Deep within a long, low, windowless building, of a military laboratory in the South of England, a scientist packs up for the day. He runs his eye over a bank of screens that display various scenes outside his lab and outside the building. On the monitor showing the western end of the facility, he sees a soldier patrolling inside the triple-perimeter fence with a German Shepherd dog. The soldier disappears out of range of that swivelling camera and reappears on the next monitor. The dog stops to sniff at the base of the floodlit expanse of razor-wire. A rabbit burrow. Another monitor shows him that the front desk security guard's head has lolled forward. There is no other sign of life outside. It's late.

Seeing that the coast is clear, the scientist removes a box from a refrigerator, places it in his briefcase and leaves, locking the door behind him. He walks down the corridor to another laboratory and punches in a security code to unlock it. The room is in darkness. Everybody left hours ago. He doesn't bother to switch on the light but walks briskly to the refrigerator, that also requires a security code for access. By its light, he takes the box from his briefcase, removes two vials, then takes another box from the fridge and swaps his vials for two from this box. His hands are shaking and his heart rate has doubled since he entered the lab. With the lethal vials safely in the cold box inside his briefcase, he takes a deep breath and tries to slow his racing heart. He can't afford another slip-up. Time is running out.

At the front desk, he eases past the sedated guard and sabotages the video recording for the biosafety level 4 laboratory building. It will look like a technical fault. No-one will suspect. Then he takes the coffee cup containing the barbiturate, bags it in polythene, places it in his briefcase and replaces it with an innocent one.

With three weeks holiday ahead of him, all the loose ends of his work projects tied and tidy, and the last few materials he requires for his personal project harvested, he leaves the premises.

Charlie waits on a crowded railway platform. The train is late, as usual. He clamps his teeth together and resists the temptation to push the jostling bodies away.

Ugh! A model for Hell on Earth. Like heaving, squirming rats in a bucket.

When the train arrives and they all pile in, it's worse. Standing room only. It feels claustrophobic, unnatural – all these strangers with their noses in other people's ears and arm-pits, breathing into each other's faces.

Let me... BREATHE! I'm drowning!

He tries to surface – to drag his thoughts away from the sensation of asphyxiation. Instead, he focuses his mind on diseases that spread in such conditions – plagues. He knows all about plagues.

A plague on this nightmare!

Such things are rare now though. Overcrowded conditions made populations more combustible, before modern medicine helped to make this sort of abomination common. The diseases are in need of a little assistance, if they are to perform their 'pruning' function efficiently once more.

The time for modest pruning has passed. Mother Nature was a hard-hearted bitch, but she did the job. Not any more.

He bites at his lower lip, frowning.

Radical pruning. Right back. Hard! Right back to the roots. It's the only way – the only cure.

It's like a mantra. He begins to relax, eyes fixed unfocused on the back of a head. It soothes him to meditate on the work he's doing.

Soon be over. Not long now. Soon be finished. No more worrying. Almost done.

His mind drifts away from the crowded railway carriage as he retraces the steps of the journey so far, then rehearses the final steps in his head. He's off to hire a car and travel around the country, delivering 'samples' in a variety of forms, to members of the target groups.

Set a thing to catch a thing.

He hasn't given it a name, but he thinks of it as one of those viruses that infect bacteria: a bacteriophage. It's not actually a bacteriophage. That's just the way he thinks of it.

Set a disease to destroy a disease.

He thinks of humanity as a disease that's destroying the biosphere it depends upon for its existence. Some diseases do destroy their hosts like that. It's a silly thing to do, but surely they don't mean to do it. It would suit them better if they could keep their host alive, but they're just dumb viruses. They obviously haven't thought their strategy out properly. He can sympathise with that, but...

What's our excuse?

Like everything else that he has ever done, this is all planned meticulously, including backup plans to cover all foreseeable contingencies. The most challenging problem now is his own increasing agitation and the mood fluctuations. And that's getting harder to control.

The press of bodies in the train generally fills him with hostility, but now that it's nearly over, he tries to assume a placid state of mind. It takes an effort to suppress the anxiety and animosity, but he reassures himself that he feels almost mellow. He's sorry for them in this suffocating crush. His sympathy – when he allows it to intrude – is usually detached, as though he has no real connection with people.

All their disappointed ambitions. High hopes. Blind desires. Mindless drives. All blind alleys. The final reality. What a shock. To discover that, after all, you are unimportant. Worse: you are nothing more than an insignificant part of a massive, unnecessary duplication. Part of the problem.

Detachment falters. For a few moments, the crowd shifts. People leave the train. More people shove their way on board. Sympathy changes to repugnance, then eases slowly back again.

They can't help themselves. It's not their fault. They're just normal animals doing what normal animals do.

The thoughts seem unhelpful at this stage, but he doesn't try to divert the stream. It slows to a trickled and drips to a stop, naturally.

Any species that evolves a level of intelligence where they can develop a complex technology, will do the same. They'll over-populate, over-consume and make life a misery for their own kind and every other sentient life-form on the planet. They'll bring about mass-extinction in just the same way. They won't be able to control themselves. It's foolish of me to despise them for following their nature.

Sympathy and loathing. Sometimes he feels like a split personality – except that he's aware of both sides of his black and white mind, at all times. In order to carry out the work, he's had to suppress the one and inflate the other. It's the only way.

Poor things. They're only following their nature. WE. OUR nature. The 'rational animal'. Rational!

The endless argument, that has raged to and fro in the back of his mind, all through the years, has pushed its way to the front. It's still unresolved. Mostly, he thinks of it as a kind of madness. Sometimes he blames stupidity. Occasionally he gives Homo sapiens the benefit of the doubt. A species driven by animal instincts but enabled to travel to the outer extremity of instinctive behaviour, by a large brain. What he's doing – is about to do – is end the madness, or the stupidity, or just rectify nature's big mistake before it's too late.

Is it madness? It is! At least... it's not sane by any rational standard.

He feels his new calm leaking away as his mind wanders back home, to the place of anger and confusion.

No. It's not really madness, is it? You don't call plagues of rats, mice or locusts 'mad'. If the conditions are right – as happens when we cause an imbalance in nature – certain species over-populate, strip the environment of resources, then starve. In overcrowded conditions, diseases take hold and sweep through populations already weakened by starvation. Occasionally there's cannibalism. A few even eat their own young. It's natures way. The animals are not mad.

Conditions around him are improving. The crush of bodies is reducing as more passengers leave the train. He deliberately relaxes his face muscles.

At least we don't eat our own young in times of stress. No. We just blight their future, without malice aforethought.

It's no good. He's drifted out of the zone. His mind is incapable of staying in the light. Every positive thought is crushed almost instantly, by an angry retort. The furrow returns to his brow.

Overpopulation and environmental destruction. The horrible inevitability of it, if I don't act. It's who we are. Having such an extravagantly high encephalization quotient, just allows my species to delay the inevitable consequences, whilst building their teetering house of cards ever higher and simultaneously undermining the foundations that support it.

Charlie sees that he still needs to maintain detachment. Sympathy and antipathy can do battle later. He's allowed his mind to wander too far. This seething, roiling confusion of emotions could cause another slip. He must refocus. His eyes drift to the passengers in the immediate press. All are staring into space, the middle-distance, resting unfocused on the heads and shoulders of strangers.

They probably hate this situation as much as I do. They should be happy to learn that no new generation is going to have to go through this misery.

Focus remains elusive.

Once we're gone, then all the world will heave a great sigh of relief to be rid of us and able to get on with following their own natures instead of being perverted and side-tracked by ours.

He begins to relax again.

The mindless destruction and uglifying of the planet will end. There'll still be death and suffering, blood will still be spilled, but there won't be any more deliberate torture and heartless exploitation. Pain and suffering will fall to normal levels. A pilot light in the cooker – not the house on fire.

He smiles to himself, looking forward to the end of his own confusion – the end of the long war between disparate elements of his own mind.

The train gradually empties. He sits by a window for the last few miles, still clutching his case, and looking out at the dirty streets. At the next station he disembarks to collect his hire car.

It's almost all up with Charlie. He's visited airports, railway stations, hospitals, conference halls, markets – wherever crowds gather – all over the country, distributing the agent. He tested the virus on himself several months ago, accidentally (his one major slip-up). It's working more quickly than he anticipated, but then, it was a massive dose - and he's modified it slightly since then, to slow it down.

At a crowded market in the Midlands, the closely packed shoppers ooze slowly around the stalls. Charlie avoids this area and goes straight to the livestock pens. He has some treats for the pigs. Early on in the project, he discovered that pigs are an exceptionally fine vector for one of the viruses he employed in the making of this new disease, and he built on that. The virus is designed to be rapidly reproduced in pigs and chickens, as well as humans – though most other species are suitable carriers. The pigs become highly infectious within days, but show no symptoms at all.

A couple of farmers are haggling on one side of the pig pens, so he goes to the other side and walks slowly up the row, throwing a few small cakes into each pen. When all the cakes have gone he strolls to the market and mingles with the shoppers. He doesn't look well.

People try to make a space around him as he pushes his way through the press of bodies, coughing and spluttering. It takes him a couple of hours to go round the whole market. By the end, he's stumbling and coughing up blood. When he finally collapses, there's a smile on his face despite the pain. It looks like a bloody, grinning death mask. His work is done.

He will not be present to witness the final result of his work, but he knows how it will end. People, chickens and pigs will radiate out from the places he's visited. A few of the people may have some mild, short-lived symptoms. People will go on holiday. The animals will go to farms, markets and abattoirs. Some will be exported. The disease will spread all over the planet.

He is not carrying any form of identification. All traces of his work have been cleaned from his home laboratory and it will look like an ordinary basement store room to anyone who examines it. By the time they suspect anything at all, it will be far too late.

The disease is a real slow-burner. He has taken sections of DNA from adeno and avian viruses, ebola, small-pox (which everyone supposed had been completely destroyed – every last virion) and foot and mouth, and spliced them together into something new and virulent: a patient, selective stalker, a psychopathic misanthrope, that can be carried by almost any species of animal, but will destroy just one species. Its first serious symptom, irreversible damage to the reproductive organs and infertility, will not be recognised for what it is. Only in a matter of years, when most of the human population has succumbed to the later and more dramatically lethal effects of the disease, might the truth be recognised. By then, the population will be too small and sick to sustain itself.

That is the plan.

Diseases mutate of course. So only time can tell whether the plan will succeed.