Ibrahim was baffled and disbelieving. 'Who?'

'Joan. Our cousin of the Outremer. I learned this through Ali Gurdu, who is working with the Christians, and functions as a conduit of information… Joan, to whom I wrote about the Engines of God, and who never replied, has now come here. It's obvious why. I told you I feared I had made her a rival. She wants the Codex for herself.'

Ibrahim remembered the almond-faced woman in the turayya. Could that truly have been his distant cousin from the Outremer, come all this way from lost Jerusalem to dying Seville? How remarkable.

'And,' his mother went on, 'do you know what Ali Gurdu tells me she has done? She has worked her way into the court of King Fernando. She may have warmed the bed of Fernando himself for all I know. And it was she who hatched the idea of forcing the evacuation of the city.'

Ibrahim was baffled. 'But why would she want this?'

'Isn't it obvious? She wants the city cleared, so the mosque will be emptied. And there, in the hours before Fernando's first mass, she will have her chance to get her hands on the engine designs. My designs.'

'That's absurd.'

'Absurd? The promise of the designs is not absurd. Joan takes it as seriously as I do. Indeed she has travelled from one end of the Mediterranean to the other for the sake of it. I need you to help me thwart her.'

Ibrahim, dismayed, looked into her face, and recognised the same sort of madness that seemed to drive the Christians, a plain, single-minded determination, uncomplicated and unsubtle and uncluttered by reflection. Her purpose had already destroyed Peter. Now the thought of scrabbling around in pursuit of this foolish dream on the floor of an abandoned mosque, in the midst of an epochal tragedy, disgusted him. 'I won't help you with this, mother. I told you. It's over.'

She nodded. 'Then what will you do?'

'As soon as this round is done I'm fetching my pack and leaving with Ibn Shaprut.'

'Then this is where we must say goodbye.'

Something inside Ibrahim broke. 'Give up this madness, Mother, I beg you. Come with us. It's a long walk to Granada. But you will have me.' He gestured at the girl. 'Obona will walk with you. Perhaps you will be able to help her with her baby.' He hoped that would touch his mother's heart.

But Subh looked down at the girl and sneered. 'If you take her you really are a fool. She doesn't need you, don't you see that? She has a pretty face. She can whore herself to the Christians as she once whored for Ali Gurdu. That's what rat-people like her do. But not us, Ibrahim. We're better than the rest. Come with me now, and we will fulfil our family's destiny together. We will change the world.'

Ibrahim looked down at the girl, who was weeping over her baby. 'You shame me, Mother. Go now.' He kept his head bowed.

When he looked up she had gone.

So King Fernando walked at last into the city he had besieged for so long. His soldiers lined the way, and cheered. But their voices were small in a city that was an echoing stone shell, and some of the soldiers looked about warily, fearful of Moorish ghosts.

It was three days before Saladin was finally allowed into the city, for the first time since he had accompanied the King's envoys during their negotiations with the emir. He was stunned by the emptiness of the place, compared to the vibrancy he had witnessed only days before. Walking through the innards of this vast stone corpse crushed his soul.

'Not bad work for a Christmas Day,' Michael said to Saladin. 'I mean, the emir's whores have all cleared off to Granada. Nothing left but old ladies and babies. But the lads say that some of those old Saracen chickens have got a bit of juice left in them, if you know what I mean. And there might be a bit of loot to be had, though we're supposed to give it all to the King to pay his mercenaries.' He sniffed, hawked up some snot, spat, and swung his arms as they walked down the empty street. 'There's nothing to buck you up quite so much as a right good sacking on Christmas Day. But still this isn't bad work, not bad at all.'

'Shut up,' said Saladin. 'Oh, shut up.'

'All right. I'm just saying-'

'I know what you're saying-'

'Saladin. Here you are.'

Saladin swung round at the woman's voice. It was his mother.

Michael bowed. 'Lady Joan.' He looked up and leered at her.

'Happy Christmas, Michael,' she said drily. She looked tense, anxious.

'What are you doing here, mother?'

'Looking for you. It took me the devil's own time to find you. That sergeant of yours barely knows where his own backside is, let alone his soldiers. Well, let's get moving. We don't have much time.' She set off down the street, without looking back.

She gave Saladin no choice. He trotted after her. Michael, grinning, followed.

'Mother – where are we going?'

'The mosque, of course. Where else? Thomas Busshe will meet us there. Time is short. The bishops are going to reconsecrate the building this morning. Then the King will hear mass in it this evening. We've only got an hour before the clerics will be swarming all over it.'

Michael asked, 'An hour to do what?'

Joan said, 'To dig up the Codex.'

Saladin had told Michael nothing about his family's strange secret from the past. But Michael picked up those words 'dig up'. 'Buried treasure, he said, his grin widening. 'Now that's what I'm talking about.'

When they reached the mosque's outer wall, they met Thomas at a gateway that led through to a broad patio where dried-up fountains stood like dead flowers. Thomas was out of breath, and looked anxious. 'This way,' he said, and he hustled them across the patio and through an arched doorway that took them into the mosque itself. 'But,' he panted, 'it isn't good news…'

The mosque was immense. Like its great sister in Cordoba, it was a complex of pillars and arches that extended off to infinity in every direction. Just days ago, this place would have been crowded with the Muslim faithful, Saladin supposed, perhaps still praying that Allah would save their city for them. Now there were only soldiers, all of them blazoned with the cross of Christ. In one comer he saw soldiers sleeping, leaning up against the wall. In another, more of them gambled with dice on the polished floor. And in the very heart of the mosque a fire had been built, right in the middle of the floor, and the soldiers were roasting a pig they had robbed from somewhere, no doubt brought here as a deliberate act of disrespect to the vanished Muslims. The smoke licked up and was blackening the fine plasterwork above.

Thomas had brought a few workers with him, off-duty soldiers standing idle with picks and shovels. 'The difficulty is,' he said, 'where are we supposed to dig?'

'I hadn't thought this far ahead,' Joan said. She strode about, looking around at the mosque. 'I think I imagined it would be obvious. That those who buried the designs would leave some clue.'

'None that is apparent,' Thomas said. 'We haven't the time to dig the whole place up, and nor would the King spare us even if we did.'

'Then what are we to do?'

'Ask me.' The woman walked towards them, out of the deeper shadows of the mosque. She wore a veil and a djellaba. She was obviously a Muslim, and had obviously been hiding. The woman removed her veil. She looked about fifty; her face was stern, determined – and familiar.

Michael made a deep growling noise. 'Now that's more like it. Old bones, but well worth jumping on, Saladin my friend, you mark my words-'

'Shut up.'

Joan said, 'I know you. You were in the meeting at the palace with the vizier's staff.'

'In the turayya, yes.' She spoke a clear but accented English.

'What do you want here?'