A light appeared suddenly in the woods ahead of me, then another alongside it. I tried to cry out or make my presence known, but in my dazed state I could not make my mouth move with sufficient coordination. The mysterious lights advanced towards me, looking very similar to those on the front of the HGV I have swerved to avoid and, now that had a moment to consider it, seemed to have a greenish tinge that no self-respecting trucker would contemplate.

Against the light, I could just make out a shadowy figure approaching, once which I was just awake enough to realise was not entirely human in shape. Oh, it was humanoid, with two arms, two legs and a head at the top, but the alien - I could not help but think of it as one - moved in a fluid way which would require more joints in each limb than the norm for bipedal primates.

In my dazed state, it seemed to me that the alien was looking behind itself. There was another shadowy figure, almost invisible against the brightness of the lighting. This one was, perhaps, larger and appeared to move more sedately. It waved its limbs in a complex way that suggested communication: a language rather than just agitation. In a sudden flash of inspiration, it occurred to me that the first figure looked as if it was being told, in no uncertain terms, to undertake a particularly menial and tedious job.

The smaller figure moved towards me, with just a suggestion of stomping in the movements of its limbs, despite its generally fluid and undulating motion. In one hand - I suppose the word is still applicable - the being held an object of some kind, and it gestured frantically with the other. Suddenly, something flew from the object in its hand, surrounding me in a green glow which crawled over my skin, penetrating the wound on my arm, somehow healing and repairing flesh and skin. The substance went to work on face and hands as well. I was fascinated, watching the glowing substance through eyelids barely opened and further obscured by congealing blood that the goo made no attempt to remove.

The alien must have assumed I was still unconscious and turned away, I imagined, addressing the other figure. With more presence of mind than I thought I possessed, I managed to move my uninjured arm and discreetly scoop up a little of the green goo in a discarded water bottle I found littering the foot-well of the car.

The smaller alien returned its attention to me. It made another gesture and the goo swiftly returned to its container. From the corner of my eye, I could see the goop in the bottle trying to move in the same way, but it was unable to escape from its plastic prison.

As I watched, still feigning unconsciousness, the figure retreated, the lights dimming as it went. Shortly afterwards, the lights disappeared entirely, blinking out as if they had never existed. There was no sound, not even of the wind, but I got the strangest feeling of a sudden and powerful movement in a direction I do not know how to describe.

I sat still for a minute or two, marvelling at what I had just seen and wondering what I should do. I was able to force open the driver's door and climb out. For another long moment, I stood breathing hard and looking around me. No movement, no lights, nothing.

I had the presence of mind to fumble with the boot, which was much less damaged than the rest of the car, and retrieve the large bag I carry everywhere with me. It contains my mobile office: laptop computer, notebook and a thick wad of papers most of which I never quite get around to reading. I concealed the plastic water-bottle inside the bag, and slid and struggled my way up the bank, following the trail of mangled foliage back to the road.

The rest of the evening was uneventful. I dragged out my mobile phone, which mercifully worked even here in the middle of nowhere, and put in an emergency call. An ambulance arrived surprisingly quickly, and I was attended by the two paramedics. Later, I would be described as "miraculously uninjured" but at the time I let them carry out their standard procedures, culminating in them wheeling me on a stretcher in a neck brace to the waiting ambulance.

Before we left for the hospital, I explained to the police constable who turned up in a patrol car shortly afterwards that I had swerved to avoid an oncoming lorry in the middle of the road. It had not stopped and I was, frankly, not in a position to describe the vehicle or the number plate because of the darkness and the speed at which the vehicle was moving. The copper seemed to accept this and went away muttering something disparaging about lorry drivers from Eastern Europe.

I failed to make the training course, which was no great loss, having spent all night in the hospital undergoing tests "for observation" as well as having no working transport. I made my way back home the following morning, this time by taxi and train. The car was a write-off, of course, and I would later plod my way through the interminable processes around insurance claims and detailed explanations to the vehicle lease management team.

The following day I was back at work, already in receipt of a loan car provided by my company, and immediately immersed in the numerous details of my job. For a time, I was threatened with having to attend the next training course, but nothing was scheduled - perhaps the monkeys in Human Resources finally realised it was a complete waste of time and money. So, everything was back to normal.

Unsurprisingly, I found myself thinking about the incident on the bridge and the mysterious substance I had encountered. I toyed with many hypotheses, but always came back to the view I had formed instinctively on the bridge: that I had had a genuine UFO encounter. I imagined that the aliens had been as surprised as I was at the unexpected intrusion of a local - myself - on some vital and no doubt secret mission, and had made amends by healing my injuries.

As for the green material I had secreted, I could not help but experiment with it. I rubbed a little into a gash on my finger, which disappeared immediately, the cut melting away as if it had never existed. I found that it was capable of healing everything that I tried it on - nothing life-threatening, fortunately - but minor cats and burns, the ache in my shoulders which I suspected was incipient arthritis. It even managed to reverse the hair loss on my scalp.

One of my earliest serious applications was to cause a new tooth to grow on the roots of the old one when I lost a crown, probably loosened by the impact of the crash. That was quite a challenge, I can tell you, pouring just a drop of the green goop into my mouth. I had expected some foul taste, but in the event the stuff had neither flavour nor smell. Indeed, once I had got over the initial revulsion, I could barely detect the stuff in my mouth at all.

Since that night, I've thought a lot about the green substance I acquired. I think it must be some kind of nanotechnology, semi-intelligence and self-organising. I cannot believe it is of human origin. From everything I have read since the event - and I've read a lot - we just cannot make anything that sophisticated. So it must be alien, but clearly designed - or at least adapted - to work on people, ordinary human beings.

*

"Thank you for your statement, Mister Gray," the more talkative of the two men in black suits said, "It seems to cover most aspects."

He consulted the notepad in his hand momentarily, then looked up at me.

"You know we've been looking for you for nearly thirty years," he said levelly, "You've been very clever, hiding and moving around for so long. And so very few rumours and clues for us to go on."

He nodded thoughtfully, taking in my unshaven chin and the grubby overalls I have been wearing when the men had grabbed me.

"There's only one thing left to cover," he continued, leaning over the interview room desk to look directly at me, "What have you done with the goop, the nanotechnology?"

"It's all gone," I answered, "There's none left."

The two men gave each other a meaningful glance, as least as far as I could tell with the sunglasses.

"That's why I gave myself up," I said with a hint of smugness.

I held up a hand for forestall the inevitable objection.

"You don't think you could have got this close if I hadn't wanted you too, do you?"

"Now that's a real shame," the hitherto silent man in the corner said laconically, "Our friends really, really wanted their technology back."

Without warning, the entire wall of the interview room, the one covered by a mirror which I had assumed was actually one-way glass, began to slide silently upwards. From under the widening crack, a dazzlingly bright light shone - a light that suddenly seemed all too familiar.

"So they'll have to settle for second-best," the first man said.

I could not move, abruptly locked rigid in the worn steel-and-plastic chair I was offered when we first entered the room. Then, the seat itself started sliding inexorably towards the source of the bright light. Glancing back, I could see the two men standing, watching impassively, the greenish light reflecting from their sunglasses.

"After all, this is the only way our friends will now be able to determine," the talkative one said, "The exact long-term effect of their formula on the human body."

Part 1 Afterword