He moved down to the 13th-floor landing. Through the walls of the elevator shaft he could hear the rails and cables humming. Passengers were stepping out of the elevators on to their floors. But the doors into the 13th-floor lobby had been bolted. A face frowned out at him, a well-groomed hand curtly waved him away.

All the way down to the 10th floor the communicating doors had been locked or barricaded. Frustrated, Wilder returned to the shopping mall. A large crowd was still waiting by the elevators. They formed clearly demarked groups from different floors, each commandeering its own transit system.

Wilder left them and strode towards the supermarket. The shelves had been stripped, and the staff had left after locking the turnstiles. Wilder vaulted over a check-out counter and made his way to the store-room at the rear. Beyond the pyramids of empty cartons was one of the three service cores of the high-rise, containing a freight elevator, and the water, air-conditioning and electrical supply trunks.

Wilder waited as the elevator descended cumbrously down its shaft. The size of a carrier's aircraft lift, it had been designed to carry kitchen-appliance islands, bathroom units, and the huge pop-art and abstract-expressionist paintings favoured by the residents of the high-rise.

As he pulled back the steel grille he noticed a thin-shouldered young woman hiding behind the control panel. She was pallid and undernourished, but she watched Wilder with interest, as if glad to welcome him to this private domain.

"How far do you want to go?" she asked him. "We can travel anywhere. I'll ride with you."

Wilder recognized her as a masseuse from the 5th floor, one of the vagrants who spent their time wandering around the high-rise, the denizens of an interior world who formed a second invisible population. "All right-what about the 35th floor?"

"The people on the 30th are nicer." Expertly she pressed the control buttons, activating the heavy doors. Within seconds the elevator was carrying them ponderously aloft. The young masseuse smiled at him encouragingly, alive now that they were moving. "If you want to go higher, I'll show you. There are a lot of air-shafts, you know. The trouble is, dogs have got into them-they're getting hungry…"

An hour later, when Wilder stepped out into the lavishly carpeted lobby of the 37th floor, he realized that he had discovered a second building inside the one that he had originally occupied. He left behind the young masseuse, endlessly climbing the service shafts and freight wells of the high-rise, transits that externalized an odyssey taking place inside her head. During his roundabout route with her-changing to a second freight elevator to climb three floors to the 28th, moving up and down a maze of corridors on the borders of hostile enclaves, until finally taking an upper-level elevator a journey of one storey-Wilder had seen the way in which the middle and upper levels of the building had organized themselves.

While his neighbours on the lower floors remained a confused rabble united only by their sense of impotence, here everyone had joined a local group of thirty adjacent apartments, informal clans spanning two or three floors based on the architecture of corridors, lobbies and elevators. There were now some twenty of these groups, each of which had formed local alliances with those on either side. There was a marked increase in vigilante activity of all kinds. Barriers were being set up, fire-doors locked, garbage thrown down the stairwells or dumped on rival landings.

On the 29th floor Wilder came across a commune composed exclusively of women, a cluster of apartments dominated by an elderly children's-story writer, a woman of intimidating physique and personality. Sharing an apartment with her were three air-hostesses from the 1st floor. Wilder walked gingerly down the corridor between their apartments, glad of the company of the young masseuse. What unsettled Wilder, as the women questioned him in pairs from their half-open doors, was their hostility to him, not only because he was a man, but because he was so obviously trying to climb to a level above their own.

He stepped out with relief into the deserted lobby of the 37th floor. He stood by the staircase doors, suspicious that no one was guarding the lobby. Conceivably the residents here were unaware of what was going on beneath their feet. The carpets in the silent corridors were thick enough to insulate them from hell itself.

He walked down the corridor towards Jane Sheridan's apartment. She might be surprised to see him, but Wilder was confident that he would spend the night with her. The next day he would move in permanently, and visit Helen and the boys on his way to and from the television studios.

As he pressed the bell he could hear her strong, masculine voice through the door, its tone familiar from countless television costume-dramas. At last the door opened, held on its latch chain. When she looked out at Wilder, recognizing him immediately, he knew that she had been waiting for him to arrive. She was detached and uneasy at the same time, like a spectator forced to watch someone about to be involved in an accident. Wilder remembered that he had given his destination to one of the women's vigilante groups.

"Jane, you're expecting me. I'm flattered."

"Wilder… I can't-"

Before Wilder could speak the door of the next apartment opened sharply. Staring at Wilder with undisguised hostility were a tax specialist from the 40th floor and an over-muscled choreographer with whom Wilder had often heaved a medicine ball in the 10th-floor gymnasium.

Realizing that his arrival had been anticipated by all these people, Wilder turned to leave, but the corridor behind him was blocked. A group of six residents had emerged together from the elevator lobby. They wore track suits and white sneakers, and at first sight looked like a middle-aged gymnasium dumb-bell team, each carrying his polished wooden clubs. Leading this antique but spritely troupe, which consisted of a stockbroker, two paediatricians and three senior academics, was Anthony Royal. As usual he wore his white safari-jacket, a costume which always irritated Wilder, the kind of garment that might be affected by an eccentric camp-commander or zoo-keeper. The corridor lighting flushed his blond hair and picked out the scars on his forehead, a confusing notation that hung like a series of mocking question marks over his stern expression. As he approached Wilder the chromium walking-stick flicked in his hand like a cane. Wilder watched the polished shaft catch the light, looking forward with pleasure to wrapping it around Royal's neck.

Although well aware that he had been trapped, Wilder found himself laughing aloud at the sight of this lunatic troupe. When the lights failed, first dipping warningly and then going out altogether, he backed against the wall to allow the group to pass. The wooden clubs clicked around him in the darkness, beating out a well-rehearsed tattoo. From the open door of Jane Sheridan's apartment a torch flared at him.

Around Wilder the dumb-bell troupe was beginning its act. The first clubs whirled in the torch-light. Without any warning, he felt a flurry of blows on his shoulders. Before he fell Wilder seized one of the clubs, but the others struck him to the carpeted floor at Anthony Royal's feet.

When he woke he was lying outstretched on a sofa in the ground-floor entrance lobby. Fluorescent lights shone around him, reflected in the glass ceiling-panels. With their toneless glow they seemed to have been shining for ever somewhere inside his head. Two residents returning late to the high-rise waited by the elevators. Holding tightly to their briefcases, they ignored Wilder, whom they clearly assumed to be drunk.

Aware of his bruised shoulders, Wilder reached up and nursed the swollen mastoid bone behind his right ear. When he could stand, he wandered away from the sofa towards the entrance and steadied himself against the glass doors. The lines of parked cars stretched through the darkness, enough transport to evacuate him to a thousand and one destinations. He walked out into the cold night air. Holding his neck, he looked up at the face of the high-rise. He could almost pick out the lights of the 37th floor. He felt suddenly exhausted, as much by the building's weight and mass as by his own failure. His casual and unthought-out attempt to scale the building had ended humiliatingly. In a sense he had been rejected more by the high-rise than by Royal and his friends.

Lowering his eyes from the roof, he saw that his wife, fifty feet above him, was watching from the balcony of their apartment. Despite his dishevelled clothes and bruised face she showed no concern, as if she no longer recognized him.