The first thing of which she is aware is the dark. Formless, seemingly endless. Inconceivable space within which she hangs. Then, that there is life, and light, stirring somewhere beyond her in the blackness and changing its boundary.

The first thing that she sees is an enormous pale face, close to her own. Not having seen her own face yet, she sees the other's. It swims, flat and round, like a mirror with fins. Like a moon at the bottom of the sea. Blank. White. Shimmering.

They touch one another through curtains of skin. They dance in the womb, within their separate sacs, tumbling gracefully over each other like Chinese acrobats. They stretch out their tiny arms, and waltz in slow motion in their world of darkness and fluid. Two loving punch-balls, each a balloon for the other.

Their mother's body divides them. It calls them out. From their happy room where they float and kick like waterlilies swaying on stems, they are expelled. The world, their underwater cave, is after all the belly of the whale. Twin Jonases, they shiver, unable to distinguish between earthquakes, indigestion, death. The walls of their sanctuary contract. A disaster is set in motion.

She loses him. He is sucked away from her; he escapes through the hatch, small perfect astronaut shooting towards a cold foreign star, the outside, where gravity rules and he can no longer fly and freely dance. Brother, come bask. Come back.

She has no choice. Where he goes, she must follow. She plunges after him. Wildly, awkwardly. She is crying her way out, her feet slipping in blood, in mucus. She does not want to leave. But muscles, hands, and the season of birth, propel her. And the determination not to let go of him.

There is a tenderness in the London suburb of Edgware in spring. The air carries the warm chirrup of birds, the scent of flowering redcurrant, a haze of pollen and dust. Helen aged ten walks home from the primary school playing fields, tired, sweaty and content after the last hockey match of the season, her knees bare under grey flannel shorts cut to look like a skirt. She comes, mindless and voluptuous, down the long gentle hill of Broadflelds Avenue, through the long tunnel of ornamental cherry trees clotted with pink, unable to see the sky, a thick litter of pink petals under her feet, a mass of pink blossom over her head, raftered with reddish-brown.

The suburb has only recently been snatched from the open countryside, as the street names attest: Bullscroft Road, Harrowes Meade, Bushfleld Crescent, Hartland Drive. From the top of the hill, where Helen's primary school stands, you can see beyond the compact loops and crescents of the suburb, beyond the council estate at its boundary, to the green fields, farms and woods that stretch towards Hertfordshire. Helen, unless accompanied by her twin brother Felix, rarely ventures that far. She confines her childhood mainly to the back garden of her home and the streets immediately around it. Run into the garden and play, their mother constantly urges the two children, and Helen obeys.

Felix makes forays into the unknown, climbing over the high wooden fences espaliered with fruit trees separating their house from the street and from the council estate at its end. He makes friends up there, returns, eyes glowing, for scolds at his trespassing, with stories of derelict cars, waste-sites, bomb-shelters, children whose parents do not know where they are for whole hours at a time, who swear and who steal. Felix comes home with new words, dangerous knowledge, toys swapped or picked up casually in Woolworths, to be told off for being so late in, to Helen who never is, and who waits for him.

Today, she is alone out on the street, on the half-mile descent between school and the front garden gate of home, safe and familiar territory crossed four times a day. On either side of the road are houses in rows, detached, some done up with timbered fronts in fake Tudor, others glazed with pastel gravel chips, with tubs of miniature fir trees set on front yards of coloured concrete; others bulge with bays and sunset gates and stained-glass window-panes.

On either side of the pavement, under the trees, is a continuous green strip, as neat as the front gardens and their flower-beds. The wide road is swept and garnished every week; nevertheless, in this spring weather, the gutters are choked with pink blossom, with furry flower dust. Well-dressed children venture from their homes to build the clean rubbish into castles and heaps, squat to poke at the drains with budding green sticks. The prams are out, pale frilly battleships done up with bows, and with nets to keep cats and birds away. Some are parked in front gardens, and whether occupied or not it is hard to say, since here babies do not seem to cry; others are wheeled along the road by au pair girls, and again, whether it is the pram or the baby which is taken out for an airing, no one can say, for the pram's sunshade is down, and its front veiled in gauze.

It is three in the afternoon, that peaceful, somnolent, after-lunch time. Helen pads down the hill, her blue aertex sports shirt flattening agreeably against her tiny sprouts of breasts in the warm wind. She is dreaming of sports day next term, when two hundred parents will see her win the girls hundred yards race; she is dreaming of teatime today, of marmite and cream-cheese sandwiches in the back garden under the apple trees, the sofa swing newly out from its winter sleep in the shed, and her mother, in a flowered cotton frock, settling the wide wooden tea-tray on the shabby wrought-iron table under the kitchen window. Her mother will perhaps have driven the big white family saloon up the hill to Grodzinski's to buy egg bread in seeded plaits, or, for a special treat, soggy dark chocolate cake flavoured with black cherries and rum, or vanilla ice-cream topped with walnuts and golden syrup. Her mother will listen raptly, utterly spellbound, to Helen's tale of the match; she will drink tea from a blue willow-pattern cup with a slice of lemon floating on the top; she will turn to her husband and son, full of pride in her daughter, to await their applause.

Then, as Helen turns the corner into her street, she sees the crazy man waiting for her.

She clutches her hockey stick tight. He dances on the pavement between her front-garden gate and the little square busy with shrubs which separates their house from the road, the folds of his open overcoat flapping loosely, his arms outstretched to catch her.

This has been happening regularly for six months now, on those afternoons when Helen walks home alone. It is a favourite game of Helen's, this encounter: enjoyable, because kept a secret from her brother and parents; daring, because played with a stranger; safe, because she always manages to get away. The sense of herself as agile and strong, which she has begun to develop through netball, hockey and tennis at school, is here confirmed, with an excitement added for which she has no name. Invincible and perfect, she dodges him cheerfully, bounds under his arm and runs through the privet arch framing the open gate, up the path bordered with neatly tended lavender bushes, and around the side of the house to the back door. Words are already spilling off her tongue: Mummy, our side won, I scored a goal, Miss Parry said I've improved.

In the same second, she realises that the house is empty.

It confronts her blandly, enquiring why she should be so surprised to find doors and windows locked, all motion of curtains stilled, the garden hushed. The shock of this unexpected setback sets her heart racing, her knees trembling, blood rushing to her face and banging in her ears. She is going to be sick. She has been abandoned by her family. They have gone somewhere far away without her, have forgotten all about her. She has lost them, and will never see them again.

Then sanity returns. She remembers that it is Saturday, the day that the family goes off for the traditional afternoon tea at her grandparents' house a mile away. She was supposed to go there straight from the match.

With this realisation come shock and dismay. She will be late, she, the triumphant heroine of the hockey match. The glories of the playing field drop away; she is merely a child feeling frightened, feeling a fool at the thought of being late, at the thought of her parents worrying about where she is. Helen is never late home from anywhere; by being perfect in her obedience she manages to feel superior to the rules that require it. Now, for the first time in her life, she has let herself down: her parents have no idea where she is.

Hasty with panic, she turns to run back to the front of the house and on to the street again, but hesitates, remembering the crazy man lurking there, suddenly no longer her friend. She is uncertain, this time, that she will be able to get past him. She turns back and climbs the low fence by the back door into the back garden, dodges through rose bushes and under young weeping willows, hauls herself over the fence into the neighbour's garden, and climbs their fence in turn to bring her out into the crescent enclosing her own street and from which she can escape without being seen.