'The years look to have been kind to you, Mr Nevinson.' It was true. The man seemed scarcely to have aged at all. He was balder, though not much. That was the only detectable change. They sat down. 'It was good of you to come.'

'Oh, any excuse to tuck into one of the Polly's fruit cakes. That's why I arrived early. In hopes of polishing off a slice before you joined me.'

'Carry on.'

'Thank you. And, please, call me Percy.'

'OK. I'm David.'

'Yes. Of course. It's odd, isn't it, to wait twenty-three years before getting onto first-name terms?'

'It was a brief acquaintance.'

'But a memorable one.'

'True.' Umber broke off as a waitress approached. He ordered coffee. 'It was certainly memorable.' Nevinson had by now embarked on a last mouthful of cake, too large to permit coherent speech. 'Your sister told you why Mr Sharp and I are here?' Nevinson nodded affirmatively. 'He retired from the Force years ago, you know. You have nothing to fear from him.'

'A representative of the authorities never truly retires, David,' Nevinson responded after a final swallow and a gulp of tea. 'You should tread carefully.'

'He's simply trying to establish whether there were any clues he missed – any leads he should have followed.'

'I gather neither of you believes Brian Radd was responsible.'

'Do you?'

'Certainly not. But who does, apart from the police? The authorities, you see. They're not to be trusted.'

'Who is, Percy?'

'You and I, of course.' Nevinson held up a hand to signal for silence as the waitress returned with Umber's coffee. Then he leaned forward in his seat and resumed, in a subdued tone. 'We were there. We know what we saw. The question we must both consider – have both had to live with ever since – is what did it mean?'

'Mean?'

'Why was the child taken, David?'

'Because some sicko got it into his head to do such a thing.'

'You believe that?'

'What else can I believe?'

'And your wife? Did she believe that? Please accept my condolences on your loss, by the way. She seemed… a charming person.'

'She was. And thank you. As for what she believed, well, she could never quite bring herself to accept that Tamsin was dead.'

'Perhaps she was right not to.'

'Do you have some reason, Percy – some good reason – to say that?'

'I think I do, yes.'

'Care to share it with me?'

'It's in your own best interests that I should.' Nevinson pulled out a roll of thickish paper from the inside pocket of his jacket, slid off the rubber band securing it and spread out in front of Umber a large black-and-white photograph, which he proceeded to anchor down using the sugar bowl and the teapot.

It appeared to Umber to be an aerial photograph of some desert landscape, buttes and mesas widely spaced and varied by what looked like craters and a couple of strange conical formations. Most striking of all, however, was a mound close to the centre of the picture so contoured that it looked for all the world like a relief depiction of a human face.

'Ever seen this before, David?' Nevinson asked.

'I don't think so, no.'

'What do you suppose you're looking at?'

'The desert somewhere. Egypt, maybe.'

'Why Egypt?'

'The formations… don't look completely natural. And this…' He pointed to the face. 'I don't know. I suppose it reminds me of the Great Sphinx in some way.'

'Interesting you should say that. In fact, this is a photograph of the surface of Mars taken by the Voyager One orbiter in July 1976. It's a site in the northern hemisphere known as the Cydonia complex.'

'Really?'

'Really and truly.'

'So, these… shapes… are just freaks of nature.' Sharp had warned him about Nevinson's Martian fixation, but he had not expected it to exhibit itself so swiftly. 'Unless you're going to tell me they're not natural.'

'You said as much yourself.'

'I said they didn't look natural. That's not the same thing. This… face… could be an optical illusion caused by… the angle of the sun.'

'What about this?' Nevinson pointed to a large circular rimmed depression near the right-hand edge of the photograph.

'A crater.'

'And this?' Nevinson's finger moved to one of the conical mounds, below and slightly to the left of the crater.

'An extinct volcano?'

'NASA would be proud of you.'

'Percy, what has this to do with -'

'Avebury? Simple. It is Avebury. What you call a crater and an extinct volcano are perfectly scaled representations of the stone circle at Avebury and the artificial hill at Silbury. Or vice versa. They are precisely proportionate and have the same geometric relationship. Trace a line due north from the centres of the volcano and Silbury Hill and you'll find that the centres of the crater and the Avebury circle are offset by exactly the same angle. Nineteen point four seven degrees. Does that ring any bells? 1947?'

'The Roswell incident.' Umber's heart sank. This was worse than he had first thought. Far worse.

'July's been a busy month over the years. Roswell. Apollo Eleven. Voyager One. And our own strange experience.'

Alien abduction. That was it, then. Nevinson's theory of choice to explain two men in a white van, one missing girl and one dead one. Umber sighed. 'Do you really believe this, Percy?'

'I've been compelled to. The evidence is overwhelming.'

'So… Tamsin was kidnapped by Martians?'

'Certainly not.' Nevinson frowned pityingly at him. 'Have you taken leave of your senses?'

Umber smiled grimly. 'I'm not sure.'

'Wessex is an encoded landscape, David. That's what you have to understand. Avebury. Silbury. Stonehenge. Woodhenge. The long barrows. The linking avenues. There're repositories of information – of ancient secrets. But not everybody wants those secrets to be uncovered.' Nevinson's voice dropped to a whisper. 'By the summer of 1981, I'd gone a long way towards cracking the code. I notified the authorities of my preliminary conclusions. I thought it my duty to do so. That was a mistake. Sadly, I fear Tamsin and Miranda Hall paid the penalty for my mistake.'

'How do you figure that out?'

'I believe the incident was staged to demonstrate to me that innocent people would suffer if I continued with my researches. Of course, no-one was intended to die. The driver of the van simply panicked. But Miranda's death complicated matters. I believe it's the reason why Tamsin was never returned.'

'What became of her, then?'

'I don't know. I imagine she's alive and well somewhere, with no conscious memory of what occurred that day. She was only two years old, after all.'

'Did you tell Sharp any of this at the time?'

'I hinted at it. But I was left in no doubt that he'd been warned off by the powers that be. Hence the need for us to meet… a deux.'

'I see.'

'I suspect you've been manoeuvred into accepting his assistance. His role is to ensure you don't find what you're looking for. And, before you ask, I'm afraid I can't disclose what I've learned from my study of the henges. Naturally, I've continued to work on the subject since 1981. But to share my findings with others would only be to endanger them.'

'Of course.'

'I strongly advise you to abandon your investigation. If you must persist, do so alone. But be aware of the risks you'll be running. They're considerable. Although…'

Nevinson's voice trailed off into a silence Umber felt no inclination to break. The man was mad. That was clear. Not barking. But mad nonetheless. Yet his madness at least ruled him out as Sharp's correspondent. His obsession left no room for Junian diversions. Even Percy Nevinson could not suppose that Junius was a Martian.

'I don't really need to tell you, do I?' Nevinson appeared disappointed that some prompting was required. 'Your wife's sad example is a salutary one.'