In the stations where they stopped there were lots of uniforms, of the conventional services of Britain, the Commonwealth and the US, and of Britain's vast volunteer armies, like Mary's own WVS. Mary didn't like the transformation of Britain into a country of uniforms. It was as if the German way of thinking had infected everybody, as if the Germans had already won.

And the towns were scarred by the war. Though repair work was underway, you could see gaps in the terraced streets, holes colonised by weeds rather than people. There were defensive emplacements everywhere, anti-aircraft guns and barrage balloons. And factories were hastily being erected, relocated from London and the south. There was plenty of labour to do all this work; all of England's major towns had taken refugees from London, homeless and unemployed.

Outside the counties of the protectorate itself, it was the capital that had suffered most severely from the invasion. London was in English hands, but, bombed and oppressively threatened, it was slowly bleeding to death. Its people and factories were being shipped out, its docks barricaded or blown up, and its many state functions transplanted elsewhere: York was now the seat of government, Manchester was Britain's financial centre, the royal family had migrated to Holyrood in Edinburgh, and the seat of the Church of England had been moved to Liverpool. London's museums and galleries had been stripped, their precious contents scattered and hidden. The city itself was turning into an abandoned museum, with only its immovable architectural treasures remaining.

There were some commentators who said that London might never recover from this cruel shutting-down – George Orwell, for instance. 'Oh yes it will,' Mary had heard the old crusties say in the Colchester pubs. 'We'll ship the Cockney buggers back ourselves.'

So the journey passed. Everybody was quiet, save for the children. Mary thought she understood why people were subdued. All the adults in the carriage faced an uncertain future. And everybody in England had lost somebody in the war, even Mary, who didn't belong here at all.

It was a relief when the train pulled into Newcastle, and she was able to leave the stuffy compartment, to find another perky WAAF waiting for her on the platform.

XI

15 October

Wednesday morning at Birdoswald was clean and sharp, the start of a bright fall day. Tom Mackie had requisitioned the farmhouse here to serve as his base of operations, he told Mary as he welcomed her from her hotel. But as he escorted her around the site, Mary saw how the farmhouse nestled at the heart of much older ruins, the remains of a Roman fort set on a bluff of high ground.

'I can see why the Romans came here,' she said.

'Oh, yes, a military man would make the same decision again. Birdoswald – actually they called it Banna – was an integral part of the system of defence based around Hadrian's Wall. Housed a thousand troops at its peak. Seems they had to drain the land, clear a forest, and import the limestone to build it. Kept the peace for three hundred years – which, if you think about it, is longer than modern Britain has existed, since the Act of Union. We'll be doing well if we last so long, eh?'

They walked back to the farmhouse. 'Rather ugly, isn't it?' Its most recent renovation was Victorian. The architects had added crenellations, amid a general look that Mary thought of as 'Gothicised'. 'But they reused Roman stone. I can show you an altar of Jupiter that's been built into the wall of a stable. And there's evidence of occupation of the site before the Romans… But I apologise,' he said.

'For what?'

'For treading on your toes. You're the historian, after all.'

'Not at all, Captain. I'm impressed you know so much.'

'Well, history's always been something of a hobby of mine. I took nat sci at Cambridge – that is, natural sciences, specialising in physics. But I did do rather well at history at my school matriculation. And I've been somewhat keen to find out more about the history of this place since your researches directed me here.'

Something of a hobby. Somewhat keen. After so long in Britain Mary was used to decoding the circumlocutory language of upper-crust types like this Captain Mackie: the more self-deprecating the words, the deeper the passion. 'I'm flattered you took me so seriously. To open up a new Military Intelligence branch here, all on my say-so.'

'Well, this does seem to be a pivotal site for your anachronistic conspiracy theories, doesn't it? And it wasn't all that hard to get the funding, actually, at least for intelligence work. For one thing there's been a general withdrawal from the south-east, as you can imagine. Sites like Bletchley Park are suddenly a lot more vulnerable.'

'Bletchley Park? Where's that? What are they up to there?'

'Oh, you know, war work, the usual. I worked there for a bit myself. And as for the project we're engaged in here, well, the threat you've hinted at seems rather bizarre, but the country is awash with rumours about Hitler's super-weapons. A Nazi time machine isn't even top of the league table of outlandishness, believe it or not. If only for the sake of morale, the government must be seen to be making an effort to investigate all these threats, and, if necessary, put a stop to them. And you do have a certain notoriety in government, Mrs Wooler, thanks to your pieces on Peter's Well. I actually have some quite high-level support. I've a line to Frederick Lindemann, Churchill's personal science advisor. Winston calls him "Prof."

'But even so I do have to compete for funding. Even now, as things seem to be moving towards a certain denouement, I'm desperately short of anything resembling solid evidence to show my superiors. One reason I wanted to talk to you today.'

'What kind of "denouement"?'

'All will be revealed.' He extended his arm and escorted her indoors. 'But first, Mrs Wooler, let me offer you a coffee – generously supplied by your own government as it happens…'

'If you're making me coffee, you can call me Mary.'

He smiled. 'And I'm Tom.'

Mackie had set up his office in the farmhouse kitchen; it was by far the best room in the building, he said, and the warmest in winter. The most striking feature was a spear of blackened wood and heavily rusted iron, suspended on the wall over the fireplace. Mary was no expert on the period, but it looked Roman to her.

They sat at a big scuffed wooden table, over which generations must have broken bread together, and spoke of the mysteries of space and time.

He folded his arms. 'Let's start from the beginning. You developed your interest in all this because of your contact with this Austrian fellow, Benjamin Kamen.'

'It was at the time of the invasion.'

'Yes. He was picked up as a prisoner during those eventful days. We believe he was taken at first to a POW camp in Kent. Place near Richborough, on the coast.'

'That's where my Gary is,' she said in a rush.

'Yes, we know,' he said, not unkindly. 'May not be a coincidence; a lot of their more "valuable" prisoners seem to be held there – and that includes Gary, an American citizen. We actually have some intelligence that Gary protected Ben – helped him conceal his racial identity.'

Mary wondered: helped? Why the past tense? Had something changed, for Ben or Gary?

Mackie went on, 'Now, your encounter with Kamen set you off on an historical investigation. But once you tipped me off I followed the chap's career from a different direction. Before the war he was, in fact, a promising young physicist… Bags I go first about all that, and then you can tell me about the history.' He grinned at her. 'You know, I think I'll enjoy this. All rather a jolly game, isn't it?'