The sea withdraws its protection from the bycatch child. The white mouth of the sky gapes open. He catches a last glimpse of his mother's upturned face before he is dragged out of the water and cast down on a hard deck. He lies there, gasping, among dying squid and scuttling crabs. All around him thrash thousands of fish, their gills gaping open. His skin burns with light and air.

The crew would throw him overboard, throw him overboard and deny they ever saw him, but a fisheries observer from the Ministry is aboard. What she sees cannot be unseen. She points, shouts. On the point of suffocation, the bycatch child is scooped up and carried to water.

The water is pale, it tastes strange, it is held in by something that hurts when he thrashes against it, but it is life to him. Exhausted, he ceases his struggles and sleeps, despite the vibration and the noise and the slopping from side to side.

Through that day and the next day they come to look at him:

- What is it?

- What does it eat?

They put dead fish in the water with him. The taste turns his stomach, and he pushes them away. At last, at last, when he is knotted up with hunger, a lively little mackerel is dropped into his tank. He seizes it and strips the tender flesh from its bones with relish: one bite, two, three, four. He looks up at round, astonished mouths. Another mackerel, another. He ignores the fourth, which swims round him morosely, as out of place as he is.

Later, when he needs to defecate, there is nowhere to swim to. They change the fouled water before it kills him.

Suddenly there are hands again, voices, the choking blanket of air. They transfer him to a bigger tank, inside which they have rigged a little compartment where - as they show by hand signals - he can meet his bodily needs without fouling his home.

Already, the faces of his clan are beginning to fade, though they still swim unbidden to him in his dreams. His mother, her tenderness - and then he wakes to light, to noise, to faces peering in.

They have differing faces, these strange air-swimmers with their wide nostrils and their blunt, useless teeth. There are three he sees most often: the old man with the array of sharp tools, who makes him nervous with his endless cutting and chopping but seldom pays him any attention; the young man who dumps mackerel in his tank; and the woman. Even with the coverings they wear, he can tell woman from man. The woman spends a lot of time watching him. He thinks that she is unhappy, or perplexed. She watches him, and taps on a flat rectangular box she holds in her left hand.

They talk among themselves, that is clear, although the sounds they make mean no more to him than the roaring of surf heard from far below. Sometimes the woman talks directly to him. He tries to reproduce the sounds she makes, a e i o u.

For two days there is a storm. He has never heard of seasickness, does not understand the wretchedness he feels, the spasms that void his stomach. They change the water in his tank as often as they can. The storm passes, and the water in the tank goes back to its usual sloshing: unpleasant, but bearable.

One morning he wakes to the unfamiliar sensation of stillness. The boat he is on, b o a t, is no longer rocking - or rather, it is, but so faintly he can scarcely feel it. The noise of the engines has gone. Overhead, things shriek and clank.

Boots clang on a metal deck. Two shapes approach him. They have puffy white arms, and where their faces should be is only a reflective surface made from some shiny material. The arms reach down for him. He struggles and swims to the deepest portion of the tank, voiding himself in his fear. They seize him all the same and lift him towards their blank faces. When they drop him into yet another tank, it takes him a moment to realise that he has not already been devoured. He cowers.

Then everything begins to move. The light grows dim, then brightens immensely. Sunlight burns through the water.

For a moment, he catches sight of the woman, talking to two men he does not recognise, waving her little rectangle about. Then his tank comes to a halt, and faces press in on every side. Bright lights pin him in place.

The woman advances, and a cover is placed over his tank. In the blessed darkness, he no longer has to look at a world he does not comprehend. His little world begins to rock again. When they open up the travel tank at the lab, he is fast asleep.

In the mornings Henry eats and swims and looks at his tablet. In the afternoons Ms Morris comes. 'Hello, Henry,' says Ms Morris. 'How are you today?'

Ms Morris is careful not to get too close to the edge of the tank. She has seen how quickly Henry can strike at those who frighten or upset him. Still, the boy seems to be settling down now. He had been a holy terror when she had first met him, eighteen months ago, after she had signed the non-disclosure agreement to end all non-disclosure agreements.

'Today, Henry,' says Ms Morris, 'we're going to talk about the sky. Can you see the sky?'

Henry shifts restlessly in the water. It has taken him a long time to get over his fear of the sky. The windows of the lab are high and small, yet at certain times of the day, when the sun is shining, they become rectangles of heat that probe the recesses of his tank, driving him to the darkest corners for shelter.

But Henry is a big boy now, and however painstakingly, he has taught his tongue and vocal chords to say their words. 'Sky,' says Henry.

'Good boy, Henry,' said Ms Morris. 'Good boy. Now, can you find a picture of the sky?'

Henry picks up his waterproof tablet. Orangutans can master hundreds of symbols on the tablets researchers have created for them. Henry has mastered over 1000, and shows no sign of stopping. Fishing across a wide area of the Southern Ocean has been prohibited until more about Henry and his kind can be discovered.

So yes, Henry can call up a picture of the sky: a bright sky, and a cloudy sky, and a night sky. (Henry prefers the cloudy sky.) 'Clouds,' says Henry, and Ms Morris tells Henry about the types of clouds, about how sometimes they cover the whole sky as far as you can see, and other times they are puffy and broken. She tells him about lightning, which he has seen, and thunder, which he has heard. He still finds the concept of rain remarkable: one world falling into another.

Ms Morris says goodbye and closes the lab door behind her. Henry does not know that she goes into another office, where she files her daily report on Henry before going home. She works quickly. She has a dinner date tonight, and unlike the last two losers, she doesn't plan to let Eric slip through her fingers.

'Henry continues to make good progress,' types Ms Morris. 'He is curious about the world beyond the lab, and I am introducing material that relates to his origins as laid out in the research programme. So far, Henry has not volunteered any further information about his home environment, but I remain confident that he will do so when the appropriate stimuli are presented.'

She pauses for a moment, considering. Then she says 'Screw it' under her breath and adds another paragraph. 'As I have previously indicated in my verbal reports to the Director, I remain concerned that Henry's complete lack of social contact, either with his own kind or with humans in his age cohort, will prove a crucial barrier to his further development, especially as he matures towards adolescence. I consider that this poses a risk to Henry's wellbeing and mental health.'

She knows she's going out on a limb by putting this in writing. At her last evaluation meeting, when she expressed her concerns to the Director, he wasn't having any of it. 'You just do your bloody job and get me some results,' he had said. 'Those bastards at Lamont Seafoods have got deep pockets, but limited patience. If they can't get the ban lifted, they at least need it narrowed. All their research vessels have managed to find is a few new fish species and a squid that looks like Lady Gaga. Show him a picture of that and ask him if he recognises it.'

She did. He didn't. She felt disappointed, in Henry, for him. She cares about her water baby.

So she adds a final paragraph. 'Henry's environment offers limited stimulation. I recommend that Henry be periodically exposed to new and enriching experiences.'

Save, send. Take that, you smug pricks, she thinks. With new energy she tugs on her coat and heads for the outer exit. Eric is in for a long night.

Henry is surprised when Ms Morris arrives early one day, just after breakfast. 'We have a special treat for you today,' she says.

Henry is not sure he likes the sound of that. He is even less sure when his transport tank is wheeled into the room. He wriggles and squirms and bites, and one of the men curses, fresh blood dripping from a deep cut in the palm of his hand. But then he is in the tank, in the truck, on his way. Once more, he falls asleep in the darkness and does not wake until the truck stops.

When they wheel him out of the truck, he is in a place of trees and fields and bright light, and in the foreground a lake. People in black shiny suits are standing between Henry and the water.

'Now watch out, Henry,' calls Ms Morris. 'These lakes are full of water weed. Don't get caught up, and if you do, put your arm up out of the water and wave your hand. We have divers to help you. And we have some lovely fresh fish waiting for you once you've had a good swim.'

So that is what they are, thinks Henry. Divers. They stand nearby, unsmiling.

And then he is in the water … the lake. He has grown used to fresh water by now, but never so much at once. When he dives, he can almost make himself believe that he is back in the ocean, that far below his clan are waiting for him. But when he dives deeper, all he finds is mud and more of the choking weed. He kicks himself free and swims to the surface. This is not the sea. But he can smell it, faint, tantalising, the brine of a breaking shore somewhere beyond low hills, covered in small shrubs and sharp grasses, that rise where the sun will bathe tonight.

An arm of the lake runs under trees, towards those hills. Drifting away from the divers, whose clumsiness is equalled only by the racket they make, Henry dives deep and swims along the centre of the channel, avoiding the clutching weeds. This arm of the lake is fed by a marshy stream. It is shallow, but there are enough pools that he can catch breaths as he moves upstream. The stream narrows to a ditch. He wrinkles his nose at the sharp smell of decay. Behind him, they are calling: the deep voices of the divers, the higher-pitched voice of Ms Morris: 'Henry!' He can hear the fear in her voice. 'Henry bad,' he says to himself, and presses on. Such strange smells from the land - and far behind, the sound of divers, splashing in vain.

The ditch ends in a concrete lip. He looks up and sees a dark mouth, a round 'O' from which water dribbles, with a tiny circle of light at its far end. His memory is firing with long-ago warnings of what can lurk in dark undersea caves: he does not want to enter the darkness of the culvert. He hauls himself out of the water, squeezes under a fence that has come loose from one of its strainers where it crosses a damp hollow, and finds himself on the edge of a rectangle of short, evenly clipped green grass. His tablet has shown him pictures of these things: this is a lawn, and beyond it, a house. Near the house, a young human, a girl, is using a hose to fill a low round object he does not recognise with water. Henry is not made to walk on land. He propels himself across the lawn towards her.

The girl looks up. She puts down the hose. The nozzle snakes from side to side, spraying water into the air. He moves gratefully under the life-giving flow, letting it moisten his gills, then keeps moving towards the rubber-sided pool. He flops into it gratefully.

The girl regards him. 'You're funny,' she says.

'Funny Henry,' says Henry.

The girl picks something up from the ground. 'I have two sticks,' she says, 'and one of them is a boy stick. Do you want it?'

Henry says 'Stick please,' and reaches for it. At that moment, the girl's mother walks round the corner of the house.

The Director looks at Henry with a theatrically furrowed brow. 'What are we going to do with you, Henry?' he asks.

'Do nothing,' says Henry.

'That little girl's mother was very unhappy with you, Henry. That cost us a lot of money.'

'Girl. Boy stick,' says Henry.

'That's probably just what her mother was thinking.'

Henry stares at the Director silently. The Director often says things with his body that he does not say with his mouth.

'What it comes down to, Henry, is that we need results. We need to know where you come from. We need to know how many of you there are. And we need to figure out what we can do with you. A whole swathe of the Southern Ocean has been closed thanks to you, Henry. That's costing some serious money.'

'Boat,' says Henry.

'You're smarter than you look, aren't you, Henry? That's right, you're going to go on a boat. Soon, Henry, soon.'

Months pass. 'Soon' turns into now.

Henry is excited, because he is going on a boat, and they are going to find his mother, his clan, his people. The Director is pleased. The man from Lamont Seafoods is pleased. Everyone is pleased, except Ms Morris. Ms Morris is sad.

'I'll miss you, Henry,' says Ms Morris.

'Henry will come back,' says Henry.

Ms Morris smiles, but her cheeks are wet.

At the dockside, Henry's tank is unloaded from the truck onto a wharf, protected by a screen of big men in puffer jackets. The gangway is before him, the sea below it. The big men step aside and the Director advances towards Henry, smiling.

Then there is a disturbance, a knot of men and women running, papers being waved in the air. The men in their jackets face outwards. Henry hears raised voices, a furious argument, as if all the people want to bite each other. A police car turns up, and then another one. The sirens hurt Henry's ears.

'A ward of the Court?' says the Director. 'You have to be shitting me.'

Now the Director's mouth and his body are saying the same thing.

Then Henry's tank is moving backwards, back towards the truck. He thinks about slithering over the side and heading for the sea, but the big men are watching him. The tank is receding, receding, and then he is in darkness again.

Ms Morris still comes to visit him, but only twice a week. Sometimes she does not have the heart to teach him anything. The news of the world flows into Henry's tank through his tablet, and the news is not good.

Everything is older, and everything is shabbier. The high, barred windows are still there, but they are grimier now. Less sunlight comes in.

Henry now has to wear swim trunks whenever he has visitors, and he has been told about some private things he is not supposed to do unless he's by himself. Sometimes, Henry wonders if they will simply stop feeding him. He swims until he is exhausted, round and round, round and round, in the tank that is now much too small for him. Round and round.

Henry never sees the Director, and does not even know if he is the Director any more. Every month - Henry marks the dates in his calendar app - a stern-faced man comes to look at Henry. He looks at Henry as a shark would look at a mackerel. Then he nods his head and turns away. His most frequent visitor is a woman who talks a lot. She pulls up a chair and sits near his tank, smiling, trying to sound calm. Henry can tell that she wishes she did not have to sit there, that he makes her nervous. But she tries so hard, so very hard, to hide it, and her voice is smooth and soothing. 'How are we today?' she asks Henry, and 'Is there anything you'd like to tell me?'

Henry would like to tell her about the sea, but all he has of the sea are the images that come through his tablet. For a long time, his mother's face would well up in his dreams. It no longer does.

'Nothing,' says Henry, and stares her down until she goes away.

Ms Morris knows how sad Henry is, and she tries to make things better. She says she has written to the Minister, that she has made an appointment to see the judge. 'You'd be better off at sea, Henry,' says Ms Morris.

The desire for the sea, the desire for home, burns all the stronger within Henry now that he can no longer remember either. Sometimes, Henry thinks about finding a way to make a hole in his tank, and letting all the water out, and lying on the floor until he doesn't have to think any more.

He is swimming in his tank, round and round, when the door is flung open and a little knot of people comes in. Henry is surprised and pleased to see his old Director is among them.

'We're in business, Henry,' says his old Director. 'We had to take those pricks all the way to the Supreme Court, but we won. Want to go fishing?'

There is a lot to do before Henry can go on the boat. He is poked, prodded, probed, immunised. They even knock him out for a couple of hours. 'This won't hurt, Henry,' they say, and 'you won't suffocate, don't worry,' and 'you'll open your eyes and you won't feel a thing.' It does hurt, though. His skin stings in the water when they put him back in his tank.

But it doesn't matter, none of it matters, because Henry is going home. The months and years of neglect are over, and now he has visitors every day, important visitors who pay attention to what he tries to tell them, even though he cannot tell them much. He says goodbye to Ms Morris - to Daphne. She cries. He says goodbye to his tank, to the lab, to the high, barred windows. He does not say goodbye to the stern man and the nervous woman, since they have vanished as abruptly as they arrived. He says goodbye to his tablet: though he wishes it could come with him, they say it will not handle the pressure. Henry is going home.

It is summer. It is very cold. They are a long way south.

The vessel has been anchored in these gelid waters, just outside the summer ice limit, for three days, during which the wind has held light and southerly. But there is a storm coming, the Captain says, and by nightfall they must sail north and out of its path.

And for three days, Henry has been swimming, enjoying the limitless freedom of ocean. He was slow at first, uncertain, but then the muscle memory came flooding back - though those are the memories of a much younger body, and Henry is older now, and powerful. He has broken free of the shackles that captivity had placed on his body and his mind. Darting fish fall to his hands and jaws. He plays in the space between the chop of the waves and the long roil of the currents, and as his confidence grows, he dives deeper, dives so deep that he can barely see.

They have given him a camera, and when he is tired he floats with neutral buoyancy and runs his fingers over the controls. It is the nearest he will come to having his tablet in his hand. He would like it to be his link to the world he is leaving behind, but all it can do is cycle through its menu of commands and show him the photos he has already taken. He has other tools, clipped to a belt round his waist: a knife, a torch.

Henry floats beneath the swells. Above him, the horizon is rising to meet the sun as the long southern summer day nears its end. Below, the darkness of the deep ocean waits. Henry dives again, one swimmer among billions, from the lowliest diatom to the mightiest whale. He has not been in danger yet, and if a shark or a leopard seal swims close and takes a fancy to him, well, he has the advantage of the knife.

Years above ocean, years of exposure to sunlight, have damaged Henry's eyesight so that he can no longer see to the deepest depths. He has been trying to adapt, but this time, as he continues to descend the long column of water upwelling from the depths, he is forced to fit his headband to his head and turn on the torch. The beam of light knifes into the depths.

Blinded by his own light, it takes him several moments to realise that he is surrounded. He turns the light off and floats in the water column. He sees that they are pale, and large, and all around him. They are his people, yet he finds his hand creeping to the tool belt, closing on the knife.

Too late. Abruptly, they are on him, biting. He struggles, but strong hands grasp him, and sharp teeth penetrate his left bicep, the side of his neck, his right thigh. He screams in pain, but as suddenly as the attack began, it is over. Now the hands are gentler, holding him. Something soft is pressed against his wounds. Small objects, metal streaked with blood, are passed from hand to hand, then let fall into the abyssal depths below. The clan soothes Henry, waits until he has recovered his composure. Then they swim south.

On the Seamaster, the Director wrenches off his headphones and throws them to the floor. 'Shit!', he says, and then 'shit' some more.

Shit shit shit.

The man from Lamont Seafoods just stands there. That's that, he thinks. Something must have got him, and now the three transmitters are on their way to the bottom of the sea with whatever remains of Henry's lifeless body. All that investment wiped out by some passing predator.

'Maybe we should have used a different frequency,' says the Director. 'Maybe this one attracted something Henry couldn't handle.'

The man from Lamont shrugs. 'It was always a gamble. All we need is a change of Government and this ban will be gone by lunchtime.'

'You don't give a shit about Henry, do you?' accuses the Director.

The man from Lamont doesn't answer. Voices sound above. The ship's engines come to life and the Seamaster begins its long voyage back to Wellington Harbour.

Far below and well to the South, under the protecting ice, the clan welcomes Henry home. His wounds are beginning to heal, and while he must be taught again about everything that keeps the clans alive in these waters, he has much to teach them. About the world above. About Lamont Seafoods, who think themselves lords of the sea. About knives.