One

This is it, he thought. The moment of my death. And he was not afraid; he felt a little foolish, that he had dodged the hammer blow that marked the end of humankind only to be snuffed out by a pratfall. He closed his eyes. The rats could have him, but not while he was breathing.

A gust of wind caught him, fed him to the wall with a smack. He snatched at the parapet and was dismayed at how the coping stones shifted under his fingers. Thankful for the first time at how much weight he had lost, he managed to hoist himself onto the roof before the stones slipped free. He paused for breath and took off, scuttling onto the rooftops of Regent Street before descending at Piccadilly Circus. He circled back and picked up his rucksack; the ledge writhed with shadow. It put a chill through the girdle of bone around his groin to see how far up the window was.

He took a detour around to the front of the hotel and with a blue stick of chalk slashed the orange mark with shaking hands, thinking all the while that the open doorway would suddenly bloat with the slithering bodies of millions of rats. Then he began walking north-west.

Every morning was a different route, more or less, to the same destination. He must have trodden the streets around W9 to a point where he'd be able to see his own path worn out of the pavements. Hunger, exhaustion and a build-up of lactic acid in his muscles conspired to bring him to the point of collapse. That and the projected disappointment. But he could not give up on his son. He would never give up.

His fingers strayed to the pocket of his jeans. The letter was there; he could make out its edges. It had become so worn by his constant folding and unfolding over the years, the salt and oil from his skin, that he'd ended up sealing it in a plastic wallet that he'd found in the Cabinet War Rooms under Whitehall. He felt cheated somehow, as if the closeness to his son was reduced by these microns of polythene. His fingertips could no longer press against the ink that had been so close to Stanley's own. They could no longer feel the faint pattern of words in the paper that had been shaped by his son's brain. He felt as if he were being gradually detached from him, like the dovetail join in two pieces of wood that has begun to fail over time.

Each step he took was accompanied by a crunch of broken glass; the boots he'd found on a platform at Paddington station had become studded with splinters picked up as he crossed the basin and its density of office blocks that was now little more than a desert of glittering scimitars and framework skeletons. He hurried under the Marylebone Flyover, shadowing the water, so aware of the pools of darkness that he felt himself involuntarily shrink, as if he were trying to withdraw like something crushed on a beach, retreating into a shell that was no longer there.

The bodies crammed into the basin had turned it into a slurry of rotting clothes, hair and fat. It disguised his own smell. You had to grasp that wherever you could, despite the inevitable unpleasantness. He skirted Little Venice and its houseboats, all of them floating coffins with their blinds drawn on secrets he wanted no knowledge of. People at a café were tumbled over their empty plates as if drunk, dull bones and mirrored grins, everybody having a whale of a time.

Stanley wasn't home. Jane waited outside the door, kidding himself that he could hear Stanley playing with his battery-powered trains and wooden track or pestering Cherry for a beaker of apple juice. He stood at the door, listening. He stood at the foot of the stairs, listening. He stopped at every landing and waited. The building was empty.

He pushed his way into his son's bedroom. Plaster and glass on the floor. Part of the ceiling was bowed and cracked; water created stalactites at its lowest point. The posters on the walls had been bleached to white oblongs; he could not remember what had hung there.

What do you want to be when you grow up, Stanley?

Lying in bed, this tiny boy, the duvet up to his chin. Hand on his soft blond hair, the heat of him rising through his fingers. A smell of soap. Sleepy-eyed. Some toy, some cheap piece of plastic that was his current favourite twisting in his fingers. Warm. Happy.

Umm, I want to be an actor in Star Wars. Because I want a real light saver.

Would you protect your mum and me if you had a light sabre?

It's light saver, Dad. But yeah, I'd cut Darth Vader's head off if he tried to hit you. And then push him off a clift.

A cliff?

No, I said a clift.

Good to know, Stan. Thanks.