Orm and Sihtric sat on floor cushions inside the priest's study, as he called it, in a corner of the palace complex. It was a nest of shelves heaped with books and parchments, and there was a lingering smell of lamp oil and candle soot. The room was in poor condition, the ceiling blackened by some ancient fire, the wall hangings musty and frayed; this part of the ruined palace was only poorly restored. But the room was far from the bustle of the vizier's court, and Sihtric said he liked it this way.

'For I have secrets in this room,' said Sihtric. 'Secrets I've shared with no one – certainly not the vizier. You want to know why I stay here, why I live among Moors who speak of Allah, and Christians who speak no Latin? Why I have let myself become locked into a damaging relationship with a snake like the vizier…' He glanced upwards to the darkened ceiling, as if challenging God. 'You see, Orm, I've found a rent in the tapestry of time. Another one, a third or a fourth, to add to the ripping-open of the Menologium of Isolde, and the Codex, and your poor wife's Testament. And through that rent I have glimpsed horror. But from that horror I have conceived an ambition as big as the world, Orm. It is nothing less than the final defeat of Islam, and the preservation of Christendom into the far future. What higher goal can there be than that? Is a man justified in giving up his very soul to achieve such an aim?'

A month had passed since the incident of Ghalib and the waterwheel. A month in which Orm had continued to learn uncomfortable details of Sihtric's murky career in Cordoba. And now, he said, Sihtric was going to tell him the whole truth. Orm wasn't sure he wanted to hear it. He shivered, obscurely frightened. 'You always did talk in riddles, priest.'

'Well, the whole business is a riddle, isn't it? But then it always was.'

Sihtric got to his feet and crossed to the wall. He pulled away a crate stacked with books, then hauled aside a shabby hanging to expose tiles with geometric patterns, a kind of trefoil in black and white that, repeated, covered the wall area. Sihtric dug his fingers into the edge of a tile and with some effort picked it away from the wall. 'I have a habit of biting my nails,' he said. 'Makes this tricky.' A hatch, concealed by tiles, now hinged down to reveal an iron door. Sihtric extracted a key from his robe, unlocked the door, and it swung back to reveal a compartment inside the wall. Sihtric began to rummage in the dark space, which Orm saw was full of books, scrolls, heaps of parchment and vellum. There was a musty smell, of rot and age.

Sihtric drew a flat wooden box from the wall compartment. He placed this on a table, unpicked ties of copper wire, and opened up the box like the covers of a book. Leather hinges creaked slightly, and a smell like stale meat flooded the room.

Inside the box was a wooden frame over which was stretched a sheet of what looked like vellum. Orm peered closely. Words had been marked onto the vellum, pricked in some black ink. The small, closely spaced letters were lined up in neat rows, but had been distorted by the stretching of the vellum, and in places the skin was pocked and broken, crudely stitched. There was nothing else in the box.

With faint dread Orm reached out and touched the vellum. It was dry, rough. 'What is this, calfskin?'

Sihtric would not say. 'When I found this object it was rolled up inside a wooden cylinder, for it had been preserved as a holy relic. It is old, three centuries or more.'

'So you stretched it on this frame.'

'With infinite care, yes. But I couldn't help a little distortion of the letters.'

Orm looked closely at the first few lines. 'Is this Latin? "My name is al-Hafredi, as the scribes tell it, and Alfred, as my family knew me. That liveth, was dead, evermore…" I don't understand. A riddle? The name, though. Al-Hafredi is a Moorish name. But Alfred-'

'English, of course. The name of our greatest king.'

'Here is a man who lived under the Moors, then,' Orm said. 'His name, Alfred, was rendered in a Moorish way. Just as our guide Ibn Hafsun's name was a corruption of the old family name of Alfonso.'

'You should have been a scholar.'

'Don't mock me,' Orm said mildly. 'However my scholarship doesn't extend to puzzling out the rest. "That liveth, was dead, evermore." It's not even a sentence. What does it mean?'

'There lies the cunning. The manuscript isn't in any kind of code. But it does contain fragments like this. They puzzled me too, until I saw that the intention of this man, a Christian living under the Moors, was to speak clearly to other Christians, in a way that his Moorish masters would not understand. So, pagan, what piece of literature do all Christians share?'

'The Bible.'

'Correct. And I realised that what we have in this line is a fragment of the Bible, a quotation, compressed and embedded.'

'What quotation?'

'From the Book of the Revelation of Saint John.' He closed his eyes. '"I am he that liveth, and was dead; and behold, I am alive for evermore." Liveth, dead, evermore. You see?'

'If you say so.'

'The quotations come from both the Old and New Testaments. With patience I riddled them all out. It would take a Christian to do so. The Muslims do study the Bible, you know; they call it the "Holy Book". However no Muslim scholar will know the Bible as intimately as a Christian. Of course the quotations are allusive rather than literal. Puzzling out the story al-Hafredi wished to tell; that was the real challenge.'

'But you did it.'

'Oh, yes. Given time.'

'Then tell me what you learned.'

'Ironically it was an assignment by the vizier himself that set me on this course…'

After the fitnah the taifas, coalescing and competing, had struggled to develop scholarship as a way of achieving dominance over each other; there had even been something of a renaissance as the monopolistic power of a corrupt caliphate collapsed. Seville was not to fall behind. Ibn Tufayl had the backing of his emir, and the funds to progress scholarly projects.

'Among other things, Ibn Tufayl decided he wanted a new history of al-Andalus to be written, from the day three centuries past when Tariq led his armies of Arabs and Berbers across the strait, up to the present. It would be the first significant survey for more than a century, since the time of one Ahmad ar-Razi.'

'And he commissioned you to do that.'

'It was part of the condition of his funding my work on the Engines of God. I think it amused Ibn Tufayl to have a Christian working on a history of the greatest enemy of Christendom itself. But I was happy enough to take the job, as it was an excuse for me to burrow into the mountain of scholarship the Muslims accumulated over the centuries of the caliphate. Knowledge, Orm, knowledge, the greatest power of all. You never know where it might lead! And so the vizier and I came to a mutually satisfactory arrangement.

'The work went smoothly enough. But I soon became aware of a great mystery.'

'What mystery?'

'Why it is that I was born a Christian and not a Muslim,' Sihtric said. 'Why I grew up, in England, speaking English. Why Christianity survives at all.'