And almost at once, it seemed (though he'd been asleep for nearly three hours), he woke up knowing two things simultaneously.

First, he knew where the case was. And second, he knew that the men were downstairs, opening the kitchen door.

He lifted Moxie out of the way and softly hushed her sleepy protest. Then he swung his legs over the side of the bed and put on his shoes, straining every nerve to hear the sounds from downstairs. They were very quiet sounds: a chair being lifted and replaced, a short whisper, the creak of a floorboard.

Moving more silently than the men were, he left his bedroom and tiptoed to the spare room at the top of the stairs. It wasn't quite pitch-dark, and in the ghostly gray predawn light he could see the old treadle sewing machine. He'd been through the room thoroughly only hours before, but he'd forgotten the compartment at the side of the sewing machine, where all the patterns and bobbins were kept.

He felt for it delicately, listening all the while. The men were moving about downstairs, and Will could see a dim flicker of light that might have been a flashlight at the edge of the door.

Then he found the catch of the compartment and clicked it open, and there, just as he'd known it would be, was the leather writing case.

And now what could he do? He crouched in the dimness, heart pounding, listening hard.

The two men were in the hall downstairs. He heard one of them say quietly, "Come on. I can hear the milkman down the road."

"It's not here, though," said the other voice. "We'll have to look upstairs."

"Go on, then. Don't hang about."

Will braced himself as he heard the quiet creak of the top step. The man was making no noise at all, but he couldn't help the creak if he wasn't expecting it. Then there was a pause. A very thin beam of flashlight swept along the floor outside. Will saw it through the crack.

Then the door began to move. Will waited till the man was framed in the open doorway, and then exploded up out of the dark and crashed into the intruder's belly.

But neither of them saw the cat.

As the man had reached the top step, Moxie had come silently out of the bedroom and stood with raised tail just behind the man's legs, ready to rub herself against them. The man, who was trained and fit and hard, could have dealt with Will, but the cat was in the way, and as the man tried to move back, he tripped over her. With a sharp gasp he fell backward down the stairs and crashed his head brutally against the hall table.

Will heard a hideous crack, and didn't stop to wonder about it. Clutching the writing case, he swung himself down the banister, leaping over the man's body that lay twitching and crumpled at the foot of the flight, seized the tattered tote bag from the table, and was out of the front door and away before the other man could do more than come out of the living room and stare.

Even in his fear and haste Will wondered why the other man didn't shout after him, or chase him. They'd be after him soon, though, with their cars and their cell phones. The only thing to do was run.

He saw the milkman turning into the close, the lights of his electric cart pallid in the dawn glimmer that was already filling the sky. Will jumped over the fence into the next-door garden, down the passage beside the house, over the next garden wall, across a dew-wet lawn, through the hedge, and into the tangle of shrubs and trees between the housing estate and the main road. There he crawled under a bush and lay panting and trembling. It was too early to be out on the road: wait till later, when the rush hour started.

He couldn't get out of his mind the crack as the man's head struck the table, and the way his neck was bent so far and in such a wrong way, and the dreadful twitching of his limbs. The man was dead. He'd killed him.

He couldn't get it out of his mind, but he had to. There was quite enough to think about. His mother: would she really be safe where she was? Mrs. Cooper wouldn't tell, would she? Even if Will didn't turn up as he'd said he would? Because he couldn't, now that he'd killed someone.

And Moxie. Who'd feed Moxie? Would Moxie worry about where they were? Would she try to follow them?

It was getting lighter by the minute. It was light enough already to check through the things in the tote bag: his mother's purse, the latest letter from the lawyer, the road map of southern England, chocolate bars, toothpaste, spare socks and pants. And the green leather writing case.

Everything was there. Everything was going according to plan, really.

Except that he'd killed someone.

Will had first realized his mother was different from other people, and that he had to look after her, when he was seven. They were in a supermarket, and they were playing a game: they were allowed to put an item in the cart only when no one was looking. It was Will's job to look all around and whisper "Now," and she would snatch a tin or a packet from the shelf and put it silently into the cart. When things were in there they were safe, because they became invisible.

It was a good game, and it went on for a long time, because this was a Saturday morning and the shop was full, but they were good at it and worked well together. They trusted each other. Will loved his mother very much and often told her so, and she told him the same.

So when they reached the checkout Will was excited and happy because they'd nearly won. And when his mother couldn't find her purse, that was part of the game too, even when she said the enemies must have stolen it; but Will was getting tired by this time, and hungry too, and Mummy wasn't so happy anymore. She was really frightened, and they went around and around putting things back on the shelves, but this time they had to be extra careful because the enemies were tracking them down by means of her credit card numbers, which they knew because they had her purse…

And Will got more and more frightened himself. He realized how clever his mother had been to make this real danger into a game so that he wouldn't be alarmed, and how, now that he knew the truth, he had to pretend not to be frightened, so as to reassure her.

So the little boy pretended it was a game still, so she didn't have to worry that he was frightened, and they went home without any shopping, but safe from the enemies; and then Will found the purse on the hall table anyway. On Monday they went to the bank and closed her account, and opened another somewhere else, just to be sure. Thus the danger passed.

But sometime during the next few months, Will realized slowly and unwillingly that those enemies of his mother's were not in the world out there, but in her mind. That made them no less real, no less frightening and dangerous; it just meant he had to protect her even more carefully. And from the moment in the supermarket when he had realized he must pretend in order not to worry his mother, part of Will's mind was always alert to her anxieties. He loved her so much he would have died to protect her.

As for Will's father, he had vanished long before Will was able to remember him. Will was passionately curious about his father, and he used to plague his mother with questions, most of which she couldn't answer.

"Was he a rich man?"

"Where did he go?"

"Why did he go?"

"Is he dead?"

"Will he come back?"

"What was he like?"

The last question was the only one she could help him with. John Parry had been a handsome man, a brave and clever officer in the Royal Marines, who had left the army to become an explorer and lead expeditions to remote parts of the world. Will thrilled to hear about this. No father could be more exciting than an explorer. From then on, in all his games he had an invisible companion: he and his father were together hacking through the jungle, shading their eyes to gaze out across stormy seas from the deck of their schooner, holding up a torch to decipher mysterious inscriptions in a bat-infested cave… They were the best of friends, they saved each other's life countless times, they laughed and talked together over campfires long into the night.