Shooting this press conference is the first worthwhile thing Francis Thorndyke will do in twenty-three years of life, and if he buggers it up he'll forfeit his right ever to shoot anything more worthwhile than country bumpkins chasing cheeses down hills. His editor has vouchsafed this to him with the aid of an evocative metaphor involving bollocks, barbecues and bamboo skewers.

From my cubbyhole in the back of his mind, I admire his response to the pressure: converting the nervous twisting in his ample gut into a kind of elastic band motor that powers his progress through the muttering throng. One hand holds his camera high above his head, safe from carelessly or maliciously jutting elbows. His journey towards the podium (where the Right Honourable Lady Farnaby, Minister of State for Science and Innovation, will shortly appear) resembles that of a stone sinking through thick mud. His weight - relative to his surroundings - lends him irresistible momentum. A venerable old science correspondent tuts his disapproval, and a black-haired girl wearing a sound recordist's bulky headphones opens her mouth to protest; Francis ignores them, and the crowd slops stickily back together behind him.

Counting down the seconds until the Minister's appearance, I rehearse over and over the speech I've composed for young Francis. It's a very short speech. At one point I caught myself thinking of it as an ejaculation, but only a madman would ejaculate in front of Lady Farnaby, and convincing her that Francis is not a madman is perhaps the most vital task of the speech. The last thing I want is for it to be discounted as the ravings of a drug addict (which it is only in the most unforgivingly literal sense).

Francis gets his muffin-top settled comfortably on the crowd barrier just as Lady Farnaby toddles up to the lectern. The Minister of State for Science and Innovation is periodically accused, by fledgling journalists short of common sense or overburdened by ambitious gumption, of being out of touch; exhibit A being her area of expertise (theoretical physics) and exhibit B, her age (fantastically old). In fact, time has boiled off only the Right Honourable Lady's weakest and wateriest ingredients. Though old age has steel-plated her hair and reduced her body to little more than five feet tall, she still retains a stare capable of undermining confidence in any assertion (up to and including 'Hello, my name is Francis'); a grip capable of liquefying small household pets (up to and including guinea pigs); and a library of killer put-downs (including an extensive subsection devoted solely to dealing with claims that she's out of touch). To the gumptious or senseless fledglings, these characteristics constitute an unpredictable (and therefore unfair) breach of journalistic archetyping. To their editors, they look more like evidence of natural selection in the workplace.

Lady Farnaby steps up onto a box and her head sprouts like a furled silver bud from the thicket of mikes and cables suffocating the lectern. From his vantage point on the Minister's left, Francis can capture her serrated profile unobscured by either the lectern or his contemporaries, whose grouching has given way to a mixture of focused professionalism and competitive jostling. He points his camera and keeps the shutter chattering. He knows great photography is about the law of averages: the more shots he takes, the more likely he is to discover a masterpiece amongst his negatives.

The Right Honourable Lady clears her throat; not because there's anything in it that shouldn't be, but as a polite indicator to the assembled press that now is the time to shut up and pay attention.

'Ladies and gentlemen of the press,' she begins. Like molten metal, her voice flows smoothly but contains the potential for impenetrable strength. 'I am here to distil fact from conjecture. Please curtail your enthusiasm momentarily and ask no questions until you can be sure they are not answered by this statement. Of course, I need not explain that you cannot be sure of this until the statement is delivered in full.' Her legendary stare sweeps across the crowd like the green needle of a radarscope, calibrated to expose the electromagnetic signature of disobedience. Apparently satisfied, she continues.

'One week ago today an object entered the Earth's atmosphere and splash-landed in the Pacific Ocean. As has been thoroughly sensationalised by the nation's hobbyists and armchair experts, speaking through many of your own esteemed programmes and publications, the object was no commonplace meteorite. A meteor of similar size striking the Pacific Ocean would, as you are no doubt already aware, have created mega-tsunamis capable of drowning the majority of East Asia, Oceania and the West Coast of the United States of America. Such waves did not materialise, for which fact we are all thankful, and from which fact we can deduce that the object's speed decreased as it descended. I believe the popular conclusion from the available data is that the object possessed, or was under the control of, some form of intelligence. I can neither support nor refute this claim at the present moment.' It's testament to Lady Farnaby's reputation that no one objects to this shameless display of scientific and governmental fence-sitting.

'I can, however, offer the nation a small amount of new data. Four days ago a United Nations Taskforce successfully retrieved the object from the ocean floor. Now, my first instinct--and many others'--suggested that the object was most likely a classified Chinese or Korean satellite whose orbit had decayed, and which had been guided down by its parent nation in a controlled crash landing. Having had the privilege of personally examining the object, as part of a delegation of scientists from a diverse selection of nations and disciplines, I can now officially discount this theory. Though, in my professional opinion, its structure is too regular to have occurred naturally, the object was not constructed by humankind; and whatever agency slowed its descent and prevented a global catastrophe was not of human origin.'

With a look that acknowledges the implausibility of the implication and simultaneously dares anyone present to repudiate its logic, Lady Farnaby says, 'I shall now attempt to answer your questions.'

In half a second the crowd transforms from meek schoolchildren into brass-lunged, pogoing ravers. Half the hands in the hall erupt into the air, and accusations, entreaties and exclamations blurt from every throat. The Lady scans the horde with narrowed eyes. To her, the cacophony is mere unintelligible static waiting to be tuned.

Young Francis fools himself that photography is enough for him, but the same ambitious streak that propelled him to the barrier longs in secret for a Pulitzer. I know exactly where he hides that ambition, and it requires very little effort on my part to push it to the forefront of his mind. His free hand leaps high just as the Minister's appraising gaze bathes the front row.

I defrost the reserved politeness Francis's Mum instilled from birth, which he's been keeping on ice for the sake of his career, and waft it about, infusing his psyche with its sweaty musk. That ought to keep his mouth shut. I know the Right Honourable Lady values manners, and there's a good chance she'll look favourably on the one person in the entire crowd that waits his turn to speak.

'You. Yes, you,' booms Lady Farnaby's amplified voice. Her finger points to Francis. Elation suffuses both our minds. I've done it! After a week's training, I'm about to do what professional retconners have been failing to do for a decade. I'm about to save the world.

As the room falls silent I scream the speech into Francis's head. For a moment the words flood his neurons and synapses, displacing memories, drives, even the basic reflex to breathe.

'You mustn't activate it!' I roar. 'It's an oily-looking ribbed cylinder the size of a small car! How could I know that? Same way I know this: shoot it back into space or it'll kill us all!'

The words arc towards Francis's speech centre - and evaporate.

Suddenly I have no purchase on his mind. I was a mountaineer clinging by my fingertips, and the rockface just fused into flawless diamond. I'm falling, without a vessel to hold my mind together, without an anchor to secure me against the inhospitable winds of the continuum.

My mind is on the rack and the cranks are belt-driven by the wheels of drag racers. In an instant my essence is dispersed across the infinite reaches of space and the eternal span of history. I'm rolled out as flat as rice paper, shredded and scattered to the wind.

Time races backwards, tortured elastic springs back into shape and the amused voice of a young woman says, 'Whoa now, Mister Thorndyke, calm down.'

I'm whole. I'm stable. I'm contained. I'm frozen in mid-thrash, with one elbow and the opposite knee thrust towards the ceiling. Firm futon padding under my back, chilly alpine breeze on my face, dusky light slotting through Venetian blinds to barcode the bare white walls. Leaning over me with a knowing look is a young woman in a white coat. She has unruly black ringlets barely constrained by an assortment of woggles and scrunchies. I relax. This is the training compound. She is Tamara, my trainer.

'We call that a rebound,' Tamara explains. 'It happens when you try something that would result in your never having taken the capsule that sent you back. The continuum throws you out and you reel straight back to the present.'

I feel like the rebound ran a few thousand volts through my mind before cramming it too hastily back inside my brain. Experimentally, I blink, touch my temples and roll my head from side to side. Nothing I do either relieves or exacerbates the feeling: a squeezing, fizzing ache.

'Bloody inconvenient,' I opine. 'Another minute and I could've put this whole business to bed.'

Tamara wrings out an imaginary teatowel and makes a frustrated noise in the back of her throat. 'Why do none of you people listen to me?' she demands. 'What did I tell you at the induction?'

'No, I know, but listen,' I explain, 'I was at the press conference when I was twenty-three, stood not two metres from Lady Farnaby-'

'What did I tell you?'

'"No grand gestures",' I recite. 'But-'

'No buts!' There's a weary aspect to Tamara's ire that suggests she's rehashed this point a few times before. 'You think you're the only one with a golden opportunity somewhere in your past? That angle is played out. It's a waste of capsules and all it earns you is a head full of static.'

'How long is that going to last, anyway?' I ask.

'Until your body finishes processing the drugs.' Tamara checks her watch. 'Call it another hour. And don't change the subject. Now the programme's open to all comers, there are millions of people working towards the same thing you are. It's not a race. Millions of little alterations: that's all the continuum will let you get away with, and it's the only way we're ever going to achieve anything worthwhile. So what's the golden rule?'

'"No grand gestures",' I repeat, trying my best to look chastised.

'That's right. You can't fix this alone,' says Tamara briskly. 'Now, once the headache wears off, you've got twelve hours until you can safely dose again. I suggest you use the time to reconsider your game plan.'

Before I can retort she moves over to the futon on my left, where Harry's wasted body lies while his mind moonlights in a younger, healthier model. I watch her back as she takes his pulse. She can't be more than half my age, which means she was born around the same year the cylinder fell, which makes her nearly useless as a retconner. How can she claim to understand the whims of the continuum when, in all likelihood, she's never taken a capsule herself?

I shuffle up the futon until I can sit up, then use the wall to heave myself to my feet. Time is gradually robbing my body of structural integrity. I feel like a water balloon on legs - and the water keeps on trickling in. After a couple of hours being twenty-three again, waking up in this body feels like a grievous mix-up. I wonder whether the continuum would let me prompt young Francis to take a few more trips to the gym.

I scuff one foot and harrumph to make it clear I'm out of bed. Tamara remains preoccupied with Harry, so I reckon I've been given permission to roam. On the futon to my right is Katharine, who, like Harry, is currently braindead in most of the important respects. I step over her carefully on my way to the window. With my fingers crossed, I prise open a peephole in the blind and take a look outside.

It's another cloudless day, and I can see clear down the mountain to the plateau below. Hundreds of identical prefab cabins cling, with chains, pitons and sturdy bolts, to every scrap of level ground in sight. Young couples hang out laundry on bungees strung between rooftops. Small children chase one another down the scree slopes. Teenagers clump in the patchy shade under scrubby mountain trees, industriously rolling and chain-smoking homegrown greenery. There's no one over thirty visible. They're all closeted up here in the training compound, where it's quiet, drugging themselves braindead and learning the hard way what the continuum will and won't tolerate.

From this height, the plateau looks blanketed with virgin snow, but I know that if I still had my twenty-three year old eyes and I peered down there with my nose to the glass I'd see that white layer stirring and swirling in the breeze. One-nil to Tamara. The fog's still there; the cylinder's still active.

Next time.

However delightful a scene of human endurance this may be, it won't keep me entertained for twelve hours. It can't even take my mind off the sizzling sherbet dusting my bloated brain.

Sitting down cross-legged against the wall, wincing at the splintery noises my knees make, I study Katharine's unconscious face. I put her in her early forties, a year or three younger than me. There are no grey hairs amongst her wavy red ones, and her face has only the compulsory set of wrinkles, between her eyebrows and at the corners of her eyes and mouth. If I hadn't lived through the last fifteen years I'd say hers was a face that had lived a life without hardship. She's breathing, but the muscles in her face are as still as a cadaver's.

When I first met Katharine at the recruitment event a week ago, my libido poked its perky head out of the box where I keep it, with a hopeful glance in her direction and an attempt at butter-wouldn't-melt innocence. One look at its troublesome little face and I found myself taping the box shut again with all my usual excuses: I'm past the age where, 'Can I take you to dinner?' starts to sound like, 'You're the last one in the room, so prepare to lower your standards'; I was never an elegant suitor, and a long famine will have done nothing to improve my table manners; and it's the responsibility of our generation to give up on our future and concentrate on fixing the past.

Perhaps my judgement's impaired by the ball of tin foil being microwaved inside my head, but all it takes is this one lingering look at Katharine for my libido to slit through all those excuses with mocking ease, spring out of its box and settle its sweaty behind in the driving seat of my mind.

Tamara, Harry and the rest of the new recruits obligingly cease to exist. I lean forward and my back doesn't so much as pop. My sagging gut abdicates its role as psychological cold shower and drags me encouragingly downwards. My breath sidles between Katharine's parted lips.

She gasps, sneezes, shudders, and her eyelids retract like shop-window shutters. My vision doubles and my jaw flaps like a puppet's. In that instant my headache is gone, as abruptly as if yanked out with a fishing line.

I convulse awake in my bunk, fending off a double-strength rebound headache. My first one ever, which I have just obligingly shouldered in place of my forty-five year old self, forms a crackling core around which a fresh one - a punishment for wasting capsules on my personal life - can agglomerate. My skull becomes a plasma globe: heavy, fragile and full of greasy purple sparks.

Gritting my teeth, I roll over, dangle one arm over the side of the bunk and lucky-dip through the empty meal trays, foil blister-packs and laundry for some socks and slippers. The ones I find don't match, but hey, I'm only going down the corridor. Everyone I might meet has seen me looking worse.

I shut my door behind me and set my slider to OUT. The others all have theirs set to DOSED. I try not to make too much noise as I pass, so naturally my every step shakes the building to its foundations a hundred storeys below.

The galley at the end of the corridor serves as our storecupboard, kitchen and study. At one end of the long room is our last cardboard crate of microwave meals; at the other, a stack of flattened cardboard high enough to block the window. Two trestle tables and an assortment of stools and chairs occupy the floor in between. Spread out on the trestles is our team timeline, a long concertina of grainy brown recycled paper weighted by the microwave at one end and by a dirty glass jar full of coloured marker pens at the other. The scale printed on the concertina covers thirty-six years: from the birth of the oldest living retconner, before which meddling is impossible, to the day the United Nations Taskforce successfully activated the cylinder, after which meddling becomes pointless.

I pick the red pen marked FRANK from the jar and chew the lid while I work out how to mark up my latest excursion. As far as I know, there's no standard notation for what I just did: emerging somewhere off the useless end of the timeline, then hitching a ride further back on a dose I took decades ago.

Time was, one capsule could hurl me back twenty years or more. Now I need at least ten just to enter the timeline's purview. But if I can repeat this trick I could jump unprecedented spans while conserving capsules. They could call it the Thorndyke Manoeuvre. The accompanying double-strength headache could be Thorndyke's Folly.

I flip the timeline over and sketch in another twenty years, then mark my emergence point with an arrow and the first stroke of a five-bar gate - indicating that I've made an attempt and rebounded, but that I've yet to fully explore every opportunity at that point. Protocol demands that I summarise the cause of the rebound, but Katharine's going to see this, and I refuse to undo forty-plus years of emotional subterfuge for the sake of protocol.

I improvise a swoopy bracketing line to represent the Thorndyke Manoeuvre and write below it, 'host dosed, passenger piggybacked'. Finally, I mark the date of the press conference with an x, confirming that all avenues at that juncture are still dead ends, good for nothing but bouncing me prematurely back to the present.

As I'm recapping my marker, I notice the other pens in the jar clicking together. As I lean in to confirm that my old eyes aren't just playing silly buggers, a rustling to one side announces that the stack of cardboard is shaking as well.

I fling the door open and stomp all the way down the corridor towards the stairwell. I'm halfway there before I feel the vibrations in my feet, and I'm hauling myself up the stairs when I feel the bump-and-judder of touchdown.

By the time I've struggled to the top of the stairs, slid the bolts and thrown open the door, the only sound on the roof is the wheeze of the breeze. The rotors of the elderly Chinook are spinning slowly enough to be visible even to my dodgy old eyes. The helicourier and her co-pilot drop down from the cockpit, crouching low as they slide open the cargo doors and start unloading crates of microwave meals.

'Hello!' I shout. I haven't used my voice in weeks and my throat has grown a scaly coating. I cough as I start towards the Chinook. Not one cloud smudges the sky. The breeze, whisked up by the rotors, is frigid, but not enough to numb the static buzz of Thorndyke's Folly. 'Hello!'

The helicourier turns. For a moment I think it's Tamara; her hair is just as dark and unruly, if a little shorter. But this girl can't be older than twenty-five, and Tamara'd be well into her sixties by now.

'Morning!' she calls. 'One floor down?'

'That's right. Stairs only, I'm afraid. The doors are all open. Need a hand?'

'You're all right.' She waves me off. 'Thanks for opening up. Let us handle this lot.'

Gratefully, I withdraw to the balustrade and watch the two healthy young things carry a crate of food across the roof. When they disappear down the staircase, I turn to survey our little archipelago. It looks the way some people used to imagine Heaven: a world built, impossibly, on unspoiled white clouds.

At the last survey, fifty storeys of our building were below the fog layer, leaving another fifty standing proud. From the balustrade I can see only six other buildings, and over the past two years, one of those has become nothing more than a concrete plaque set into the fog. When the wind gets up, ethereal white waves sweep across it and break around the water tower. In six months that, too, will be swallowed.

Looking down into the fog gives the others vertigo, but it just gets me wondering about all the strange new flora flourishing down there. The fog isn't toxic. It just isn't air. Taking a walk below the fiftieth floor would be like being smothered by a white pillow the size of the world. But people with breathing gear have been all the way down to ground level, gathering samples to aid research into a chemical solution, and I'm told it looks like the surface of an alien world down there. What they can see of it through the fog, anyway.

I miss the alpine communities sometimes. We could take walks or talk to people during the comedowns in those days. But as years passed and the fog crept higher up the mountain, the presence of our compounds ceased to be reassuring and instead became a reminder of our continuing failure. No one can blame the younger generations, trapped in a closing vice between us and the fog, for becoming disgruntled. I miss being able to look outside and see the people I'm trying to save; I don't miss being too scared to dose in case one of them throws a rock through the window and brains me while my body's vacant and defenceless.

'Mister ... Thorndyke?'

The helicourier has a clipboard in one hand and a cardboard palette stacked with little orange pillboxes under her other arm. Her co-pilot is already back in the Chinook, spooling up the rotors ready for takeoff.

'You're the only one awake,' she shouts over the crescendoing whoosh and whine. 'All the other rooms are locked. Can you sign for these, and distribute them once the others wake up?'

'I don't see why not.'

She hands over the clipboard and I make my mark. We swap, clipboard for capsules, and I stand watch by the stairwell door as she jogs back to the copter, runs her preflight checks and lifts off, heading for the next rooftop.

Back downstairs, I toss next month's capsules on my bunk and head to the galley to take stock. The stack of flattened crates is gone and a stack of full ones blocks the opposite window instead. Our much scribbled upon timeline is gone, replaced with a clean copy, folded into a concertina we will soon unfold and never manage to recreate. I puncture the lid of a meal tray and watch it rotate in the microwave.

Soon after the beep, I hear movement in the corridor. The door opens just a crack and Katharine edges in. Whereas I've expanded outwards over the years, Katharine has scrunched in on herself. A vacuum inside her body pulls all her skin inwards, hollowing her cheeks and stomach and arching her spine. Her waist-length hair is white tipped with red. One of the regular helicouriers is an old friend of hers, and he hides bottles of dye in the meal crates when he can, but he hasn't been for a couple of months.

'Morning,' she says.

'Morning,' I agree. After all these years living and working together, this is still the limit of our interaction. The usual excuses stoppered my throat for the first few years, and long after my urges have cooled and mouldered I still find it a nearly impossible habit to break. I can't start from small talk at this stage. I can't admit that I still don't know whether she has family, or which is her favourite microwave meal, or what her middle name is.

Katharine goes for her green marker while I start the recycling collection afresh with the lid from my meal. I'm about to circumvent the inevitable awkward silence and head back to my bunk to eat when she says, 'Did I miss the courier?'

I pause with one hand on the door handle. She's already deconstructed the concertina. The tip of her marker taps against her chin.

Eventually I reply, 'By a matter of minutes.'

'Was it Lawrence?'

'No. A new girl.'

Katharine marks a neat green x on the timeline. Apparently she doesn't feel obliged to respond, but I'm pretty sure what we're doing qualifies as a conversation, so I can't leave. Which means I'm left with no choice but to speak.

'Were you expecting Lawrence?'

'Hmm.' It might be an affirmative or it might just be a contemplative noise. 'He promised me another bottle this month.'

'Maybe he's been reassigned.' I've already said more words today than in a typical week, and more to Katharine than in a typical month.

'Hmm,' she sighs again. She attempts some experimental origami on the timeline. It's futile, but it saves her having to make eye contact. 'You know, I used to dye my hair a different colour nearly every month.'

'Really?'

'Hmm. Acid green, electric blue ... I kept going back to black, jet black. I liked how I looked with black hair.'

Abruptly as always, my headache contracts to a bright pinpoint and vanishes like the picture on a dying television. Where before there was only a blizzard of static, sunlight now glints off slender threads of association.

'I think I did too,' I say.

Katharine turns around and scans me, head to foot, for the palest glimmer of irony.

'Were you ever a TV sound recordist?'

A slowly spreading smile stretches and smoothes her wrinkled features, momentarily revealing the face of a black-haired girl I last saw wearing a sound recordist's bulky headphones and opening her mouth as if to protest. 'Radio,' she corrects.

'The press conference-'

'Lady Farnaby's statement,' she confirms. 'I knew it was you - I knew right from the very first time!'

'What are the chances?' I move fully back into the room and set my meal down on top of the microwave. 'That was, what - twenty years before we met at induction?'

'Twenty-two.' Katharine's hands flutter from her sides to her throat and back, and she hovers back and forth as if unsure whether to move closer. 'I've been back there so many times. I used to try and make myself talk to you.' Her smile turns sheepish. 'I gave myself a lot of headaches that way.'

'Isn't that...' I understand what a struggle it must be for Katharine to render herself this emotionally vulnerable, and the last thing I want is to send her back into her shell at this crucial stage, but I can't leave the question unasked. 'Isn't that sort of a waste of capsules?'

'A waste?' She decides against drawing closer. 'I suppose I was worried it might be. I didn't know whether you'd ever go back there, or whether you'd recognise me if you did. But you did go back there and you did recognise me, and now we're finally talking, so it wasn't a waste, was it?'

It's not a rhetorical question. She's asking for reassurance, for my blessing on her romantic endeavour; and I want to give it. There's hope clinging to her words and features, more hope than I thought was left in this place, and I can't stand to be the one to smother it. But what I want isn't important. I have a responsibility, and so does Katharine.

'There aren't so many of us left these days,' I remind her gently. 'We have to make the most of every chance. Just think about all the good you could've done with that many capsules. I mean, what would happen if all of us just used them for ... nostalgia trips?'

I brace myself for a hostile reaction. Instead, sympathy floods the deep channels of Katharine's face and she advances on me with arms outstretched. I flinch back, still half-expecting a slap.

'Oh, Frank,' she sighs, placing her hands on my shoulders and her head on my chest, the closest she can get to hugging me. 'It's wonderful that you still believe. It really is.' My every heartbeat must be a knockout blow to the side of her head. 'But we were never going to change anything, Frank. Harry, bless his soul, you remember Harry? All he ever did was go back to before he got ill. He never tried to change anything in case he bounced back into that wasted body of his. You're the only idealist left, Frank. The rest of us just take the pills and enjoy remembering what it was like before.'

We stand in silence for a long minute. Katharine hardly breathes the whole time, as if influencing the air currents in the room is the same as influencing my reaction.

I can sympathise. I can. I understand the temptations the drugs put in front of us. I've wasted doses before; I wasted one just now, trying to reboot my relationship with Katharine. It's natural to want to seize missed opportunities, to feel like we're twenty-three again. I realise they didn't mean anything malicious by it.

I pluck her bony wrists off my shoulders, pick up my meal and, without looking back, carry it down the corridor to my room.

The rest takes me several trips. Katharine fusses and pleads around me the whole time, but there's so little of her that it's easy to pretend she doesn't exist. By the time I'm finished, sweating through my shirt and on the verge of an aneurysm, there's barely enough space in my room for me. The clutter covering the floor is crushed under two cardboard crates of microwave meals, enough to last me until the next helicourier arrives. The timeline, the jar of markers and the microwave sit scattered on my bunk alongside the palette of pill boxes.

'Frank, if you must do this, then at least-'

I set my slider to DOSED and lock Katharine out. Perhaps with only the view to entertain them, she and the others might remember what they're supposed to be doing here. The absence of malice doesn't excuse their apathy. At least when I wasted my doses I was trying to improve something.

If Tamara was right, and I can't prevent the fog alone, then the fog wins. There's no way I'm accepting that. I have everything I need. I'm going to do what retconners worldwide have been failing to do for decades. I'm going to disprove the golden rule.