"Was it fortunate that I arrived?" The jeweller peered at Laing, as if puzzled by his presence in this private domain. "You're here to play squash with Anthony Royal? I'm afraid he's decided to decline." He added, as much to himself as to Laing, "My wife should have been here. She was treated appallingly, you know-they were like animals…"

Slightly shaken, Laing accompanied him to the stairway. He looked back at the cocktail party, with its well-bred guests, uncertain whether he had imagined the imminent attack on him. After all, what could they have actually done-hardly tossed him over the edge?

As he pondered this, he noticed a familiar pale-haired figure in a white safari-jacket standing with one hand on the callisthenics machine in the penthouse overlooking the northern end of the roof. Resting at his feet was Royal's alsatian with its arctic coat, without doubt the premier dog in the high-rise. Making no attempt to hide himself, Anthony Royal was watching Laing with a thoughtful gaze. As always, his expression was an uneasy mixture of arrogance and defensiveness, as if he were all too aware of the built-in flaws of this huge building he had helped to design, but was determined to outstare any criticism, even at the price of theatrical gestures such as the alsatian and his white-hunter's jacket. Although he was over fifty, his shoulder-length fair hair made him look uncannily youthful, as if the cooler air at these great heights had somehow preserved him from the ordinary processes of ageing. His bony forehead, still marked by the scars of his accident, was tilted to one side, and he seemed to be checking that an experiment he had set up had now been concluded.

Laing raised one hand and signalled to him as the jeweller ushered him briskly below, but Royal made no reply. Why had he not cancelled their squash game by telephone? For a moment Laing was certain that Royal had deliberately let him come up to the roof, knowing that the party was in progress, simply out of interest in the guests' reactions and behaviour.

The next morning Laing rose early, eager to get on. He felt fresh and clear-headed, but without realizing why he decided to take the day off. Promptly at nine, after pacing about for two hours, he telephoned his secretary at the medical school and postponed that afternoon's supervision. When she expressed regret at Laing's illness he brushed this aside. "It's all right, I'm not ill. Something important has come up."

What? Puzzled by his own behaviour, Laing wandered around the small apartment. Charlotte Melville was also at home. She was dressed for the office in a formal business suit, but made no attempt to leave. She invited Laing over for coffee, but when he arrived an hour later she absent-mindedly handed him a glass of sherry. His visit, Laing soon discovered, was a pretext for him to examine her son. The boy was playing in his room, but according to Charlotte was not feeling well enough to go to the junior school on the 10th floor. Annoyingly, the young sister of an airline pilot's wife on the 1st floor had declined to baby-sit.

"It's a nuisance, she's usually only too keen. I've relied on her for months. She sounded rather vague on the phone, as if she was being evasive…"

Laing listened sympathetically, wondering whether he should volunteer to look after the child. But there was no hint of this in Charlotte's voice. Playing with the boy, Laing realized that there was nothing wrong with him. Lively as ever, he asked his mother if he could go to his 3rd-floor playgroup that afternoon. Without thinking, she refused. Laing watched her with growing interest. Like himself, Charlotte was waiting for something to happen.

They did not have long to wait. In the early afternoon the first of a fresh series of provocations took place between the rival floors, setting in motion again the dormant machinery of disruption and hostility. The incidents were trivial enough, but Laing knew already that they reflected deep-rooted antagonisms that were breaking through the surface of life within the high-rise at more and more points. Many of the factors involved had long been obvious-complaints about noise and the abuse of the building's facilities, rivalries over the better-sited apartments (those away from elevator lobbies and the service shafts, with their eternal rumbling). There was even a certain petty envy of the more attractive women who were supposed to inhabit the upper floors, a widely held belief that Laing had enjoyed testing. During the electricity blackout the eighteen-year-old wife of a fashion photographer on the 38th floor had been assaulted in the hairdressing salon by an unknown woman. Presumably in retaliation, three air-hostesses from the 2nd floor were aggressively jostled by ta party of marauding top-floor matrons led by the strong-shouldered wife of the jeweller.

Watching from Charlotte's balcony, Laing waited as the first of these incidents took place. Standing there with a pretty woman, a drink in one hand, he felt pleasantly light-headed. Below them, on the 9th floor, a children's party was in full swing. The parents made no attempt to restrain their offspring, in effect urging them to make as much noise as possible. Within half an hour, fuelled by a constant flow of alcohol, the parents took over from their children. Charlotte laughed openly as soft drinks were poured on to the cars below, drenching the windscreens and roofs of the expensive limousines and sports saloons in the front ranks.

These lively proceedings were watched by hundreds of residents who had come out on to their balconies. Playing up to their audience, the parents egged on their children. The party was soon out of control. Drunken children tottered about helplessly. High above them, on the 37th floor, a woman barrister began to shout angrily, outraged by the damage to her open-topped sports-car, whose black leather seats were covered with melting ice-cream.

A pleasant carnival atmosphere reigned. At least it made a change, Laing felt, from the formal behaviour of the high-rise. Despite themselves, he and Charlotte joined in the laughter and applause as if they were spectators at an impromptu amateur circus.

A remarkable number of parties were being held that evening. Usually, few parties took place other than at weekends, but on this Wednesday evening everyone was involved in one revel or another. Telephones rang continuously, and Charlotte and Laing were invited to no less than six separate parties.

"I ought to get my hair done." Charlotte took his arm happily, almost embracing Laing. "What exactly are we celebrating?"

The question surprised Laing. He held Charlotte's shoulder, as if protecting her. "God only knows-nothing to do with fun and games."

One of the invitations had come from Richard Wilder. Instantly, both he and Charlotte declined.

"Why did we refuse?" Charlotte asked, her hand still on the receiver. "He was expecting us to say no."

"The Wilders live on the and floor," Laing explained. "Things are rather rowdy down there…"

"Robert, that's a rationalization."

Behind Charlotte, as she spoke, her television set was showing the newsreel of an attempted prison break-out. The sound had been turned down, and the silent images of crouching warders and police, and the tiers of barricaded cells, nickered between her legs. Everyone in the high-rise, Laing reflected, watched television with the sound down. The same images glowed through his neighbours' doorways when he returned to his apartment. For the first time,people were leaving their front doors ajar and moving casually in and out of each other's apartments.

However, these intimacies did not extend beyond each resident's immediate floor. Elsewhere the polarization of the building proceeded apace. Finding that he had run out of liquor, Laing took the elevator down to the loth-floor concourse. As he expected, there was a heavy run on alcohol, and long lines of impatient residents stood outside the liquor store. Seeing his sister Alice near the counter, Laing tried to enlist her help. Without hesitating, she turned him down, and promptly launched into a vigorous denunciation of the tomfoolery that afternoon. In some way she clearly associated Laing with the lower-floor tenants responsible, identifying him with Richard Wilder and his rowdies.

As Laing waited to be served, what resembled a punitive expedition from the upper floors caused a fracas in the swimming-pool. A party of residents from the top three floors arrived in a belligerent mood. Among them was the actress whose Afghan hound had drowned in the pool. She and her companions began by fooling about in the water, drinking champagne on a rubber raft against the swimming-pool rules and splashing people leaving the changing cubicles. After a futile attempt to intercede, the elderly attendant gave up and retreated to his booth behind the diving-boards.

The elevators were full of aggressive pushing and heaving. The signal buttons behaved erratically, and the elevator shafts drummed as people pounded impatiently on the doors. On their way to a party on the 27th floor Laing and Charlotte were jostled when their elevator was carried down to the 3rd floor by a trio of drunken pilots. Bottles in hand, they had been trying for half an hour to reach the 10th floor. Seizing Charlotte good-humouredly around the waist, one of the pilots almost dragged her off to the small projection theatre beside the school which had previously been used for showing children's films. The theatre was now screening a private programme of blue movies, including one apparently made on the premises with locally recruited performers.

At the party on the 27th floor, given by Adrian Talbot, an effeminate but likeable psychiatrist at the medical school, Laing began to relax for the first time that day. He noticed immediately that all the guests were drawn from the apartments nearby. Their faces and voices were reassuringly familiar. In a sense, as he remarked to Talbot, they constituted the members of a village.

"Perhaps a clan would be more exact," Talbot commented. "The population of this apartment block is nowhere near so homogeneous as it looks at first sight. We'll soon be refusing to speak to anyone outside our own enclave." He added, "My car had its windscreen smashed this afternoon by a falling bottle. Could I move it back to where you people are?" As a qualified physician, Talbot was entitled to park in the ranks closest to the building. Laing, perhaps anticipating the dangers of proximity, had never made use of this concession. The psychiatrist's request was instantly granted by his fellow residents, an appeal to solidarity that no member of his clan could deny.