So Subh, descendant of a vizier, abandoned Cordoba, once the capital of a caliphate.

When Peter crossed the city on the day of her departure, the air was already hot, the sun intense, even so early in the morning. It had been late spring when Peter had arrived in Cordoba, with his mixture of hope and devastating bad news for Subh. Now it was midsummer and the fresh greenness had burned away, leaving the city parched and dusty, the blossom fallen, the patio gardens weary.

At the house Subh had already flung open the gates. Goods were heaped up in the narrow road: bags and packs and rolled-up carpets and wall hangings, even pot plants from the patio. Subh's household, with the usual gaggle of relatives, milled around. It was a day of defeat for Subh, of course. But she seemed as serene as always as she glided through the crowd, resolving disputes, solving problems, managing this last project in Cordoba as efficiently as she had handled all the other details of her life.

And as Subh supervised the abandonment of her home, Alfonso 'the Fat' and his ratty little granddaughter stood and watched. Alfonso didn't try to hide the look of triumph on his face.

The fleecing of departing Moorish refugees had become something of an industry in the conquered city. There seemed to be endless tithes to pay before you could get one mule-load of goods outside the walls. And Christians, never Muslims, were encouraged to buy up abandoned businesses and properties, usually at prices ruinous to the Muslim owners.

Even so Peter had been surprised when Subh sold her property to Alfonso, her rival.

But she had told Peter she was glad to do it. 'Alfonso was so determined to push my face into the dirt that he outbid everybody else and paid too much. Not as much as I'd have made if I'd sold up ten or fifteen years ago, before the siege, but far more than I expected. So let him have his victory; let him watch me walk away, no doubt fiddling with himself under that ugly cloak in his excitement. I'll take the fat fool's money.'

Muleteers drove their animals into the street to join the chaos, and gradually the backs of the patient beasts were loaded up. Peter, in his travelling clothes and carrying his pack, tried to master the mule he had to ride. It was a surly, truculent slab of muscle with sharp bristly fur and a stink of dried dung, and it was resolutely uninterested in Peter's plans for it.

Subh said to him, 'You don't have to come, you know. After all we are travelling out of the Christian territories. You could return to Toledo, and burrow back into your libraries like a worm into a book. If you decide to come with us you will leave behind everything you know, everything that is familiar.'

'I don't really make decisions,' he confessed. 'Not in that way. I do things step by step, depending on what seems right at the time. I left my birthplace near Bath to go to Oxford to study, and then on to London. And then I travelled to Toledo, where every scholar in Christendom wants to be for its translation schools. Then, when under your sponsorship I unearthed the story of your family's past, I felt I had to travel to Cordoba to meet you in person. And so on.'

She waved a hand. 'This is how the young plot their lives. You think you will live for ever; you think the future is full of endless possibility. So you follow impulses. You aren't old enough yet to understand that each choice you make in life in fact shuts as many doors for you as it opens.'

He felt put down by that; he had expected more gratitude, perhaps, for his loyalty. 'Well, lady, I have come across a trail of unanswered questions that I feel it's my duty to follow, regardless of where it may lead. That is not impulse, it's scholarship. And it is my instinct too,' he said boldly, 'to be at your side in the coming adventure.'

She looked him over. 'So sweet,' she murmured. And she cupped his cheek.

His flesh burned where she had touched it. But she had patted him like a child, not a lover. He turned to his mule, which seemed to look at him with sympathy, though it still resisted Peter's every attempt to climb on its back.

The caravan set off. There wasn't a single wheeled vehicle; everything and everybody was loaded on the back of a mule or a horse or a camel – even the magnificent Subh, who rode a delicate palfrey as if she had been born in the saddle.

They bumped their way out of Cordoba, met up with more mules and their drivers, and the caravan formed up for its passage along the banks of the Guadalquivir south-west to Seville – Ishbiliya – for the great river passed through both cities. The muleteers took over now, weather-beaten men with ragged clothes and faces like leather. They walked beside their lead mules, and the chiming of the bells around the animals' necks rose up into the dense, still air.

Peter looked back at the city as it receded, wondering if he would come this way again. Its walls were battered after the long months of siege, and the flags of Christ fluttered over its gates. But still the Roman bridge arched handsomely over the glimmering river, and when the muezzin calls sounded all the party paused to turn to the east and pray, all save Peter himself, who contented himself with the devotions of his childhood.

The maps which Thomas Busshe had studied in the monastery all showed the Holy Land as the centre of the world. But he felt as if his extraordinary journey took him, not to the centre, but to the very edge of reality.

Even to cross the water to France was gruelling for a man who had sailed nothing more ambitious than a leather-stitched rowboat across the Thames. Then came the slog through the splintered kingdoms of France to embark again at Marseilles, and a sea journey ever further eastward across a hot, flat Mediterranean which he knew to be largely a Muslim lake. He followed his maps as he passed along the coastline owned by the East Romans, Christians who did not bow to the Pope, and whose ancient city of Constantinople was now, shamefully, in the hands of the crusaders who had sacked it. But as he ploughed ever further east there was only the huge mass of the Sultanate of Rum to the north, under the Turks who had taken Asia Minor from the East Romans, and the Fatimid Caliphate to the south, where the crescent of Muhammad fluttered over the cities of the Nile. Great sprawls on the map these were, like enormous Muslim hands ready to crush his frail ship like a fly.

In Palestine, true, there was the Outremer, his destination, the remnants of the Christian kingdoms planted bravely by the soldiers of the First Crusade. But these domains were shrunken now, split up and reduced to fragments by the dramatic conquests of Saladin half a century before. Even Jerusalem itself was only nominally in the hands of Christians. Seeing these little islands of the faithful on his maps served only to convince Thomas Busshe that despite the Pope's ardent preaching, echoed in every pulpit in western Christendom, two centuries of crusading had resulted in little solid achievement, indeed perhaps the very opposite.

But that could all change.

Thomas, over fifty years old, was no warrior himself. But he had formulated for himself a mission that he believed might yet reverse the fortunes of Christendom, a mission inspired by a relic of the past that had come swimming fortuitously to him out of the dark, like the finger-bone of a saint emerging from the muck of a drained pond. A gift that, if he used his intellect well, might yet win the epochal war of civilisations for Christ.

And so he drove on, determined, clinging to the ship's rail and trying not to vomit.

The climax of his extraordinary journey was at its very end. He landed at Jaffa, once more a Muslim city, and submitted himself to the ordeal of a trek across the dusty land to Jerusalem. And he met Joan and her son Saladin before the walls of the city itself.