'To meet you. You are here because of me.' The woman smiled, but it was a stern, chilling expression. 'I wrote to you, many years ago. I told you of the existence of the Codex of the Engines of God. I told you where the designs were buried. I hoped for cooperation. We are cousins.'

'You are Subh of Cordoba.'

'And you are Joan of the Outremer.'

The women faced each other. Saladin had rarely sensed such tension between two human beings, even in combat.

'You should have fled with the others,' Saladin said to Subh. 'You must know you have put your life at stake by staying here.'

Joan introduced him. 'This is my son, Saladin.'

'Another relative.' Subh smiled at Saladin, and turned back to Joan. 'I would not leave without what is mine,' she said. 'No – ours.'

Joan said, 'If the Codex exists at all, it is lost under this ocean of flooring.'

'Not lost,' said Subh. 'I know where it is. Precisely.' And she told them of a memoir left by Ibn Hafsun, the muwallad who, more than a century ago, had been the man actually to bury the Codex at this site. 'I had a scholar, Peter, who analysed his record and calculated precisely where under the mosque the cache must be. I had hoped we could work together,' she said. 'That is why I wrote to you. In trust. With the two scraps of knowledge that have come down to us, the engine designs and the enigma of the Incendium Dei-'

'Tell me where the Codex is.'

'Not until we discuss terms.'

Joan laughed in her face. She turned to Michael. 'Hold her.'

Michael drew his sword. He stepped forward and took Subh's arm. Thomas flinched, upset by the bit of violence.

Subh's face was coldly furious. 'I came here for a civil negotiation.'

Joan said, 'I will not bargain with a defeated Moor.'

'You will not find the designs without me.'

'Oh, I will. Perhaps not today. Perhaps not for a year. But with time, there will be a way. Do you doubt that? The only question is, will you cooperate? For, you see, the only chance you have to gain anything from this situation is if you tell me what I wish to know.'

Saladin put a hand on her arm. 'Mother-'

But Joan shook him off.

Subh was beaten, but she was unafraid. 'Very well.' She glared at Michael until he let her go. Then she turned and pointed to the bonfire blazing in the middle of the mosque. 'There. About six paces beyond that bit of arson by your brutish soldiers.'

'Good,' said Joan. 'Let's get on with it.'

Subh stood before her. 'And what of me?'

'What of you? You are Muslim, I am Christian. All over the world, we are at war. And the engine designs are the spoils of war. I told you whatever was necessary to take those spoils.'

Subh stared back at her. 'So you betray me.' She wasn't begging, Saladin saw with grudging admiration. Unarmed, alone, surrounded, she was thinking, trying to find a way into Joan's soul. 'Cousin. We have different faiths. But we are family, you and I – and Saladin. I have a son too, called Ibrahim.'

'I am not like you,' Joan said coldly.

Subh insisted, 'We are the same blood. With roots in this very country, where once a foolish boy called Robert met a girl called Moraima. Is that not a deeper unity than anything else, even than differences in faith? Must it go on and on, Joan, Christian against Muslim, century after century as it has already for half a millennium in Spain, until none of us are left alive?' And she reached out a hand.

Joan pulled back, her face twisted with fury. 'Don't touch me, you witch. You talk to me of blood? Your kind forced me from my home, from Jerusalem. Do you imagine I will forgive that? Do you imagine I will rest until I see the land of Christ restored to Christian hands? Take her out of here,' Joan said to Michael. 'Hand her over to your sergeant, or-'

Without warning Subh let out an extraordinary animal howl. She leapt at Joan and dug fingers arched like claws into her face. Joan screamed and fell backward.

Saladin and Michael rushed forward. They grabbed Subh and hauled her off Joan, but with difficulty, for she was a heavy woman animated by utter rage. At last Michael got his arms clasped around her, pinning her hands to her body.

Joan would have attacked Subh in turn, had not Saladin held her back. Her face was streaming with blood from gouges under her eyes. 'Look at me. Look at me! I'm lucky she didn't take out an eye.'

Michael called, 'What do you want me to do with her, lady?'

Saladin said quickly, 'Mother, she is our cousin.'

'She's a boil that needs to be lanced. Take her,' she said to Michael. 'Do what you want with her, you and your lads. Then throw her out of the city, naked.'

Subh struggled, but Michael grinned and hauled her away to the squad of soldiers playing dice. They laid their hands on her, and pushed her down on the floor of the mosque.

Thomas was ashen. But his eyes were alive with anticipation. 'The Codex. Come. We may not have much time.' He led the way back to the bonfire.

Joan dabbed her torn face with a bit of cloth.

Saladin said, 'You need to find a doctor.'

'Oh, stop your fussing, boy.'

'Did you mean what you said? About the designs – the weapons, and using them to take the Holy Land?'

'Of course I did. It is the most sacred place in the world, Saladin. And our home. We will use the engines for the purpose for which they were intended.'

As a boy in Jerusalem, a 'warrior cub' as Thomas had once called him, Saladin would have applauded such an ambition. But he had grown since those days, grown into a man of twenty-one who had seen much more of the world – and had seen much suffering. 'Mother, are you sure that was the intention? And are you sure that's what we should do?'

'What do you mean?'

'Remember the Dove.' He meant his family's other prophecy, the Testament of Eadgyth, handed down since the days of Robert and his father Orm, a commandment that seemed to warn against the use of Sihtric's Engines of God.

'Gibberish,' she said. 'Meaningless. I care nothing for prophecies. All I care about is acquiring the power to achieve my goals. All I care about is getting hold of those designs, and building the weapons, and turning them on Muslim flesh.'

Saladin heard screams, and the angry shouts of the men. Subh was putting up a fight. 'And what of your cousin?'

'She deserves what's coming to her,' Joan spat. 'I hope they split her open.'

Saladin decided in that moment that he would not follow his mother, not any more, not after this. He would fight for the Holy Land, yes. But he would do it the honourable way, the Pope's way. He would take the Cross again, and join King Louis's crusade in Egypt.

And he would not forget the prophecy of the Dove as long as he lived, and he would pass it on to his own children, and instruct them to pass it on to theirs, so that in the unimaginable future they might make their own judgements about the Engines of God.

They reached the fire. Thomas was rubbing podgy hands. 'Perfect, perfect. All we have to do is retrieve the designs, ship them back to England, and let Roger Bacon get to work.'

Joan, her blood leaking between her fingers, snapped, 'Must we clear this fire first?'

'No need. In fact the fire will help us. Perhaps it was sent by God for that very purpose.' And from his sleeve he drew a packet.

'What's that?'

'A present from Bacon. Black powder.'

Joan grinned, and held out her hand. 'Let me.'

There were howls from the soldiers with Subh. 'Ow!' Saladin heard Michael call. 'The old witch has bitten through my cheek!… By God's wounds. She's dead! Now, how did she manage that? Poison under her tongue? I think she's defeated us, lads…'

Joan sprinkled the powder on the floor, on the spot Subh had indicated. Thomas took an ember from the fire and threw it. Fire blossomed, its noise echoing like thunder, beneath the mosque's low ceiling, and the floor broke open.