I come out on top of a fluffy cloud. Through a break to my left I see land far below. My stomach drops as I picture myself falling through the mist, then the sky, hitting the ground hard and splattering. But the cloud holds, supporting me like the water in the first world I visited.

Shark steps through after me, Dervish just behind him. They yell with shock when they see what we're standing on. Turn to dive back through the window. "It's OK!" I shout. "We won't fall."

They pause, glance at me uncertainly, then realise I'm telling the truth - otherwise we'd have already dropped.

The now two-headed demon is on another bank of cloud ahead of us. When it spots us, it squeals with fear and bounds away. Shark starts after it, but Dervish calls him back. "We're here to find Beranabus, not kill a stray demon."

Shark pulls a face, loath to let the demon escape. Then he sighs. "OK, kid. Tell us how you plan to find him."

"I'll open a window," I say, as the one we stepped through comes apart. "Just give me a few minutes to find the patches."

" 'Patches'?" Dervish echoes, but I don't answer. Looking around, I'm pleased to note that there are loads of patches of light in the air, despite the fact that we're standing on top of a cloud. I start thinking about Beranabus, muttering his name softly, hoping he's still alive and that I have the power to make the lights pulse.

For a few seconds - nothing. But before panic has a chance to set in, a pink square by my left foot blinks. Then a brown octagon. Soon, dozens of the patches are pulsing and I merrily set to work.

I'm aware of Dervish and Shark talking while I build the window. Dervish is complaining about the cold. His leather jacket is too small to button up properly - it's for style, not warmth - and his bare chest is freezing in these icy heights.

"Use magic," I tell him, recalling the way I instinctively repaired my broken arm. "You can warm up if you think yourself warm."

Dervish is sceptical, but gives it a go, and moments later he's beaming, even taking his jacket off and tying it around his waist.

"You must have been here a long time to know so much," Shark says.

"Actually, I don't think it's been more than half a day," I reply. "Though it feels longer. I'll tell you about it later, if we have time."

The window comes together smoothly under my guidance. I don't hurry. Pleased to note I'm no longer hungry or tired. Marvelling at the way this universe works. I start wondering if we could float down to the ground from here, but then the assembled patches pulse as one and a brownish window opens. "Here we go," I say smugly.

"I didn't think windows could be opened that quickly or simply," Shark says.

"It's easy-peasy when you know how."

Shark steps up beside me and looks back at Dervish. "Ready for the next leg of the tour?"

"Hurm," Dervish says uncertainly. "Do you know where that leads?" he asks me.

"No. But Beranabus will be there." I hesitate. "When I left him, he was in trouble, fighting a team of demons. We might have to help him. So be prepared, OK?"

"Yes, boss," Shark laughs.

"Thanks for warning us," Dervish says, then takes up a position to my left. Shark slides into place on my right. We step through the window.

It's the same world where Nadia and I ran out on Beranabus. Night. Three moons shine, closer than the moon is to Earth in my universe. Too bright to see if there are any giant demons soaring overhead.

Corpses are scattered across the hard yellow ground. Demons in an advanced state of decomposition, most rotted to the bone. Either demons rot quickly here or this is one of those places where time runs faster than in the human universe.

I spy Beranabus working on a window. Sharmila is nearby, sitting next to a mound of freshly dug earth. I guess it's Raz's final resting place, that she and Beranabus - probably just her - dug a grave for the fallen Disciple.

I get a lump in my throat when I think about how Raz died, but there's no time to cry. I didn't believe I could be so matter of fact about the death of a friend, but I'm learning a lot here. One of the things is that in times of severe disorder, you can't worry about the dead, only the living. I still think Art is alive. He's the one I have to focus on. I can't do any good for the dead Raz Warlo.

"Beranabus!" I call. "It's me, Kernel. I'm back."

The magician's head whips round and Sharmila's jerks up. They stare at me in disbelief, then at the two men with me. Then Beranabus cheers - the first time he's acted like an ordinary human since I met him - and rushes across to pick me up and whirl me around.

"Kernel Fleck!" he booms. "You're a wonder! I've been struggling to build a window to you for days. And here you pop up, cool as a breeze! You're the most remarkable human I've met in centuries!"

He sets me down and I find myself grinning at him. I hadn't liked the cranky magician before. But now I see he can be as emotional as any normal person. He simply hides his feelings better than most.

"Hello, Beranabus," Shark says, stepping forward, hand outstretched.

Beranabus shakes the hand briefly, frowning. Then he points at Shark and says, "Octopus?"

"Shark," Shark laughs.

"Ah. I knew it was something like that." He looks at Dervish blankly.

"This is Dervish Grady," Shark introduces him. "My latest recruit."

"Another Disciple," Beranabus murmurs, nodding shortly at Dervish. "How many does that make?"

"If you don't know, I'm sure nobody does," Shark says.

Beranabus shrugs. "I never was good at numbers. Anyway, welcome to the team, Grady. I hope you last longer than some of my other followers." His eyes flash on Raz's grave.

"It is good to see you again, Kernel," Sharmila says, stepping forward to hug me. She looks drawn and miserable, but has reattached her arm and healed her wounds since the battle.

"I'm pleased you made it," I whisper.

"It was close. My injuries were fatal. If not for Beranabus, I would have died."

"Yes, yes, I'm better than any doctor," Beranabus says impatiently. "Now what about my other Disciple? Where's Nadia? Not dead, surely."

I pause. Part of me wants to cover up for her and tell him she was killed by a demon. But his small dark eyes are fixed on me and I find myself incapable of attempting a lie. "She didn't want to come back. She left me. Went off by herself. She's had enough of demons."

Beranabus' face blackens with fury. "I'll flay her skin from her back! Find her. Open a window to her. Immediately."

"I do not think you should do that," Sharmila says. "Nadia is a free agent. If she wants to - "

"I don't care about her wants!" Beranabus bellows. "We need her. Now open that window, Kernel, and don't pretend you can't. I'll know if you're lying. And I'll punish you for it."

I want to tell him to get stuffed, I'm not his servant. But those dark eyes are fierce with anger when I look at him and I wilt. "I'll try," I mutter unhappily. "I'm not sure it will work, since she's not in this universe, but I'll give it a go. If you're certain."

"Aye!" Beranabus growls, glaring at me as I clear my thoughts and concentrate on Nadia's image, feeling like a traitor of the lowest, meanest order.

We're waiting for Beranabus. He said that because of the time differences between this world and ours, he could be gone for a few hours as we experience it, even though it will only be a few minutes for him. I keep an eye on the window, holding it open. It's not hard. When I see a patch or two shimmer and start to slip free of the panel, I press them back into place.

We chat to pass the time. Sharmila tells Dervish and Shark about the Kah-Gash, Beranabus' quest, her part in the mission. I describe how I got mixed up in it, and demonstrate how I can construct windows so quickly, though I can't explain why I alone can see the patches of light.

In return, Shark and Dervish tell us of their lives. Shark's been a Disciple for several years, working with others to avert demonic crossings. He was in the army when he discovered his magical talent. On a tour of duty. A village came under attack from four demons. Shark and his team tried to stop them. A Disciple was present. He realised Shark's potential when he saw him fighting, pulled him clear of the massacre, explained about the Demonata. All of Shark's fellow soldiers were killed, so Shark became a Disciple, though he still wears his uniform out of respect for the dead.

Dervish is new to the game. Shark discovered him a few months ago while trying to stop a crossing in the city where I met them. He was successful, thanks to Dervish, who happened to be nearby when a window was about to open. Dervish saw that Shark was in trouble, ran to his aid and used magic he'd never known he possessed to knock out the woman trying to open the window. That was the end of life as he knew it.

"Is that how Disciples are normally recruited?" I ask. "There's an attack, they discover magic within themselves and a Disciple asks them to join?"

"Pretty much," Shark says. "Lots of people have magical ability, but it usually only reveals itself in the presence of demons. When windows are created, some of the magic of their universe flows through, which people like us can tap into, even without training or intent. We've spent decades trying to identify and develop the potential some other way, but no luck so far."

"Does everybody join once they know they have the power?" I ask.

"No," Sharmila answers. "Many reject their calling. I do not blame them. Ours is a harsh life, lonely, filled with peril."

Shark snorts with contempt. "If I had my way, we'd press-gang the lot of them, force them to fight."

"That would be unfair," Sharmila says.

"This isn't about fairness," Shark argues. "It's about winning a war. You can't run away from your duty during war. It's desertion."

"That's what Nadia did," I say softly, and my eyes meet Sharmila's. We're both worried about what will happen when Beranabus catches up with her.

Nadia falls hard through the window and lands heavily on the ground. She howls hatefully and tries to leap back through the panel of light. Beranabus appears before she completes the jump. Shoves her away, snarling like an animal. "Stop!" he roars.

Nadia tries to wriggle around him, fingers stretching towards the window of light. He blocks her way, standing firm like a Roman emperor, while Nadia shrieks and wails.

She tries using magic to move him, but he flicks aside her bolt of energy and holds his position. "Kernel!" he shouts. "Dismantle the window."

"I'm not sure I should - "

He flashes his teeth at me, making it very clear that he'll turn on me next if I disobey him. Feeling lousy and afraid, I slink around Nadia and Beranabus - the magician fending his assistant off, protecting me from her - and set to work on the window. After removing a few pieces, the patches slide apart and the window disappears.

Nadia throws herself flat and weeps into the yellow earth, hammering the ground with her fists. Beranabus sighs and steps aside, rubbing the back of his neck. "You'll thank me for this later," he says.

"I'll thank the demon who rips your head off and fills your skull with fire!" she screams back, then bolts upright and glares at me. "You showed him where to find me!"

"I had to," I mutter shamefully. "He said he'd - "

She spits at me.

"Do not blame the boy," Sharmila says, putting her hands on my shoulders. "He argued to leave you be, as did I. But Beranabus would not listen."

"Why?" Nadia cries, whirling on Beranabus. "Why didn't you leave me there? I could have been happy. Led a normal life. Been human again. Why rip me away from all that?"

"I need you," Beranabus says flatly.

"No you don't! I gave you what you wanted - a vision which might help you find part of the Kah-Gash."

"There will be other parts to find."

"But that could take hundreds of years! Thousands! We'll both be dead long before that happens - if it ever does."

Beranabus shrugs.

"This is slavery," Nadia snarls. "You always said I was free to leave."

"And you are," Beranabus insists. "As soon as we find the pieces of the Kah-Gash. Until then, I need you. Your universe needs you. I know you don't enjoy this, but that doesn't matter. You've been chosen, like the rest of us. If we turn our backs on our responsibility, the entire world will fall to the Demonata."

"I don't care!" Nadia shrieks. "What's the difference between fighting them here or on my own world? They might as well take it over for all it matters to me!"

"Then you would never have anywhere safe to go," Beranabus says.

"So? I can't go there anyway while you're holding me prisoner."

Beranabus exhales sharply, starts to say something, stops, then smiles tightly. "We'll make a deal. Stay with me until I find this piece of the Kah-Gash. After that, you can go. I'll open a window to the human world for you and not stand in your way."

"And if I die during the search?" Nadia retorts. "Or if it takes a hundred years to find it? Or if you never find it?"

Beranabus' smile disappears. "I'm trying to be reasonable."

Nadia laughs. "No. You know this is wrong and you want to feel better about it. You don't want to admit you're as ruthless and monstrous as the demons you claim to hate."

"I never said I hated the Demonata," Beranabus says quietly. "And I never claimed to be anything other than - as you say - ruthless and monstrous. That's how I need to be to fight them."

Nadia prepares another curse, then realises it would be a waste of breath. She looks around at us, hateful and alone. Points a finger at Beranabus. "You betrayed me. That's something I won't forget or forgive. You don't just have to watch out for demons anymore - when I'm around, you'll have to worry about me too. I hate you as much as they do now, and I'll quite gladly kill you if I ever get the chance. And anybody else who's with you."

With that, she turns her back on us, screams at the three moons, then sits and weeps while Beranabus looks on with an unconvincing, half-shameful sneer.