'But this was not the work of al-Hafredi's people.'

'I do not believe so. A different method was used to persuade the minds of men – an imp in a box rather than a human being thrown into history. And, though it is not clear, it seems that the makers of the amulet sought a different future from that described by al-Hafredi.'

Saladin struggled to absorb these dreadful ideas. He feared they were heretical, feared that even to speculate about such matters in the darkness of his own head might be to commit a sin.

But his mother briskly focused on the practicalities. 'I see your point,' she said to Thomas. 'He who sent Ogodai's imp may or may not have been our Weaver. But this does seem to prove that time can be spanned by an agent's will, be he human or divine.'

'Exactly.' Thomas's rheumy eyes were bright.

'Well, it's clear what we must do now. The veracity of the Codex is proved to be more than plausible.'

'You never wrote back to your cousin Subh?'

'I was never sure about that. After all Subh is a Muslim. Yet we need the Codex.'

'You're thinking of going to Seville yourself?' Thomas asked cautiously.

'Of course! I will dig up that mosque with my bare hands if needs must.'

'But the armies of the Castilians are moving in on the city. Soon it may be besieged.'

'All the more reason to move quickly, before some other chancer happens on the plans – or worse, Subh herself.' Her eyes were cold. 'I am sure now that this is our opportunity, our chance to revive our family's prospects. We must take it without hesitation.'

Saladin gladly put aside the strange mysteries of the ever-changing tapestry of time, and grasped the essence of this new mission. 'We are going to al-Andalus?' He bunched his fists. 'There are many Muslims there. I shall take the Cross!'

Joan stroked his cheek fondly. 'That's my boy.' She stood. 'We have much to do.' Briskly, still talking, planning, scheming, she led them out of the room.

Thomas hurried after her. 'Of course there is still the question of your enigma: Robert's scrap of cipher, which may or may not have something to do with that phrase in Subh's letter – Incendium Dei. As it happens I have heard of a young man who may be able to assist us, another Franciscan, a bright young philosopher at Oxford who is becoming notorious for his radical philosophies. His name is Roger Bacon…'

There was trouble at the pontoon bridge. A suspicious mob, a suspected spy, a near-riot – and the potential for real disaster for Seville, if its only bridge across the Guadalquivir were damaged.

This news was brought to Ibrahim's office by a sweating, panicky soldier. Ibrahim summoned Abdul, a captain of the palace guard, and told him to assemble a unit of his troops. Then he ran out of his office, without waiting to see if Abdul and his soldiers followed.

From the emir's palace, the fastest way to get to the bridge was to cut down to the river and follow the bank, and that was the way Ibrahim headed. Even so it was tough going, for every open space, every street was cluttered with refugees and their belongings. Ibrahim was forced to wade through this throng as through a sea. By the river it was almost as bad, but troops stationed here kept a path cleared along the bank, and once he was in sight of the water Ibrahim was able to make faster progress.

It was a bright spring day, he noted absently. The river water glistened prettily, and the orange trees were in bloom. But this year, the hungry would not leave the fruit on the branches long enough to ripen.

It wasn't hard to find the source of the trouble. The mob had caught their man at the abutment of the pontoon bridge. Ragged, already bleeding, his head hidden by a hood, the quarry had backed a few paces onto the bridge.

'Let me scatter them, sir.' Abdul, a veteran of the sieges of Cordoba and Valencia, was a tough, competent man of about thirty-five, who wore a black patch to hide the empty socket from which a Christian arrow had taken his right eye. 'A charge will do it.'

Ibrahim trusted Abdul with his life. But Abdul thought in a soldier's blunt, direct terms. Ibrahim's job was to see the wider picture. 'We can't risk the bridge,' he said. Seville had only two arteries to the wider Muslim world: the river, which was gradually being blockaded by King Fernando's fleet, and this pontoon bridge, which linked the city to the suburb of Triana and the Muslim communities beyond. 'You charge them and they'll be on that bridge for sure. All it will take is one of those idiots with a torch to start chucking fire around, and we're all sunk.'

Abdul pursed his lips. 'Let me guess. You want to go and talk to them.'

Ibrahim grinned at the tough soldier. But he wondered, not for the first time, how it was that he, who had always thought his destiny was to be a warrior, had finished up being a cut-price diplomat. 'Our job is to keep order, above all, captain. Let's see if we can do that without breaking any more heads.'

'And when it goes wrong, my boys will sort it all out and save your arse for you. Again. Sir.' But Abdul smiled.

'Fair enough. Wait here.'

Moving rapidly to mask his own fear, Ibrahim walked the last few paces to the bridge, and stood boldly between the mob and its prey. He glanced at the victim, who was a slim man, breathing hard, his face so bloodied it was unrecognisable.

Then Ibrahim turned to face the mob. The crowd was perhaps fifty strong, mostly men, a few women. They were all as ragged and dirty as the man they hunted. Ibrahim knew these people. Without homes, without hope, they were profoundly afraid. But fear was easier to bear if you could find somebody to hate.

He spread his hands to show he was unarmed. Abdul was watching closely, his hand on his scimitar.

Ibrahim called, 'Why are you here? Why do you keep me away from my prayers?'

There was an inchoate growl. One man waved a ragged bit of parchment in the air.

'You.' Ibrahim picked on the man and strode forward. 'Come here!'

The man instinctively stepped forward, and the crowd pressed back. Suddenly the man looked less certain, for he was once again a man, himself, and not a component of the mob.

'Tell me your name,' snapped Ibrahim. 'In me, you face the authority of the vizier. Tell me!'

'I am Gabirol,' he said reluctantly. He was probably no older than Ibrahim.

Ibrahim nodded. He turned to Abdul, who made a show of writing down the man's name. 'All right, Gabirol. My goal here is to secure the peace. That's all I care about. We can't have crowds running around with torches and knives, and we can't have citizens torn apart on our streets-'

'He's no citizen,' Gabirol snapped, and his anger surged. He waved his bit of parchment at the man on the bridge. 'He's a spy! A spy for Fernando, for the Christians.'

A city under threat was rife with rumours, riddled with imagined traitors and spies – and perhaps a few real ones. 'And how do you know that?'

'Because of this! This is what he was carrying when he was found.' He held up the parchment again.

Ibrahim took it gingerly. Streaked with blood, it was covered with sketches of what looked like fish. Perhaps they were anatomical drawings. But when he looked more closely he saw that there were bits of machinery inside each 'fish', gears and levers and pulleys, and sketches of tiny men who pulled on oars or worked at capstans.

He growled, 'Oh, Mother, what have you done?'

Abdul looked at him. 'What was that, sir?'

'Never mind. So, Gabirol, you think he's a spy because of these sketches.'

'Isn't it obvious? Everybody knows Fernando wants to block up the river. Maybe this is the way he's going to do it, with these fancy ships-'