"What's he doing?" said Specialist Stone, mostly making conversation. They were four hours into a night shift and her companion had spent much of this drumming his fingers impatiently on a table. He wanted his bed, MTV and oblivion. This shift was Master Sergeant Saez's way of telling the man he probably shouldn't have kicked the prisoner when Miles Alsdorf was there.

"Eh?" The thickset marine glanced up from his fingers and checked the screen. On it Prisoner Zero was knelt where his bed should be, his head almost touching the floor, his fingers scrabbling at something unseen.

"Reckon he's lost it?"

"Fucked if I know. When did this start?"

"About five minutes ago." Specialist Stone was lying. She had no idea when Prisoner Zero had begun this latest routine. She'd been too busy watching Corporal Thompson out of the corner of her eye.

"Wake the Master Sergeant," said Corporal Thompson, and Specialist Stone looked at him, then saluted. "Yes, sir," she said. Her smile lasted most of the way to the Sergeant's quarters.

At Miles Alsdorf's suggestion the marines had allowed Prisoner Zero a new mattress and blanket. Well, an old mattress really, stuffed with horsehair and worn down to its warp and weft along one seam. It was stained in the way old mattresses seem to get stained with a lifetime's worth of precipitous periods, spilt coffee, babies made, born and then grieved over.

Prisoner Zero wasn't sure why that mattress had been chosen. Maybe it was all the marines could find at short notice or perhaps Sergeant Saez really believed it was the most disgusting thing possible. If so, he should have seen the squat in Amsterdam.

The blanket which came with the mattress was US issue, the colour of goose shit and machine-sewn along all four edges. A label glued to one corner claimed it was made from recycled plastic bottles, thus helping the environment.

Since it was July and the room in which the cage lived had only one window and this was sealed shut, the winter-weight blanket was as useless as it was unnecessary; but Miles Alsdorf had demanded his client be given a blanket and a mattress and Colonel Borgenicht had seen to it that he had.

So tightly was the mattress squeezed between the sides of Prisoner Zero's cage that it could only be edged out a little at a time. The prisoner then had to lift free the metal frame which supported it, raising one end until he could manoeuvre the other away from the brackets welded around one end of the cage to support his bed. All of this he had to do in silence.

The floor beneath his bed was steel mesh, plastic-coated like the rest and soldered at the edges to the frame of the cage. The darkness had suggested he begin his tunnel under the bed, where four tiles met. To help himself remember this, Prisoner Zero had scratched a cross into his arm to mark the inner edges of the four tiles and then run a circle around that point to indicate the tunnel.

Having cut free the tiles, he would need to tear his way through the mesh on which he knelt before he could prise the tiles from their setting. This created so many problems that Prisoner Zero decided he'd better worry about them later.

The difficulty for Prisoner Zero was that he needed space to walk in order to focus. Itchy inside his own skin, that was how one girl had put it a very long time ago. Nail him down, sit him in a café with a latte, a spliff or that day's paper and he would drift away into dreams, complex interplays of events misremembered, rewritten memories and occasional flashes of something Prisoner Zero used to think of as genius.

In the days before he realized he didn't rate that word.

"You think we ought to stop him?"

Corporal Thompson reached for a can of Pepsi Max, ripped the tab and shook his head. "You heard the Sergeant. The guy's nuts. Get over it." Master Sergeant Saez had been and gone, barely stopping long enough to glance at the screen.

Staying with the picture for only as long as it took him to finish his Pepsi, Corporal Thompson switched his attention back to the DC comic in front of him, leaving Prisoner Zero to scrabble helplessly against the mesh of the floor.

"This is getting bad," Specialist Stone said, when another five minutes had gone. "We should pass it up the line."

"Feel free." Corporal Thompson nodded at the house phone. "I'm sure Sergeant Saez will be delighted."

Ten minutes after that Specialist Stone came to a decision. One that would have had her cleaning shithouses for the rest of her career if Master Sergeant Saez had found out about it. She telephoned the Lieutenant.

"Sir, it's the prisoner. He's trying to tear up the floor of his cage..."

"With his fingers, sir."

"No sir, he's not getting away."

"Yes sir, the steel mesh is still in place."

"The guards are still outside his door, sir."

"Why did I call you? It was my mistake, sir." Specialist Stone stood very straight as she said this, listening while Lieutenant Ashcroft provided his own answer.

"Yes, sir. Sorry, sir."

"Told you," Corporal Thompson said, passing her that week's issue of Spider-Man.

Inside the cage the prisoner was wrestling with the steel frame that supported his mattress, prior to dragging that mattress back into position against a side wall so it could hide the entrance to his tunnel. He'd cut free the grout from around the tiles, just as the darkness instructed, and made a start at worrying his way through the mesh.

All he had to do now was make sure no one thought to look under his bed and to do that he needed to put his captors off entering the cage.

"It's obvious," said the darkness.

And it was.

As Prisoner Zero stripped off his paper jump suit and squatted next to the doorway to his cage, he ran over the map of Camp Freedom he kept in his head. He was trying to work out if the others were here. He wasn't sure who the others might be, but he was pretty certain who they weren't.

The Corporal, the one who liked kicking him, had mentioned that dozens of Prisoner Zero's co-conspirators had been rounded up. Which was odd, because the prisoner was pretty sure the darkness had talked to no one but him and he had talked to no one at all.

In all, he'd known maybe five people in Paris and most of those had politely ignored him. A state of existence Prisoner Zero had worked hard to achieve and he was beginning to regret upsetting its balance.

All the same, thought Prisoner Zero as he began to smear shit onto the mesh, what choice did he have? And how could he possibly explain that lack of choice to the endless, interchangeable people who sat across tables from him and asked questions to which the answers had to be obvious?

The US President had to die because the future demanded it. Prisoner Zero knew this to be true even if he could no longer remember exactly why. And if Prisoner Zero could not go to the President then the darkness really did require that he persuade the President to come to him.

It had something to do with history.