climbed the rope anyway. He tied a second rope to the ladder, hoping it would break his fall.

And then he was in. He could smell old incense, and there was enough light in the sky to see that he was in a kitchen, and that someone had opened the grain pithoi set into the floor.

Enough light to see the row of palettes where people slept. Kitchen slaves, perhaps.

And enough light to see the sword, held at eye height, pointed at his face.

On the positive side, it was a European sword, and the man behind it looked Greek.

It’s not always easy to take note of a man’s appearance when he’s looking at you over a sword, but the Greek was very handsome, with a small pointed beard and moustache, excellent skin and a strong chin. He was heavily muscled, like an athlete or a rower.

‘I’m from Cardinal Bessarion,’ Swan said.

The young man – he was no older than Swan – breathed out. ‘Christos Anesti,’ he said. ‘Christ is Risen.’ He looked at Swan. ‘We’ve been waiting.’

Swan lost a few hundred heartbeats when he saw how precariously his grapnel had grabbed the very edge of the well cover. He vowed never, ever to do such a foolish thing again.

Peter came up the rope.

The young man’s name was Apollinaris. He spoke perfect Italian. ‘I work for the cardinal,’ he said proudly.

‘Are you his steward?’ Swan asked.

‘I’m a philosopher,’ the young man said. ‘Sometimes an actor.’ He frowned. ‘Sometimes I steal secrets. And I’m an astrologer. And a hermeticist.’

Swan looked the young man over. ‘Are you alone here?’

‘No,’ Apollinaris said. ‘My whole troupe is here.’

‘Troupe?’ Swan felt as if he was missing something.

‘We’re mimes. We perform mimes, and ancient plays.’ Apollinaris shook his head. ‘You are a barbarian, I see.’

Peter’s head emerged from the well.

‘This is Peter – my . . . friend. Peter, this young man is Apollinaris. He says he is . . .’ Swan hesitated. ‘A philosopher. And the leader of a troupe of actors.’

‘Good Christ,’ Peter said.

‘I’m not the leader,’ Apollinaris said. ‘Nikephorus is the leader.’

‘I see,’ Swan said. The young man was on edge, and Swan had the oddest feeling that the young man was an escaped lunatic. He seemed to bounce slightly on the balls of his feet, as if overfilled with spirit.

Apollinaris leaned over the well. ‘Did you really come from the sewers? I always meant to explore them.’

Swan shrugged. ‘Am I right in assuming you need to get – er – out of Constantinople?’ he asked.

Apollinaris nodded. ‘Cardinal Bessarion sent a coded message and said he was sending someone to pick us up,’ he said. He sagged. ‘But that was months ago.’

Swan delivered a long string of obscenities. Peter arched an eyebrow.

‘Of course he didn’t tell us. What else could His Eminence do? What you don’t know, you can’t reveal.’ The Fleming sounded vaguely envious.

‘Books, he said. Relics. The head of Saint George.’ Swan all but spat. ‘A troupe of actors.’

‘You know about the head?’ asked the young man. ‘We have it.’

Swan crossed himself, something he very rarely did. ‘You . . . have it?’

‘Yes. We stole it. From the Turks.’ Apollinaris seemed very matter-of-fact about the whole thing.

He led them down a hall, and up a servant’s stair. At the top, he knocked softly at a pair of double doors. They opened.

Inside stood an enormous man with a cocked crossbow, a normally sized older man with another, and two women with the muscles of dancers, wearing men’s clothing, and with Turkish bows.

‘They’re from Cardinal Bessarion,’ Apollinaris said.

The room had pigeonholes in the walls, from floor to fifteen-foot ceilings, and every pigeonhole was filled with scrolls. Scrolls lay on the floor, and more were in baskets by the chairs.

In the middle of the room was a vast table, and in the centre of the table sat a reliquary slightly smaller than a man’s helmet. It looked to be made of solid gold, studded with pearls, enamel work and jewels.

Swan took it all in.

The crossbows didn’t waver. ‘Prove it,’ said the big man, in Greek.

‘How?’ Swan asked.

The man looked confused.

‘Look, I’ve come a long way. I thought I was coming for some books, but it appears I’ve been sent to get you lot. I have an escape plan, and all I need is a boat. If you don’t want to come, that’s fine.’ The whole time Swan was speaking he was looking at the reliquary.

It was . . . incredible.

First, he wasn’t sure he’d ever seen so much gold in one place at one time.

Secondly, the workmanship was . . . exquisite. Divine. Amazing.

Thirdly, it was covered – almost vulgarly so – in jewels. Swan wasn’t a jeweller, but he was pretty sure he was looking at diamonds. And rubies.