Mary and Mackie sat on a fallen pillar in the shade of the ruined north transept, their feet in the long grass, and watched the girls working. They were cheerful enough, their young voices echoing from the stone walls.

Mary said, 'Looks as if they are fighting a losing battle against the rosebay willow herb.'

Mackie shrugged. 'I'm told it's more a symbolic effort than anything practical. Morale booster, you know. I mean it's rather too shady in here to grow anything worthwhile. Of course the archaeologists have been crawling all over this place since Hitler conveniently blew it up for them.'

'Well, they would. There are roots here going back to a Roman military headquarters, the centre of power in the whole of the north of England.'

'And now York finds itself the locus of a world empire. Remarkable how things come around. I wonder what archaeologists of the future will find of our time. A layer of ash, I suppose. Rubble and bones.'

'Geoffrey Cotesford visited the city many times, according to his memoir. In fact his first monastery was just outside the walls.'

'Ah, our friend Brother Geoffrey! I thought he might have done. So to business, Mary.' He dug out his pipe and began the usual rather theatrical business of filling it, shred by tobacco shred. It occurred to Mary that she hardly ever saw Mackie without the pipe. Perhaps he needed this prop for reassurance; perhaps he was less calm than his urbane British surface would have led her to believe. 'Tell me first how you are getting on with your counter-history. What was your hinge of fate?'

'Dunkirk,' she said immediately.

This was an exercise the two of them had set themselves. In an effort to delve into the minds of history-meddling Nazis, Mackie had proposed that they try to devise their own 'counter-histories'. If you had a Loom, what tweaks to history would you consider making? It was not so much the results that were of interest, he argued, but the habits of thinking and the types of research, a bit removed from the conventionally historical, that he wished to understand.

Mackie nodded sagely. 'Dunkirk. I should have guessed you would wish to spare your son the consequences of living through that horrendous defeat.'

She said fiercely, 'Let it be done to someone else's son, not mine.'

'Fair enough. How could that calamity have been averted?'

'If Hitler had hesitated…'

She had had access to remarkably thorough briefings. Mackie's MI-14 had moles that penetrated all the way, it seemed, to the top of the Nazi Party. And she had learned that in those dark days of May 1940, when the BEF and the remnant French forces were trapped on the beaches, it had been a full day before Guderian had been authorised to unleash his Panzers for the final assault and his resounding victory. The delay was obvious even to the allied soldiers on the beaches; Gary had spoken of it.

'There seems to have been a debate at all levels within the military and the Party,' she told Mackie. 'Guderian himself had some concerns about the nature of the ground they would have to cover. The blitzkrieg had advanced so fast he was short of proper intelligence.'

'Ah,' Mackie said around his pipe. 'Germans never did like a heavy pitch.'

'Meanwhile Guderian's superiors were well aware that France was not yet conquered. The BEF was beaten; so let it go, and keep back Guderian's forces, let them rest and re-equip for the French campaign. And then Hitler was still dreaming of peace with England. He thought that sparing the BEF from slaughter might demonstrate to you Brits that he was a civilised kind of guy after all. But in the end they concluded that the destruction of the BEF was too good an opportunity to miss.'

'So how would you make the change? You remember the rules we set. You're allowed to go back and whisper in one person's ear.' This was, from Geoffrey's evidence, how the Loom seemed to operate.

'I'd reach back to Hitler's court in those hours when they were debating whether to unleash Guderian. And I'd mess about with the head of Karl Ernst Krafft.'

'Who's he?'

'An astrologer.'

'I thought Hitler didn't believe in astrology.'

'Yes, but there are those around him who do, Himmler and Goebbels to name but two. In 1939 this guy Krafft sent a prediction to Himmler's intelligence service that there would be a bomb attempt against Hitler. Well, the prediction came true.'

Mackie snorted. 'Pure coincidence!'

'Of course. But it gave him an in at court. Mess with him, and you can influence at least two Nazi barons.'

'And if the decision about Guderian was so close, that might be enough to swing it. What would you say to him, though?'

Mary shrugged. 'Some suitable gobbledygook, couched in Aryanmythos phraseology. Hitler is a Taurus, which is supposedly ruled by the element earth. Hitler is a lion on land but lost in the water – so he should spare Britain, an island nation, and concentrate on the ground he can conquer, which after all stretches all the way to Russia. Something like that.'

'Um. So what next? If the BEF had been reprieved-'

'I think everything would have been different,' Mary said. She hesitated, then plunged on, 'I think the German invasion might not have happened at all.'

Mackie raised his eyebrows.

A different Dunkirk would have altered the mood on both sides of the Channel, she argued, and so affected the chains of decision-making that led up to the invasion itself. A saved BEF might have boosted British morale. Churchill might have survived politically – and then, emboldened, he might have forced through such belligerent actions as disabling the French fleet, to prevent it being absorbed by the German navy. As for the Germans, facing a tougher, more resolute Britain, and with the balance of power less in their favour on both land and sea, the invasion might have seemed that bit more daunting.

'They were always disunited at the command level,' she said. 'Each service seeking to shuffle off responsibilities onto the others. If the invasion had seemed more difficult, the infighting might have got that much worse.'

Mackie nodded, but looked doubtful. 'You know, actually I'm not sure how much difference the invasion has made, up to now at any rate, in the bigger picture of the war. Essentially the land war in the west has been stalled since September 1940. It doesn't matter much whether Hitler's troops were held up on this side of the Channel or the other. Without an armistice, he couldn't have withdrawn too many units for the eastern front – I doubt if he could have launched Barbarossa any earlier or more violently. And in the meantime he still had the Luftwaffe. He would have been able to strike at us even without an invasion. Hammer the cities – London especially. And he could attack the Atlantic convoys. In some ways we might have suffered more.'

'That's true,' Mary insisted, 'but with an unoccupied British mainland the allied western front would have been a hell of a lot stronger. You wouldn't have to go through a W-Day counter-invasion to scrape the Germans back into the sea before you could even contemplate going into France, for instance.'

'Perhaps. And with Britain intact, the Japanese might not have been bold enough to launch their invasion of Australia, for instance…' He pulled his lip, clearly not convinced. 'The trouble is, Mary, all the military logic of the time dictated invasion. In that summer the Germans had the momentum of blitzkrieg, and we were the last pawn to be taken. One way or another they'd have had a go, I think, whether the BEF was spared or not. Nice try, Mary, but invasion was inevitable, whatever happened at Dunkirk! Tell you what, though: if I may, I'll hand this to my tame boffins back at Birdoswald. See what they make of it. All right?'

'Sure. I'll give you my notes. So how did you get on? Where would your turning point be?'