'A very Nazi idea!'

'They seem to be trying to implant seeds of weapons technologies, anachronistically advanced, centuries before Columbus – giving them enough time to come to fruition.'

'Ready to be placed in the hands of Columbus, yes? But after centuries of development, who would know what to do with the stuff?'

'That's where the Testament of Eadgyth comes in. The second of the Columbus prophecies. Unfortunately it only survives in fragments.' She showed him some of this. The Testament", supposedly whispered into the ear of an eleventh-century Christian woman, was a kind of poem in old English. 'It refers to Columbus, I think, if elliptically.'

'Not that elliptically. The Christ-bearer" – Christopher. The Dove" – Columbus.'

'It mightn't have seemed so obvious to contemporaries, and certainly not to anybody in the eleventh century. There are lots of references to God's engines" and coming wars, and finally the main commandment: All this I have witnessed / I and my mothers. / Send the Dove east! O, send him east!"'

'And these two messages would, you're arguing, set up chains of events which will converge in the career of Christopher Columbus. And then what?'

'You have to remember that Columbus was a militant Christian as much as an explorer. He thought he was going west to Asia, yes? He was after wealth from new trading routes. But he also dreamed of taking on Islam, which was then on the march across Europe. He carried a letter from the Spanish monarchs to the Mongol Khan, hoping they could team up.'

'And squeeze Islam in a pincer movement. Good plan. Shame for him the Americas were in the way! But I think I see where you're going with this. If he had these super-weapons from friend Aethelmaer-'

'He mightn't have felt the need to enlist the Mongols. With such weapons he could conceivably have given up his dreams of sailing west, and turned east instead, to launch a direct attack on Islam. Europe would have been consumed by a new age of crusading and jihad, fuelled by anachronistic weaponry. The destruction would have been horrific. And though others would surely have sailed to the Americas, nothing like the modern United States might have emerged.'

'So, America aborted – but Europe destroyed in the process. Why would the Nazis want that?'

Mary shrugged. 'They don't approve much of Islam, or the Jewish-Christian conspiracy". The usual Aryan nonsense. It's a bit drastic, but they might be quite happy to see medieval history expunged.'

'Remarkable. Intricate. Audacious! But it didn't work, did it?'

'Apparently not,' she said. 'But then, in our present, these messages may not yet have been sent back.'

'But we see traces of them in Geoffrey's memoir.'

'Well, we saw the Menologium before Trojan sent that back. I don't pretend to understand it all, Tom!'

'All right. I'll put the squeeze on my intelligence sources, and try to find out what Trojan is up to – in particular if he's working on anything like these messages you've discovered in the record. And then we must decide what to do about all this.'

Mary folded up her notes. 'I'd say that's clear enough. Destroy the Loom before it can be used again.'

'Yes, of course,' he said sagely. 'And that's why I've asked for your son Gary. Security around Richborough has been as tight as a mouse's arsehole since our raid in '41. But Operation Walrus gives us an excellent chance. If we send in a small team, highly trained and motivated, going in perhaps ahead of the main counter-invasion front – hit them before they even know we're there.' He tapped his teeth with his pipe stem. 'But we must plan for all contingencies. Suppose, for instance, we're too late to stop this Codex being sent back. What then? Do we block the Eadgyth material?'

She frowned. 'I'm not sure. I've no idea what harm the Codex engines might do to history without the Eadgyth testament. It might be better to make a minimal change in the record. Sabotage the testament rather than destroy it. Turn it into a mandate to send Colombus west, not east.'

'In war it always pays to have back-up plans. I wonder if you'd work through these possibilities for me.'

She thought that over. 'Perhaps I could work out a warning about what might have followed a destructive fifteenth-century European war. A conflict with China, perhaps. A counter-invasion by the American cultures, the Incas or the Aztecs… But I'm no expert, Tom.'

'Well, who is, in this peculiar field?' He sucked on his pipe, and brushed bits of ash from his trousers. 'You know, all this mucking about with the past by one side or another – it's as if our modern war with the Nazis is folding down into the past. Remarkable thought. Tell me this, though,' he said. 'Purely hypothetically. If you had the power to make a change – say, your Dunkirk intervention – if it was just a matter of pushing a button – would you do it?'

She'd thought about that, long and hard. Having studied Geoffrey's agonised testimony, she'd become convinced that nobody really understood the deep structure of the tapestry of time, even though so many hands eagerly plucked at it. And when they did meddle, they left flaws. She didn't want to mention to Mackie evidence she thought she had unturned of holes, where it seemed entirely plausible a figure had been torn from the weave of centuries. Robin Hood, for instance – a shell of legend around a character that ought to have existed. Bubbles of remnant causality.

'I don't know,' she said honestly. 'I think it's possible that even the slightest change might wreak the most devastating consequences. You might be like al-Hafredi, deleting your own history entirely, cut away at the root you tamper with. You might create a world in which nobody like you would ever be born…'

'That seems a drastic point of view,' Mackie murmured. 'My gut feel is that history might be a bit more resilient than that. I mean, it seems to me it's possible that if you were to make some sort of change, the consequences would just sort of ripple through. The tapestry of time must be a hefty piece of work. The patterns would persist, wouldn't they, even if you pulled out the odd thread? The physicists have nothing to say, incidentally. Nothing sensible anyhow, which is typical of that crew.'

'So if you could push the button?'

He pursed his lips. 'I'd like more data. But it seems to me that it might be possible to calibrate the effects of interventions.'

'Calibrate?'

'It would mean turning history from an art to a science, but still! Think what a boon for good such power could be.'

And there, she thought, was the difference between herself and men like him. Mackie was an instrumentalist, who saw in this technology only a weapon. She saw horror. But then she thought of her own Dunkirk counter-history. Only if one were sufficiently desperate, she thought. Only then…

'Of course,' Mackie said, 'all this mandates us to keep this technology, if indeed it exists at all, out of the wrong hands.'

'You mean the Nazis, the Russians-'

'And the bally Americans, my dear, no offence! Now come, let's get out of here. Can I offer you a lift anywhere?' She stood. He took her arm, and guided her out of the ruins of the minster.

A liberty bus drew up outside the minster, a 'passion wagon' that took young women to dances at the GIs' bases. The Land Girls flocked that way, colourful, grimy, laughing.