"Markham-you idiot-get off me!" he grunted. "I don't want to hurt you but I will if you don't stop!"

"Hurt me?" shouted the other Monk. "As if you bloody could!"

So he lunged upright, using fists and elbows and knees to fight free. But this other Monk was desperate. Red-faced and sweating, he crushed him close in a suffocating bear hug.

And then that horribly familiar voice was whispering frantically in his ear.

"Bloody hell, Gerald, it's me. The real me. Play along with him, for God's sake. I've got a way out."

Stunned, he went limp, as though the assault had overwhelmed him. Monk shoved him to one side and found his feet. Turned on the other Gerald, sucking great rasping mouthfuls of air into his lungs.

"I'll make him help you! I swear it, all right? I'll make him do whatever you want, Gerald. Just don't hurt Melissande. Don't hurt Reg. Please."

Cautiously, Bibbie crawled out from under the bench. "I agree, actually," she said, fastidiously smoothing the wrinkles from her Fandawandi silk ensemble. "It's more fun if they're alive. It won't be the same if I have to throw rotten eggs at a stranger. And you know the bird's harmless, Gerald. It just sits in the cage and moans."

"What?" said Monk, startled. "What did you say, Bibs?"

Bibbie shot him a venomous look. "You can shut up. I don't have to listen to you any more, big brother."

Shaken, Gerald stared at the Markham siblings.

Monk? My Monk? How can that be my Monk? Bloody hell, I know he's a genius but...

It couldn't be him, surely. This had to be a trick. There was no proof that this man was who he said he was or that he had a way out of this mess.

Bloody hell. I'll have to trust him. I can't afford not to. Because if that is my Monk Markham- He wasn't going to think about how that made a difference. It just did. He'd worry about the ethics of it later, once they'd got themselves safely home.

If we can. If we don't get ourselves and everyone else in this world killed trying.

The other Gerald, ignoring Monk's staring disbelief and Bibbie's bristling resentment, considered him with narrowed eyes. He stared straight back, making sure to still look shaken. It wasn't much of an act.

But-but if this is my Monk Markham, what's happened to the other one? Oh my God, don't tell me he's hiding in the bathroom!

It took all his strength not to look through the bathroom's open door.

"Professor," his counterpart said at last. "That was stupid. And I am-I used to be-a lot of things but really, stupid isn't one of them. You've read my potentia. You know what I can do. You've seen what I will do. And you still refuse me? I'm embarrassed for both of us."

He won't believe me if I give in too easily.

"I think you are stupid, Gerald," he snapped. "I'm the Dunwoody who didn't lose his nerve, remember? The one who defeated Lional without resorting to Uffitzi's filthy grimoires. In the only way that truly counts, I'm stronger than you. So go ahead. Do your worst. You won't break me."

"Really?" said his smiling counterpart. "You know, I wouldn't bet on it."

And with a snap of his fingers he dropped Monk back to the floor.

"You have to understand, Professor," he said, sounding bored now, "that I can keep this going and going and he won't actually die. He'll want to die. He'll beg to die. But he won't. He'll just suffer until you change your mind. Remember the cave? Just like that. Days and weeks and months and years of suffering. So the question is-how noble are you, Professor, when you get down to brass tacks? How noble is it to let someone else pay the price for your principles?"

Transfixed, Gerald stared at his keening, writhing friend. His Monk. From his world. The man who'd risked his career for him. Saved his life. Made him laugh. Paid for more than his fair share of Yok Tok takeaway.

I'm not an only child. I've got a brother, and his name's Monk Markham.

Bibbie had walked away, as far as she could get, and was standing with her back to them with her close-cropped head lowered and her silk-covered arms folded tight. Beneath her ghastly new veneer she wasn't entirely indifferent. Did that mean there was hope for her? Maybe. Maybe there was.

And then he looked at his counterpart.

But there's no hope for him.

"I might even put him on display in the parade ground," said the other Gerald, still smiling. "Wouldn't that be an exhibit to make folk sit up and blink? Imagine it, Professor-the five year old brought to witness your indifference today could be the grandfather forty years from now, showing his own grandson what happens when I don't get what I want. On the other hand, I could lock you both in here and leave you. How would you like that? Just your best friend and his screams for company, year after year after year after messy, noisy year..."

Oh, lord. Eyes stinging, he looked down at Monk, his jaw clenched so hard he thought it might break. The shadbolt shackling his friend was the worst he'd ever felt. It made Melissande's look like a diamond tiara. Even those he'd sensed on Attaby and the others, they endured nothing compared to this. The urge to rip Monk free had him shaking. But he couldn't. He had to play this out, right to the bitter end. This other Gerald would never believe a swift surrender. And if he wasn't convinced-if he suspected a trap- Hang on, Monk. I'm sorry. Please. Hang on.

The other Gerald was watching him closely. He didn't dare so much as a glance at this world's Reg-but he could feel her looking at him, crammed in that dreadful cage with her long beak tied shut. On the floor, Monk started twitching. The sounds he was making were getting louder. Less bearable. And then he opened his eyes and looked straight at him.

"Gerald, come on," he whispered.

He shook his head. "I'm sorry. You know I can't."

"Yeah, you can. Come on."

He closed his eyes, briefly. "No, Monk. I can't."

Monk sobbed once. "Thought you were my friend. Everything I've done for you."

"You-" He had to fold his arms against the pain in his chest. "You haven't done anything for me. I don't know you. I know the Monk from my world. And he-he wouldn't ask this. He knows what's at stake."

"Bugger that," said Monk, choking. "We're the same man in every way that matters, Gerald, and I'm asking..."

He didn't know what to do. How long to let this play out. Sickened, and sickeningly aware of the other Gerald's scrutiny, he half-turned away.

Monk's anguished cry followed him. "Please, Gerald. Please!"

Right or wrong, he couldn't do this any more. He let the plea break him.

"Fine!" he shouted, turning to the other Gerald. "You win! For God's sake, that's enough!"

"Not quite," said his counterpart. "After all, I am trying to make a point."

"You've made it!" he said, and dropped to his knees. "You've made it! You've got me, Gerald. All right? You win. I'll do whatever you want."

His counterpart laughed. "See, Bibbie? I told you. Soft as whipped cream." And then the amusement vanished. "But Professor? Just in case this is a ruse, and you're planning to pull a fast one? Well-just don't."

Monk shuddered once, with a terrible moan. Then the other Gerald snapped his fingers again and released him.

Bibbie turned around. Her eyes were dry but her face was chalk-white. "So are you finished now, Gerald? Can we go?"

The other Gerald glanced at her. "In a minute. Professor?"

He dragged his gaze away from silently shivering Monk. "What?"

"Catch," said his counterpart, and tossed him a small, dark red crystal.

"And what's this?" he said, feeling the acid of dark magics sizzle his cold fingers.

"A present," said the other Gerald. "I made it especially for you."

"Really?" He pushed to his feet. "You shouldn't have."

"Oh, it was no trouble."

He could feel his potentia stirring, reacting to the incants sunk into the red crystal. "Don't tell me, let me guess. You want me to swallow this?"

A genial nod. "If you'd be so kind."

"Since it's not a shadbolt," he said, feeling his skin crawl, "d'you mind telling me what it is?"

"You don't know?" The other Gerald pretended shock. "Gosh. Are you the world's most powerful wizard or aren't you?"

"I meant specifically, Gerald," he said, glowering. "Under the circumstances rogue thaumaturgics is a little vague for my tastes."

The other Gerald's smile went nowhere near his eyes. "Don't worry. There's nothing in there that can hurt you, Professor. I'm just... giving you a boost, that's all. So you can help me. Although, actually, when you think about it, you're really helping yourself."

No nightmare could ever come close to this. "Really? And what-you just happened to have this little grimoire sampler lying around? How convenient."

"Convenience had nothing to do with it," his counterpart snapped. "I've been planning this for months, Professor. I knew almost from the beginning that I'd need you to make my plan work. Why d'you think I risked what I risked to get you here? Mind you-" He shot a resentful look at Monk, on the floor. "I didn't realize this idiot would fail me. But that's all right. I can spare you for a few hours."

"To do what?"

"Bloody hell, Professor," said the other Gerald, exasperated. "Weren't you listening? I need you to help him finish what he started. What he swore to me he could build. And then, once it's completed, you and I are going to change the world. This world. To start with. Today is what you might call-the overture."

Oh, wonderful. He looked down at the red hex crystal burning his fingers with malign, malevolent promise. "Using this?"

"That's right." Another wide smile, as though he and the other Gerald were friends. As though Monk wasn't curled up on the floor at their feet. "Using that. And trust me, Professor, you're going to thank me for it. Not only will the incants in that crystal help you change the world, they'll give you a taste of what you're missing out on because of some antiquated notions forced on you by men too gutless to take what they want."

He looked at Bibbie. "And that's how you hooked her, is it? How you twisted and-"

The other Gerald raised a finger. "Careful, Professor."

Of course it was. This other Gerald had dangled a morsel of forbidden fruit in front of his Bibbie, and his Bibbie-being Bibbie, and fearless-had grabbed it with glee. Because she and Monk were Markhams, and in the habit of... bending the rules. Because she was tired of being a girl and hearing No, dear, you can't. Be a good little witch and don't show up the boys. And of course, once she'd tasted it, she'd wanted more, and more, and more...

Oh, bloody hell, Bibbie. It was ridiculous to feel guilty. He wasn't the one who'd tempted her. But still. Bibs, I'm so sorry.

"Anyway," said his counterpart briskly, and clapped his hands. "I've got things to do, so let's get on, shall we? Swallow the crystal, Professor. Now. While I'm watching. Nothing personal, I just don't trust you not to flush it down the bog once my back's turned."

"You still haven't told me what incants are in this thing."

The other Gerald rolled his eyes. "Oh, Saint Snodgrass save me. Fine. There's a general etheretic enhancement hex. A trebled counter-incant that'll let you-well, never mind. You don't need to know about that yet. There's an incant that gives you the power to control any First Grade wizard-which won't work on me, so don't even bother trying. A bunch of shadbolt matrixes, always useful. A handful of compulsion hexes-and they won't work on me either, so, y'know, don't waste my time. Oh yes, and a couple of nifty punishment hexes. For when your underlings get uppity."

"I see," he said, feeling sick again. "And that's it?"

"For starters," said his counterpart. "But if you're very good, Professor, who knows? There could be more."

He shook his head. "Trust me, Gerald. I won't be wanting more."

The other Gerald laughed. "Yes, well, you say that now. But I think you'll find that once you get a taste of what's possible you won't be quite so eager to sermonize. Or turn me down."

The casually mocking comment chilled him.

What if he's right? What if I like what's in this crystal? It might not be the worst dark magic in the world, but still... if I swallow it I won't be me any more. I'll have taken the first step towards turning into him.

He could feel Monk, staring. Lifted his own gaze, just enough. His friend was hunched on his side with his back to the other Gerald. He nodded, the smallest gesture. Twitched his lips into the merest hint of a smile.

Oh, bloody hell, Monk. You'd better know how to purge me if this muck.

He closed his eyes, shuddering, and swallowed the hex crystal. Within moments his mouth filled with a raw and angry heat. Whatever he was tasting he didn't begin to understand it. It tasted of nothing, of everything, of power and pain. He felt his potentia stir to life like a banked fire kicked over. Felt the hex crystal's incants unfurling like a seed-pod in spring. All that dark promise uncoiling, expanding, pushing single-mindedly through his blood towards the sun.

"Steady now, Professor," said his counterpart, and took his arm. "Don't fight it. Let it happen. It might tickle a bit. But what's a little pain compared with undreamt of power?"

Bloody hell. A little pain? The dark incants and hexes had sharp teeth and claws and they were tearing holes in his etheretic aura. Burrowing into hispotentia. He could feel himself... changing. Could feel a shadow, encroaching.

Oh no. Oh no. What the hell have I done?

He would've fallen, if the other Gerald wasn't holding him up. He felt himself clinging. Heard himself say: "Don't let go."

"It's all right, Gerald," said his other self, so kindly. "Don't be frightened. I've got you. You're safe with me. You're safe."

And then the dark magic inside him finished unfolding and caught fire. Seared every nerve and sinew as it flashed through him, incandescent.

"There," said the other Gerald, pleased. "All finished." Letting go, he stepped back. "And doesn't that feel better?"

The pain was gone. That was better. But the shadow-the shadow- "Yes," he said, blinking. "I'm fine."

"I knew you would be," said his counterpart, who then turned and kicked Monk. "Get up, Markham. You've got work to do."

Slowly, painfully, Monk rolled to his hands and knees. Took a deep breath and staggered to his feet. "Look. I'm going to need him for more than a couple of hours. This bloody contraption of yours-I don't know if you understand how complicated it is, Gerald."

"Thank you, I'm not an idiot," the other Gerald said coldly. "How long do you need him?"

"All night would be good."

"All night? Markham-"

"Oh, Gerald, don't go on," said Bibbie. "Isn't it better that he'll be locked in here with Monk? At least this way you won't have to worry about keeping an eye on him. Once the lab doors are hexed that's it. He's just another Reg in a tiny little cage."

"I suppose so," said the other Gerald, grudging. "But let's get one thing clear, Monk-if he's staying the night that means no more excuses. I want to find my machine finished and foolproof when I come back in the morning. Is that clear?"

"Actually," said Gerald, not looking at Monk, "instead of making assumptions that I can help, maybe you should explain what this contraption of yours-"

"Monk'll explain it," snapped his counterpart. "I don't have time. Bibbie-"