I find another set of stairs and surge to the top. The others aren't far behind. We pause and listen closely. There are no footsteps. The mutants don't seem to be chasing us.

I double over and make a sighing sound. I feel like I should be panting, but of course I can't, since my lungs don't work properly.

"Who the hell was that guy in the clown outfit?" Cathy moans.

"And those freaks in the hoodies," Peder exclaims. "Were they zombies? Humans? What?"

"I don't think they're either," I tell him. "They're mutants. They were there when my school was invaded. It looks like they work for Mr. Dowling - the clown - but I've no idea what he is, or why they're here, or why the zombies obey them, or..." I shake my head and scowl. "They don't matter. We need to get out of here. We can wonder about it later."

"But what if they come after us?" Cathy whimpers.

"All the more reason to get a move on," Tiberius grunts, and off we set again.

Time seems to slow as we search for another set of stairs. We can't find any, and the longer we go on, the more our spirits dip. We're all in rough shape. The bits of brain we sucked from Dr. Cerveris's head didn't do much for us. The shooting pains are coming regularly now and I know the others are feeling them too by the way they wince and twitch every few minutes. My thoughts are starting to swim. It's getting harder to focus.

"I think we've come the wrong way," Gokhan mutters. "Just because we're underground, it don't mean the exit has to be at the top, eh? Maybe it's at the bottom, a tunnel that leads to an elevator or something."

Peder frowns. "But if we head down and don't find it, what then?"

"We come back up," Gokhan says.

"That means slipping past the clown and his mutants a couple of times," Peder growls. "I don't fancy that."

"Maybe we don't have to," Cathy says. "There must be other flights of stairs. We've been taking the first set that we've found on every floor. Let's go down a level and - "

"Wait," Mark mutters, looking at the ceiling. "Did any of you have attic stairs at home, the sort that rest inside the attic when you're not using them?"

"What sort of a question is that?" Cathy sneers.

"My parents put in a set a few years ago," Mark says stiffly, ignoring her. "If you have steps like that, you use a thin pole to open the door, which is part of the stairs. In our home, the hole you stuck the tip of the pole into looked just like that one up there."

I squint and spot the small opening that he's talking about. "So there's an attic. So what?"

"Why would they have an attic in an underground building?" Mark asks softly.

I stare harder at the hole. "You think it's something else?"

"It has to be." He shrugs. "I mean, it might just be a machine room or housing for an air-conditioning unit. But wouldn't they have signs up if that was the case, like they have elsewhere?"

"I haven't noticed any signs," Peder grunts.

"Then you haven't been paying as much attention as me," Mark says smugly, then starts looking for a pole. We search too and a minute later we find one in a nearby room, tucked away in a corner.

I try to unlock the attic door but my hands are trembling and I can't guide in the narrow tip. Peder and Tiberius try too, but both fail, their hands shaking as badly as mine.

"Give it to me," Mark snaps, losing patience, and slots it in at the first attempt. His hands are remarkably steady. He doesn't seem to be suffering like the rest of us. Maybe the doctors slipped him some nutrients on the sly when they were operating on him.

A set of steps drops smoothly as the door opens. I feel a stab of excitement. But before I can head up, Cathy pushes me aside and I stumble and fall over. "Ladies first," she chuckles.

I scowl as she trots up, Peder, Gokhan and Tiberius just behind her. Mark helps me to my feet. "Are you okay?" he asks.

"Sure. But I'm gonna give her a thumping when this is - "

Two soldiers and a female scientist spill into the corridor. The scientist is babbling, "... right here. The pole's in a room. Once we open the door, we can..." She spots Mark and me and curses.

One of the soldiers has a gun. He aims quickly and fires. A bullet rips by my head but misses.

"Up the steps!" I roar at Mark.

The soldier fires again, three times. One of the bullets strikes Mark's left arm and blood sprays from it. He cries out and whirls away from the stairs. I duck as the soldier fires again, grab Mark and hurl him at the steps. He scrambles up them. I'm about to start after him, then pause, pick up the pole and throw it up into the space ahead of me.

A bullet hits the flesh on my back where my heart used to be. If I was whole, it would open up a nasty wound. But because I'm more hole than whole in that part of my body, it only nicks a flap of skin and shoots on through the cavity and out the other side.

I hurry after Mark and pull the steps up behind me, locking them into place. The humans scream beneath us and the soldier fires a stream of bullets into the ceiling, either trying to hit us or smash the lock. I don't hang around to find out. Pushing Mark ahead of me, I scurry after the others.

We're in a corridor, not an attic, lit dimly by soft red lights. The dimness is a relief after the brightness of the complex. I hadn't realized how much the glare of the lights hurt.

We shuffle along, nobody saying anything. The corridor angles upwards, then turns back on itself and keeps rising. The noises of the complex fade the farther on we push. I feel real hope for the first time. The nurse was leading the soldiers to this place and seemed desperate to reach it. Because it's a way out? Not the main exit, but a secret escape route for those of a certain rank?

The corridor snakes around several times before the floor levels out and we step through into a large, square room. There are doorways in the middle of all four walls. Two are open, like ours, leading to corridors like the one we've stepped out of. A metal door stands in the fourth. It's like no other door that we've seen in the complex, taller, wider, more impressive.

"That must be it!" Peder whoops, racing towards the door.

"The exit!" Cathy gasps, then spins towards me. "Do you still have the eye? You didn't drop it, did you? Tell me you didn't - "

I hold up my hand and widen my fingers, letting her see the eye.

"Yes!" she shouts.

I grin and push past, giving her a sharp dig with my elbow to pay her back for pushing me aside earlier. Cathy doesn't care. She only has eyes for the door.

Gokhan presses Dr. Cerveris's fingers to the panel and it beeps. Tiberius and Peder shut their mouths as if to hold their breath, both forgetting that they can't actually breathe.

I step forward and hold up the eye. A camera scans it. There's an agonizing pause in which I convince myself that it isn't going to work, that I've shaken the eye around too much, dislodged something vital inside. Then...

Beep.

Everyone cheers as if I'd just scored the winning goal in a cup final.

The cheers stop when the system beeps again and a touch-screen calculator flashes up on the panel where Gokhan scanned in the dead doctor's fingerprints. There's a short message just below it.

Please enter six-digit authorization code.

We stare at the message, then at each other.

I clear my throat. "This is where someone says that they're a hacker and they can crack this bastard in five minutes flat."

Nobody says a word.

"Damn," I sigh, sinking to my haunches. "In that case I guess we just rot here and turn into rabid, brain-munching reviveds."