Soon after dawn the next morning, Robert Laing sat on his balcony on the 25th floor, eating a frugal breakfast and listening to the first sounds of activity in the apartments around him. Already a few residents were leaving the building on their way to work, picking their way through the debris underfoot towards their garbage-speckled cars. Several hundred people still left each day for their offices and studios, airports and auction-rooms. Despite the scarcity of water and heating, the men and women were well dressed and groomed, their appearance giving no hint of the events of the previous weeks. However, without realizing it, many of them would spend much of their time at their offices asleep at their desks.

Laing ate his slice of bread with methodical slowness. Sitting there on the cracked balcony tiles, he felt like a poor pilgrim who had set out on a hazardous vertical journey and was performing a simple but meaningful ritual at a wayside shrine.

The previous night had brought total chaos-drunken parties, brawls, the looting of empty apartments and assaults on any isolated resident. Several more floors were now in darkness, including the 22nd, where his sister Alice lived. Hardly anyone had slept. Amazingly, few people showed any signs of fatigue, as if the economy of their lives was switching from day to night. Laing half-suspected that the insomnia so many of his neighbours had suffered had been some kind of unconscious preparation for the emergency ahead. He himself felt alert and confident-despite the bruises on his shoulders and arms, he was physically in fine trim. At eight o'clock he intended to clean himself up and leave for the medical school.

Laing had spent the early part of the night straightening Charlotte Melville's apartment, which had been ransacked by intruders while she and her small son were sheltering with friends. Later, he had helped to guard an elevator which his neighbours had seized for a few hours. Not that they had gone anywhere-having commandeered the elevator what mattered was to hold it for an effective psychological interval.

The evening had begun, as usual, with a party held by Paul Crosland, television newsreader and now clan chief. Crosland had been delayed at the studios, but his guests watched him deliver the nine-o'clock news, speaking in his familiar, well-modulated voice about a rush-hour pile-up in which six people had died. As his neighbours stood around the television set, Laing waited for Crosland to refer to the equally calamitous events taking place in the high-rise, the death of the jeweller (now totally forgotten), and the division of the tenants into rival camps. Perhaps, at the end of the newscast, he would add a special message for his clan members at that moment fixing their drinks among the plastic rubbish-sacks in his living-room.

By the time Crosland arrived, swerving into the apartment in his fleece-lined jacket and boots like a returning bomber pilot, everyone was drunk. Flushed and excited, Eleanor Powell swayed up to Laing, pointing hilariously at him and accusing him of trying to break into her apartment. Everyone cheered this news, as if rape was a valuable and well-tried means of bringing clan members together.

"A low crime-rate, doctor," she told him amiably, "is a sure sign of social deprivation."

Drinking steadily and without any self-control, Laing felt the alcohol bolt through his head. He knew that he was deliberately provoking himself, repressing any reservations about the good sense of people such as Crosland. On a practical level, being drunk was almost the only way of getting close to Eleanor Powell. Sober, she soon became tiresomely maudlin, wandering about the corridors in a vacant way as if she had lost the key to her own mind. After a few cocktails she was hyper-animated, and flicked on and off like a confused TV monitor revealing glimpses of extraordinary programmes which Laing could only understand when he was drunk himself. Although she kept overruling everything he said, tripping over the plastic garbage-sacks under the bar, he held her upright, excited by the play of her hands across his lapels. Not for the first time Laing reflected that he and his neighbours were eager for trouble as the most effective means of enlarging their sex lives.

Laing emptied the coffee-percolator over the edge of the balcony. A greasy spray hung across the face of the building, the residue of the cascade of debris now heaved over the side without a care whether the wind would carry it into the apartments below. He carried his breakfast tray into the kitchen. The continuing failure of the electricity supply had destroyed the food in the refrigerator. Bottles of sour milk stood in a mould-infested line. Rancid butter dripped through the grilles. The smell of this rotting food was not without its appeal, but Laing opened a plastic sack and scooped everything into it. He slung the sack into the corridor, where it lay in the dim light with a score of others.

A group of his neighbours was arguing in the elevator lobby, voices raised. A minor confrontation was developing between them and the 28th-floor residents. Crosland was bellowing aggressively into the empty elevator shaft. Usually, at this early hour of the day, Laing would have paid no attention to him. Too often Crosland had no idea what he was arguing about-confrontation was enough. Without his make-up, the expression of outrage on his face made Crosland resemble an announcer tricked for the first time into reading an item of bad news about himself.

From the shadows outside his door the orthodontic surgeon emerged with studied casualness. Steele and his hard-faced wife had been standing among the garbage-sacks for some time, keeping an eye on everything. He sidled up to Laing and took his arm in a gentle but complex grip, the kind of hold he might have used for an unusual extraction. He pointed to the floors above.

"They want to seal the doors permanently," he explained. "They're going to re-wire two of the elevator circuits so that they move non-stop from the ground floor to the 28th."

"What about the rest of us?" Laing asked. "How do we leave the building?"

"My dear Laing, I don't suppose they care very much about us. Their real intention is to divide the building in half-here, at the 25th floor. This is a key level for the electrical services. By knocking out the three floors below us they will have a buffer zone separating the top half of the building from the lower. Let's make sure, doctor, that when this happens we are on the right side of the buffers…"

He broke off as Laing's sister approached, carrying her electric coffee-pot. With a bow, Steele moved away through the shadows, his small feet stepping deftly among the garbage sacks, the centre parting of hair gleaming in the faint light. Laing watched him slide noiselessly into his apartment. No doubt Steele would pick his way with equal skill through the hazards ahead. He never left the building now, Laing had noticed. What had happened to that ruthless ambition? After the battles of the past weeks he was presumably banking on an imminent upsurge in the demand for advanced surgery of the mouth.

As Laing greeted Alice he realized that she too would be excluded if the surgeon was right, living in the darkness on the wrong side of the dividing line with her alcoholic husband. She had come up ostensibly to plug her coffeepot into the power point in Laing's kitchen, but when they entered the apartment she left it absently on the hall table. She walked on to the balcony and stared into the morning air, as if glad to have the three extra floors beneath her.

"How is Charles?" Laing asked. "Is he at the office?"

"No… He's taken some leave. Terminal, if you ask me. What about you? You shouldn't neglect your students. At the present rate we're going to need every one of them."

"I'm going in this morning. Would you like me to have a look at Charles on my way?"

Alice ignored this offer. She grasped the handrail and began to rock herself like a child. "It's peaceful up here. Robert, you've no idea what it's like for most people."

Laing laughed aloud, amused by Alice's notion that somehow he had been unaffected by events in the high-rise-the typical assumption of a martyred older sister forced during her childhood to look after a much younger brother.

"Come whenever you want to." Laing put his arm around her shoulders, steadying her in case she lost her balance. In the past he had always felt physically distanced from Alice by her close resemblance to their mother, but for reasons not entirely sexual this resemblance now aroused him. He wanted to touch her hips, place his hand over her breast. As if aware of this, she leaned passively against him.

"Use my kitchen this evening," Laing told her. "From what I've heard, everything is going to be chaotic. You'll be safer here."

"All right-but your apartment is so dirty."

"I'll clean it for you."

Checking himself, Laing looked down at his sister. Did she realize what was happening? Without intending to, they were arranging an assignation.

All over the high-rise people were packing their bags, readying themselves for short but significant journeys, a few floors up or down, laterally to the other end of a corridor. A covert but nonetheless substantial movement of marital partners was taking place. Charlotte Melville was now involved with a statistician on the 29th floor, and had almost vacated her apartment. Laing had watched her leave without resentment. Charlotte needed someone who would bring out her forcefulness and grit.

Thinking about her, Laing felt a pang of regret that he himself had found no one. But perhaps Alice would give him the practical support he needed, with her now unfashionable dedication to the domestic virtues. Although he disliked her shrewish manner, with its unhappy reminders of their mother, it gave him an undeniable sense of security.

Holding her shoulders, he looked up at the roof of the high-rise. It seemed months since he had last visited the observation deck, but for the first time he felt no urge to do so. He would build his dwelling-place where he was, with this woman and in this cave in the cliff face.

When his sister had gone, Laing began to prepare for his visit to the medical school. Sitting on the kitchen floor, he looked up at the unwashed plates and utensils stacked in the sink. He was leaning comfortably against a plastic sack filled with rubbish. Seeing the kitchen from this unfamiliar perspective, he realized how derelict it had become. The floor was strewn with debris, scraps of food and empty cans. To his surprise, Laing counted six garbage-sacks-for some reason he had assumed that there was only one.