It begins at Avebury, in the late July of a cool, wet summer turned suddenly warm and dry. The Marlborough Downs shimmer in a haze of unfamiliar heat. Skylarks sing in the breezeless air above the sheep-cropped turf. The sun burns high and brazen. And the stones stand, lichened and eroded, sentinels over nearly five thousand years of history.

It begins, then, in a place whose origins and purposes are obscured by antiquity. Why Neolithic henge-builders should have devoted so much time and effort to constructing a great ramparted stone circle at Avebury, as well as a huge artificial hill less than a mile away, at Silbury, is as unknown as it is unknowable.

It begins, therefore, in a landscape where the unexplained and the inexplicable lie still and close, where man-made markers of a remote past mock the set and ordered world that is merely the flickering, fast-fleeing present.

Saxon settlers gave Avebury its modern name a millennium and a half ago. They founded a village within its protective ditch and bank. Over the centuries, as the village grew, many of the stones were moved or buried. Later, they were used as building material, the ditch as a rubbish-dump. The henge withered.

Then, in the 1930s, came Alexander Keiller, the marmalade millionaire and amateur archaeologist. He bought up and demolished half the village, raised the stones, cleared the ditch, restored the circle. The clock was turned back. The National Trust moved in. The henge flourished anew – a monument and a mystery.

Nearly forty years have passed since the Trust's purchase of Keiller's land holdings at Avebury. The renovated circle basks unmolested in the heat of a summer's day. A kestrel, soaring high above on a thermal, has a perfect view of the banked circumference of the henge, quartered by builders of later generations. The High Street of the surviving village runs west-east along one diameter, crossing the north-south route of the Swindon to Devizes road close to the centre of the circle. East of this junction, the buildings peter out as the effects of Keiller's demolition work become more apparent. Green Street, the lane is aptly called, dwindling as it leaves the circle and winds on towards the downs.

As it passes through the village, the main road performs a zigzag, the north-western angle of which is occupied by the thatched and limewashed Red Lion Inn. East of the inn, on the other side of the road, are the fenced-off remains of an inner circle known as the Cove – two stones, one tall and slender, the other squat and rounded, referred to locally as Adam and Eve. There is a gate in the fence, opposite the pub car park, and another gate in Green Street, on the other side of Silbury House, a four-square corner property that formerly served as the residence of Avebury's Nonconformist minister.

It is a little after noon on this last Monday of July, 1981. Custom is sparse at the Red Lion and visitors to the henge are few. When the traffic noise ebbs, as it periodically does, somnolence prevails. There is a stillness in the air and in the scene. But it is not the stillness of expectancy. There is no hint, no harbinger, of what is about to occur.

At one of the outdoor tables in front of the Red Lion, a solitary drinker sits cradling a beer glass. He is a slim, dark-haired man in his mid-twenties, dressed in blue jeans and a pale, open-necked shirt rolled up at the elbows. Beside him, on the table, lie a spiral-bound notebook and a ballpoint pen. He is gazing vacantly ahead of him, across the road, towards the remaining stones of the southern inner circle. They do not command his attention, however, as a glance at his wristwatch reveals. He is waiting for something, or someone. He takes a slurp of beer and sets the glass down on the table. It is nearly empty. Sunlight glistens on the swirling residue.

A child's voice catches his ear, drifting across from the Cove. There is, at this moment, no traffic to mask the sound. The man turns and looks. He sees a woman and three children approaching the Cove from the direction of the perimeter bank. Two of the children are running ahead, racing, perhaps, to be first to the stones: a boy and a girl. The boy is nine or ten, dressed in baseball boots, blue jeans and a red T-shirt. The girl is a couple of years younger. She is wearing sandals, white socks and a blue and white polka-dot dress. Both have fair hair that appears blond in the sunshine, cut short on the boy but worn long, in a ponytail, by the girl. The woman is lagging well behind, her pace set by the youngest child, toddling at her side. This child, a girl, is wearing grey dungarees over a striped T-shirt. There can hardly be any doubt, given the colour of her hair, tied in bunches with pink ribbon, that she is the sister of the other two children.

It is much less likely that the woman escorting her is their mother. She appears too young for the role, slim, fine-featured and dark-haired, surely not beyond her early twenties. She is dressed in cream linen trousers and a pink blouse and is carrying a straw hat. Her attention is fixed largely on the little girl beside her. The other two children are dashing ahead.

As they approach the stones, a figure steps out from the gap between Adam and Eve, hidden till then from view. He is a short, tubby man in hiking boots, brown shorts, check shirt and some kind of multi-pocketed fisherman's waistcoat. He is round-faced, balding and bespectacled, aged anything between thirty-five and fifty. The two children stop and stare at him. He says something. The boy replies and moves forward.

The man outside the Red Lion watches for lack of anything more interesting to watch. He sees nothing sinister or threatening. What he does see is a flash of sunlight on glass as the man by the stones takes something out of one of his numerous pockets. The boy steps closer.

The woman is hurrying to join them now, not running, nor even necessarily alarmed, but cautious perhaps, her attention suddenly diverted from the slow-moving infant who follows at her own dawdling pace, before abruptly sitting down on the grass to inspect a patch of buttercups.

The man outside the Red Lion sees all of this and makes nothing of it. Even when another figure enters his field of vision from behind Silbury House, he does not react. The figure is male, short-haired and stockily built. He is wearing Army surplus clothes and is moving fast, at a loping run, across the stretch of grass beyond the stones. The woman, who cannot see him moving behind her, is smiling now and talking to the man in the fisherman's waistcoat.

And then it happens. The running man stops and bends over, grasps the seated child beneath her arms, lifts her up as if she weighs little more than the buttercup in her left hand and races back with her the way he came.

The man in the fisherman's waistcoat is first to respond. He says something to the woman, raising his voice and pointing. She turns and looks. She puts her hand to her mouth. She drops her hat and begins running after the man who has grabbed the child. Screened as he is by Silbury House, he can no longer be seen by the man outside the Red Lion. The roaring passage of a southbound lorry further confuses the senses. Everything is happening very quickly and very slowly. The beer-drinker does no more than rise from his seat and gape as the next minute's events spray their poison over all who witness them.