The footlights faded and left Nick in half-darkness, leaning against the cold glass, breathing hard. He tried to twist around without moving his feet and scanned the space above the cabinets for the telltale glow of movement.

He was so busy looking up he almost didn’t see it coming. Only a sixth sense – perhaps a reflection in the glass, or something in the corner of his eye – saved him. He glanced back the way he had come and froze. The lights were coming on, rippling forward one by one as the footsteps advanced. They spilled around the corner and lapped towards his feet like a rising tide. Then stopped.

The intruder must be just around the corner. Did he know Nick was there? Was he waiting to see if the lights came on again? Nick’s body screamed at him to run, though he knew it would mean certain death. But he couldn’t stand there.

The lights were still on. Nick could move without being detected, but only towards the danger. He fought back the fear.

Terrified that the lights would fade, he edged towards the corner of the cabinets, like a child shuffling towards the end of a high diving board. He crouched down, feeling the warmth of the lights on his face.

The lights flashed up again as the gunman stepped around the corner. Nick didn’t wait: he pushed off on his feet and launched himself forward. The man fired, but too high. Nick crashed into him and brought him down, then rolled away. He wouldn’t win any sort of fight. He kicked the gun out of the man’s hand, then scrambled to his feet and ran.

Panic took over. Nick zigzagged through the cabinets, trying to work his way towards the machine room. He was trapped in a maze, unable to see more than an arm’s-length ahead or behind. Three more shots came, three bolts of lightning, terrifyingly loud in the low-ceilinged room. The last one rang on in his ears; the flash echoed in the darkness. Had he been hit? Was this what it felt like to die?

It was an alarm. Perhaps one of the bullets had hit something sensitive. It was no help to Nick. The alarm lights strobed the room; the bells drowned any hope of hearing his enemies. At the far end of the aisle, steel shutters were descending from the ceiling, blocking out the glass walls of the machine room. They were already below head height and slithering down with ominous speed. Nick had no choice. He hurled himself forward and sprinted down the corridor, praying there was no one with a gun behind him. All the floor lights were on full blast now, while a recorded voice barked urgent instructions he could neither hear nor understand. The grinding shutters and trilling bells were all around him, while in the background rose an enormous whine like a jet engine revving for take-off.

The doors sensed him coming and parted automatically. With the shutter closing, the opening was little more than a foot high: he flung himself onto the floor and slid underneath it.

The shutter touched the floor and snapped taut. Nick looked around, shivering with shock. Atheldene and Emily were peering out from behind the machine. There was no sign of the gunmen.

He pushed himself to his knees. He didn’t trust himself to stand. ‘What happened?’ He had to shout to make himself heard above the roar coming from behind the shutters.

‘The smoke from the gunshots set off the fire alarm,’ said Atheldene. ‘It’s on a hair trigger – as you can imagine, given the contents of that vault.’

‘Why didn’t the sprinklers come on?’

‘Don’t have them. Spraying water all over those books would be almost as bad as burning them. They seal the storage room, then suck out all the air.’ He rapped his fist on the freeze-drying machine. ‘Much like an overblown version of this.’

Nick rubbed his head. ‘Lucky I got out when I did.’

‘Quite.’

‘What happened to Haltung?’

‘They killed him,’ said Emily. Her face was drained. ‘Those men, whoever they were. They shot him.’

On the far side of the shutter, the roar of air died away. The alarm bell had gone off – or perhaps, Nick thought, sound couldn’t travel in a vacuum.

‘What do we do now?’

Atheldene nodded to the steel shutters. ‘Nothing. The shutters can only be released from upstairs. Even if we could open them, it would be a bad idea. It’s like outer space on the other side.’

‘Then how do we get out?’ Nick looked around. There were no doors except the one to the main vault.

‘We’ll have to wait for the police to arrive. It shouldn’t be long.’

That was no comfort to Nick. He might be in Belgium, but the moment the cops ran his name through a computer they’d surely find out everything about him. He stared wildly around the room. There had to be an air vent, an escape hatch, a service tunnel. Anything. All he saw was a concrete prison. The only hint of a break was behind the machine, where a two-foot pipe led into the wall.

Nick examined it. ‘Almost a perfect vacuum’, Haltung had said. So the air had to go somewhere. There was a steel coupling where the pipe joined the wall, studded with four wing nuts. Nick ran over and twisted one of them. It didn’t budge.

He searched for something to hit it with and found a fire axe in a recess in the wall. He grabbed it, turned it round and hammered the nut with the flat end of the axe head. It shuddered, then shifted a few degrees. He kept going, bashing desperately until he had turned it a couple of rotations. The bolt itself must have been an inch thick, but now it was loose enough to be turned by hand.

‘Over here,’ he called to Emily. He pointed to the bolt. ‘See if you can get that out.’

She understood at once. Three more to go. Nick looked back at the shutters and wondered what was going on behind them. Had the police arrived? Had the men in balaclavas been asphyxiated, or had they escaped like he had before the vault was sealed?

The fourth bolt was the hardest of all. By the time he had it unlocked, Emily had removed the other three. He knelt beside her, their hands fumbling over each other like children with a Christmas present. Despite the chill air, Nick was sweating.

The bolt came free. Nick leapt back, expecting the pipe to drop like a stone. It didn’t move.

On the other side of the room, something banged against the shutters. In fury and frustration, Nick lifted his leg and slammed his foot against the pipe. It burst free of the coupling with a pop and fell to the ground, just missing his toes. A dark hole yawned in the wall. When he put his hand up to it, he felt a current of cold air.

‘Better than nothing,’ he muttered uncertainly. He looked back at the machine. ‘Is the book done?’

Atheldene looked at the dials. ‘It’ll be at least another hour until it comes up to room temperature.’

‘We don’t have time.’

‘What do you mean?’ Atheldene grabbed his arm. ‘That book’s priceless. You can’t yank it out halfway through the process.’

Nick shook him off and ran around to the control panel. He scanned the buttons until he found a large red knob labeled NOTAUSSCHALTUNG. Emergency shutdown. He slammed his palm against it. The whirring noise inside the machine died away. The lock clicked. He opened the door and slid out the book. It was cold to the touch, but not hard.

I don’t even know why I’m taking it, he thought to himself. But someone thought it was worth killing for.

‘It doesn’t belong to you,’ Atheldene protested. ‘Just wait for the police.’

‘I can’t.’

Nick put the book in his backpack with the card and handed it to Emily. Then he pushed himself head first into the hole. He was in a narrow concrete tunnel, barely wide enough to fit his shoulders. It went straight back for a few yards, then stopped in a sheer wall.

‘Air’s still got to go somewhere.’

He flapped a hand above his head and felt emptiness. He squirmed around until he lay on his back and looked up. A few feet overhead, he saw a lattice of bars silhouetted against the city glow in the sky. He tucked up his knees and pushed off, wriggling up the shaft until he could touch the grille. It lifted free without resistance. He slid it back, hauled himself through the opening and flopped onto frozen grass.