The unidentified magician was the only unknown in the equation, the X. We could designate Egor as Y: his resistance to magic was far too high for any novice Other. But on the other hand, the boy was an already known quantity, with just one undetermined factor . . .

And that had been deliberately introduced into the problem, to make it more complicated.

'Zabulon!' I shouted. Behind my back Egor was scrabbling and sliding on the ice as he tried to stand up. Semyon was backing away from the magician, still maintaining his defences. Ilya was simply observing everything dispassionately. Bear was closing in on the twitching girl vampire as she tried to stand up. Tiger Cub and the witch Alisa were moving towards each other again. 'Zabulon!'

The demon looked at me.

'I know who you're fighting for!'

But I didn't know yet. I was just beginning to understand, because the pieces of the jigsaw had come together and shown me a familiar face . . .

The demon opened its jaws – they shifted to the left and the right, like a beetle's. He was looking more and more like some huge insect, his scales had grown together into a single carapace, his genitals and tail had retracted, new limbs had begun to sprout from his sides.

'Then you're dead.'

His voice was the same as before, in fact it sounded even more thoughtful and intelligent. Zabulon stretched his arm out towards me – it extended in jerks, growing new joints as it came.

'Come to me . . .' whispered Zabulon.

Everybody froze – apart from me. I started walking towards the Dark Magician. There was barely a trace left of the mental defences I'd nurtured for years. There was just no way I could not obey Zabulon.

'Stop!' roared Tiger Cub, turning away from the battered but still snarling witch. 'Stop!'

I really wished I could do as she said, but I couldn't.

'Anton . . .' I heard someone say behind me. 'Look back . . .'

That was something I could do. I turned my head, tearing my eyes away from the gaze of those amber eyes with their narrow, vertical slit pupils.

Egor was still squatting down, he didn't have the strength to get up. It was incredible that he was even conscious at all . . . after all, the external input into his energy reserves had been shut off. The external input that had attracted the boss's attention, that had been maintained from the very beginning. Factor Y. Introduced to complicate the situation.

A small ivory medallion on a copper chain dangled from Egor's hand.

'Catch!' the kid shouted.

'Don't take it!' Zabulon ordered me. But he was too late, I'd already bent down and grabbed the amulet as it came flying towards my feet. The carved medallion burned my hand when I touched it, as if I'd picked up a live coal.

I looked at the demon and shook my head:

'Zabulon, you no longer have power over me.'

The demon howled and came straight at me. His power over me was gone, but he still had plenty of strength.

'Tut-tut!' said Ilya.

A wall of white flame cut across the space between us. Zabulon howled as he hit the magical barrier and the sheet of pure white light flung him back. He shook his scorched paws, looking ridiculous now, not terrible at all.

'A complex move,' I said. 'But elementary really, isn't it?'

Everything on the roof went quiet. Tiger Cub and the witch Alisa stood side by side, not even trying to attack each other. Semyon looked at me, then at Ilya, and I couldn't tell which of us had surprised him most. The vampire was crying quietly, as she tried to get up. She was in the worst state of all, she'd used up all her strength to survive the fight with Bear, and now she was struggling to regenerate. With an incredible effort she left the Twilight, becoming a vague silhouette.

Even the wind seemed to have died away . . .

'How can you make a Dark Magician out of someone who is fundamentally pure?' I asked. 'How can you win over to the Dark Side a person who doesn't know how to hate? You can rain difficulties on him whichever way he turns ... a little at a time, hoping that he'll become embittered . . . But that doesn't work. This person . . . this girl ... is too pure.'

Ilya gave a quiet laugh of approval.

'The only thing that she could hate,' I said, looking into Zabulon's eyes, now filled with nothing but powerless malice, 'is herself. And that's the clever move. Unexpected. Let her mother fall ill. Let the girl devour her very soul, despising her own weakness and refusal to help. Drive her into a corner so tight, there's nothing else she can feel but hate, even if that hate is for herself. Of course, there is a divergence of probabilities. Just a slight chance that a single Night Watch agent who doesn't really know all that much about field work—'

My knees started to buckle – I wasn't used to staying in the Twilight this long. I would have fallen on my knees in front of Zabulon, something I really didn't want to do, but Semyon slipped through the Twilight and supported me by the shoulders. He'd probably been doing that for a hundred and fifty years too.

'About field work . . .' I repeated. 'Might suddenly not behave according to plan, not trying to pity and comfort a girl for whom pity is fatal. He had to be distracted. A situation had to be created that would keep him busy. He had to be given a secondary assignment, and feel obliged to carry out that assignment for professional and personal reasons – anything that came to hand would do. An ordinary vampire could be sacrificed for that, couldn't he?'

Zabulon began transforming back to human form, assuming his former appearance as a mournful-looking intellectual.

That was odd. What for? When I'd already seen what he'd become in the Twilight, what he'd become once and for ever.

'A complex manoeuvre,' I repeated. 'I'll bet Svetlana's mother doesn't really have to die from any fatal illness at all. That was a minor intervention from your side, within the permitted limits. . . But then we have rights too.'

'She ours!' said Zabulon.

'No.' I shook my head. 'The Inferno's not going to erupt. Her mother's going to get well. I'm going straight to the girl now . . . and I'm going to tell her everything. Svetlana will join the Night Watch. You've lost, Zabulon. No matter what you do, you've lost.'

The tatters of clothes scattered across the roof crept towards the Dark Magician, grew together and sprang up on to his body, reclothing the sad, charming intellectual grieving for the whole world.

'None of you will leave here,' said Zabulon. The Dark began thickening behind his back, like two immense black wings unfurling.

Ilya laughed again.

'I'm stronger than all of you,' said Zabulon. 'Your borrowed powers are not unlimited. You will stay here for ever, in the Twilight, deeper than you have ever dared to look . . .'

Semyon sighed and said:

'Anton, he still hasn't got the picture.'

I looked round and asked:

'Boris Ignatievich, don't you think you could drop the playacting now?'

The bumptious young field operative shrugged:

'Of course, Antosha. But I don't often get a chance to observe the head of the Day Watch in action. Don't hold that against an old man. I hope Ilya found it just as interesting being me . . .'

Boris Ignatievich resumed his normal form. Instantly, without any theatrical intermediate stages or light effects. He was still dressed in his gown and skullcap, but he was wearing soft moccasins on his feet, with galoshes over them.

Zabulon's face was a sight.

The dark wings didn't disappear, but they stopped growing and flapped hesitantly, as if the magician was thinking about flying away, but couldn't quite make up his mind.

'Wind up this operation, Zabulon,' the boss said. 'If you withdraw immediately from this building and from Svetlana's house, we won't lodge an official protest.'

The Dark Magician didn't hesitate.

'We'll withdraw.'

The boss nodded, as if he'd never expected any other answer. Just for a moment I thought . . . He lowered the wand, and the barrier between me and Zabulon disappeared.

'I'll remember the part you played in this. . .' the Dark Magician murmured. 'For ever.'

'Do,' I said. 'It's good to remember.'

Zabulon brought his hands together – the mighty wings flapped, and the magician disappeared. But before he went, he glanced at the witch – and she nodded.

I didn't like that at all. A spiteful parting gesture may not be fatal, but it's never pleasant.

Alisa came over to me, walking with a light, dancing step completely out of keeping with her bloody face and dangling, dislocated left arm.

'You must leave too,' said the boss.

'Of course, I'll be only too delighted,' replied the witch. 'But before I do, I have one small, very small, debt to collect. Isn't that right, Anton?'

'Yes,' I send quietly. 'A seventh-degree intervention.'

Who would she strike her blow at? Not the boss, the idea was ludicrous. Tiger Cub, Bear, Semyon . . . that would be stupid. Egor? What suggestion could she implant in him at the very weakest level of intervention?

'Open yourself,' said the witch. 'Open yourself to me, Anton. A seventh-degree intervention. The head of the Night Watch is a witness: I won't overstep the mark.'

Semyon groaned, squeezing my shoulder so tight it hurt.

'She has the right,' I said. 'Boris Ignatievich . . .'

'Whatever you say,' the boss answered softly. 'I'm watching.'

I sighed and laid myself open to the witch. There was nothing she could do. Nothing. A seventh-degree intervention – she could never turn me to the Dark with that. The idea was simply ridiculous.

'Anton,' the witch said gently. 'Tell your boss what you wanted to say. Tell the truth. Act honestly and correctly. The way you ought to act.'

'Minimal intervention . . .' the boss confirmed. If there was any pain in his voice, it was so deeply hidden that I couldn't hear it.

'A complex manoeuvre,' I said, glancing at Boris Ignatievich. 'From both sides. The Day Watch sacrifices its pawns, and the Night Watch does the same. For the great goal. In order to win over to their side a sorceress of immense, unprecedented power. A young vampire who is longing for love may die. A young kid with undetermined powers may disappear for ever in the Twilight. Operatives may be hurt. But there's an end that justifies the means. Two great magicians who have opposed each other for hundreds of years cook up another little war. And the Light Magician is in the toughest spot ... he has to stake everything. And for him to lose would be more than an inconvenience, it's a step into the Twilight, into the Twilight for all time. But still he stakes everyone's lives. His own side's and the other's. Right, Boris Ignatievich?'

'Right,' replied the boss.

Alisa laughed and walked towards the trapdoor. The witch was in no shape to fly. Tiger Cub had given her quite a mauling. But even after that she was feeling cheerful.

I looked at Semyon and he turned his eyes away. Tiger Cub slowly transformed back into a girl . . . also trying not to look me in the eye. Bear gave a short, sharp howl and trudged towards the trapdoor without changing his form. It was toughest of all for him. He was too uncompromising. Bear, the great warrior and opponent of all compromise . . .

'You're all bastards,' said Egor. He stood up, moving awkwardly – not just because he was tired, the boss was feeding his reserves now, I could see the fine thread of power running through the air – but because at first it's always hard to tear yourself out of your shadow.

I was the next out. It wasn't difficult, during the last quarter of an hour so much energy had been splashed into the Twilight that it had lost its usual aggressive clamminess.