It wasn't as bad as Jack had expected. Bazalgette's sewers were a thing of beauty. They were wide and airy – if anything could be called 'airy' in the vicinity of all that stench – and they were tiled with lovely, geometric patterns, seemingly put in place just to please the eyes of the rats and repair-crews that ventured down here. 

Still, it wasn't easy. By the time Jack had been down there an hour, the darkness and the stink had compressed the air he walked through into a kind of clay. It took a supreme effort of will just to move forwards. He didn't have much to spare for observation. 

He trailed his hand against the tunnel-wall as he walked, in case he missed an alcove or a side-tunnel that might have marked the entrance to the demon realms.

But when he saw it, he couldn't have missed it – or even mistaken it for something else, which seemed odd, because presumably the engineers had seen it when they'd built the place, and had thought of it as – what? A natural rock formation? The remains of an old church or temple? A pile of debris too big and inconvenient to move away?

It was a branch in the river – or rather, an island tall enough to reach the tunnel roof, where stalagmites and stalactites met like clenched teeth: a pale, milky grin of a place, like – well, like the smile of the Cheshire Cat.

Jack gasped – which involved taking in more of the foul air than he wanted to all in one go. He knew immediately that this was where Sita had got in to the demon realms, though he didn't quite see how.

He jumped off the walkway and splashed through the thigh-deep water until he was close enough to touch the stone. It didn't reflect much of his lantern-light. There was a curious sense of depth, as though there was an ocean of space back there, sucking in the light and the splashing sound of his footsteps. It was solid to the touch, but felt more... organic... than the tunnel-walls.

Was this like the entrance to the fire-mines, he wondered? Did you have to be a woman to get through? Or something else? Maybe you had to be under ten, or a virgin, or no more than five feet tall? Magic was so bloody whimsical.

He found the nearest sewer-grating, and very carefully measured the number of paces between it and the milk-white stone. He tried to judge the direction too, from the angle of the daylight that filtered down through the grating, but that would probably be easier once he was up in the fresh air. And it had been a long time since he'd thought of London's atmosphere as 'fresh air'.

Then he waited some minutes for a lull in passers-by and hauled himself up.

He was in Farringdon Road – the broad avenue and sooty stucco was unmistakable. It was a slightly nicer district than Camden, but this turned out to work in his favour, because the well-dressed pedestrians gave him a wide berth for fear that he might soil their clothes.

In truth, he didn't smell much worse than the average vagrant or night-soil man, but he wanted to wash before he even ventured into the same city as Ellini. 

He went to the Turkish Baths in Covent Garden. A few eyebrows were raised – and a few noses wrinkled – when he turned up in the foyer, smelling like the bottom of the Thames, but money could be very persuasive. Money always scrubbed up nicely, no matter where it had been.

He went into the opulent, tiled interior and watched the other bathers scurry for the door. Then he scrubbed himself until he was practically shining. He even paid an errand-boy to buy him new clothes, and dispose of the old ones. In London almost everything was sold on, re-packaged, broken up and made into something new, but Jack was guessing those clothes weren't fit for anything except the furnace.

It was expensive, but he felt a little better as the train chugged back towards Oxford, and the spires drew back into sight, like a spiky ceiling that was slowly but steadily being lowered on top him.

Now he was standing outside the ring of gargoyles in unfamiliar clothes, still pink-skinned from the scrubbing, and smelling emphatically – hopefully overpoweringly – of soap. And he was watching Elliott and Ellini stroll through the winter garden.

He could see the whole conversation playing out, even though he couldn't hear them. The boy was making her nervous – he looked at her too much – he was even stupid enough to stop dead while they were walking and make a grab for her hand. Good Lord, didn't he know? Hadn't she told him? Couldn't he imagine?

Jack was glad he was too far away to intervene. He knew he mustn't lay a finger on this boy. Ellini already thought he was a murderer. And he was a murderer, but he didn't have to compound the situation by attacking a harmless pianist.

She wasn't frightened, anyway. The boy was ardent, but not desperate. You couldn't mistake him for that type. It was yet another of the ways in which he was perfect.

When they headed back towards the Academy – was the boy dragging his heels a bit? Had she said something to upset him? – Jack leaned against the nearest stone gargoyle and shut his eyes.

The hairs on the back of his neck prickled, and something made him duck and roll and kick upwards a second before the knife slashed through the air. 

The shadow bearing down on him staggered back. It had a white, livid face – a face that, for the first time in all the years he'd known it, didn't look the least bit handsome. Those feline eyes were bulging. His breath was hissing in his teeth.

He dodged around Jack's flailing legs and lunged forwards. Jack just had time to think: Oh crap, he knows, before the knife sliced down beside him. It missed, but it was whipped up again so fast that Jack was showered with grass and bits of turf from where it had bitten into the lawn.

Oh, that knife! He swore it got bigger in the heat of battle, as if – ugh, horrible thought – as if it was excited.

"You smug bastard," said Robin, breathing hard. "You smug, grinning, thieving little creep."

Jack rolled again, kicked up again, this time connecting with his jaw. He felt the knife slash through his new trousers, but no deeper. He would know – he'd know the bite of that thing anywhere. And it wasn't so much the thought of its sharpness, or its wicked, serrated edge, as the thought of where it had been.

Robin staggered backwards, flicked the knife around with an audible swoosh, and plunged again. He was calmer now, starting to regain his swagger, starting to enjoy himself. But Jack had seen that livid, lurching moment, and he would never forget it. He had made Robin ugly.

"I've killed six men since we last met, golden-boy," he said, adjusting his hold on the knife. It spun in his grip as though it was magnetised to his palm. "And I didn't hate any of them as much as I hate you. What do you think your chances are?"

He swung the knife again, but Jack caught his arm, wrenched it up, and banged it repeatedly against the nearest gargoyle-statue. "Wouldn't – care – if they were non-existent," he gasped, trying to grin. "I've already won."

The knife tumbled from Robin's hands. And, as it did so, the mask slipped again – once again, his face was ugly, hissing, white and sweaty.

"You fucking arsehole, you fucking always win!" Robin spat. He kicked out at Jack's knees, and then punched him as he doubled over. "That's the most frustrating thing about you, golden-boy – and, believe me, there's some pretty stiff competition. You're always winning and never realizing it."

Jack staggered backwards, trying to cling onto consciousness. But he had been here before. He knew what to do. Staying on your feet wasn't important, keeping your eyes open wasn't even important. You listened for the swish of that bloody knife, because it moved independently of Robin, and it was more important to dodge that than to dodge him.

"But you know now, don't you?" said Robin. He had picked up the knife. He was standing a few feet away, leaning against the gargoyle, picking clumps of mud off the blade. He seemed entirely preoccupied. But it was too casual, after the fury. Even if he hadn't known him, Jack wouldn't have been fooled.

Robin gave him a smile as pale as winter sunlight and said, "Yes, you know now. Cat's out of the bag, isn't it?"

And, as he said this, Jack realized how true it was. Whatever mental block he'd had with Robin – whatever it was that had made him feel like he was sinking, over-shadowed, clinging on by his fingertips whenever he faced him – it was gone. He wasn't afraid. Well, any more than it was reasonable to be afraid of a highly-trained, knife-wielding lunatic. But he was just another opponent. There was no magic to him.

How long had it been this way, he wondered? What had done it? Was it like the scars sinking back into his chest? Had he been cured of his hang-ups too? God, that night with Ellini really was the gift that kept on giving.

And Robin knew it. Robin had spotted it first. There was something rueful and self-mocking in his expression as he went on. "Still rusty, though, aren't you? I would be too if I'd spent five years mouldering away in this city, taking pills and being that Darwin-woman's lapdog."

"You spent five years being dead," Jack reminded him, his eyes still on the knife. Robin was holding the blade tucked in at the elbow, as if he had no intention of cutting anyone but himself.

"Well, even death isn't as deadening as this place."

The knife swished round again, but Jack was ready. He leaned to one side, and then the other, as the blade rushed past, then rolled backwards as it was thrust forwards, right at stomach-height.

Robin staggered on, carried away by the momentum of the thrust – unless that knife really did have a life of its own, and was leading him on like a dog on a leash. 

Then he stopped in mid-air, as if he'd come up against some invisible obstacle. Whatever it was, it bent the blade – it might even have snapped it if Robin hadn't twisted his wrist at the last moment, causing that to thud against the barrier instead.

After two breaths – and maybe fifty thudding heartbeats – Jack took his eyes off him and glanced around. He had rolled between two gargoyle-statues, into the Academy's grounds, and Robin hadn't been able to follow. He had definitely been exhibiting hostile intent, and nobody could get past the gargoyles with hostile intent, but there was more to it than that. Robin had been momentarily surprised, but not astonished. He had already known he couldn't get through – which meant, presumably, that he couldn't get through even when he wasn't trying to murder people.

"Well, that could be a problem for you, mate," said Jack, with a cheery smile.

For a second, he saw Robin despair – actually despair. He leaned his back against the air that wouldn't let him through and shut his eyes, his jaw set grimly, as if he was weathering some kind of internal storm. Then he slid down into a sitting position, shoulders slumped, careless of the opponent behind him. 

Presumably he knew that there was no barrier on Jack's side. Was this a capitulation? Had he given in? 

No, it was a stalemate. He knew Jack needed him. There was no other source of information on Myrrha. Perhaps he even knew that Jack had promised Ellini he'd never kill again.

"I'd pretty much made up my mind not to kill you anyway," said Robin, through tight lips. He was nursing his wrist, but he hadn't dropped the knife.

"Is that right?" said Jack. He sat down on the grass a few feet away from him, leaning his back against the nearest gargoyle. It wasn't quite turning his back on Robin, but almost. He meant it as a surly signal that he acknowledged the stalemate, but Robin would probably interpret it as a sign of weakness. Sure enough, his tone was brighter when he spoke again.

"You know, golden boy, you're lucky I was so angry. If I'd thought about it – if you hadn't been so tantalisingly close – I would have gone after your loved ones instead."

"Oh yeah?" said Jack, smiling faintly. "How would you have done that? They're all in there." He jerked his head towards the Academy.

"The policeman? The mourner? The Romanian doctor?"

"He's from North Dobruja," said Jack, no longer smiling.

"Well, it's too late now, anyway. Now you'll be expecting it. Now you know what you can do, and where I can't go. That was my last chance to hurt you." He paused a fraction, and Jack could just picture the slow, hideous smile spreading across his face. "No. Maybe there's one more."

He didn't say – because he didn't need to – 'I'll be taking Ellini to Edinburgh. Your reason for living will be in my hands'. 

"The question is," Robin continued, "whether hurting you would be worth it."

"What if you could hurt me and help Ellini?" said Jack, who had been thinking about this. "What would you say if I promised that, if you help her to kill Myrrha, I'll never touch her again as long as I live? I'll disappear – she'd never find me."

"I wouldn't say anything. I'd just laugh."

"You could make sure I kept my word-"

"How? By chopping your dick off?"

Jack waved a hand irritably. "I don't know, some kind of magical oath – Elsie could show us."

"I'd rather chop your dick off."

Jack sighed, and relapsed into silence. It was a companionable silence – it always had been, with them. The kind of silence you got with someone who would never be surprised, even at the most depraved things you could come up with, because they were always, more or less loudly, thinking the same thing.

"You have no idea what you did to her," said Robin at last. "It was a work of genius, golden boy. Can you picture what it was like, watching her struggle to breathe through that punctured lung every night while we were training – watching it bleed through her dress whenever she wasn't sufficiently distracted?"

Jack clenched his fists but said nothing. This was a new tack for Robin, trying to torture him with guilt rather than inadequacy. It would have worked better coming from someone who hadn't handed out punctured lungs on a daily basis. But it worked a little, because it was true.

"You had no right," Robin went on. "Even if she offered herself to you..."

He trailed off, apparently too angry for words, but there had been a slight, questioning inflection at the end of that sentence.

"I didn't force her, if that's what you mean," said Jack sharply.

"You know what I mean. She feels sorry for people. With care, someone could use that, if they had no scruples..."

Jack scowled straight ahead of him. "You know, there's no barrier on my side of the gargoyles."

Robin shrugged. "So kill me. I know you want to."

"And I can't believe you are talking to me about having no scruples."

"I never pretended to be anything else," said Robin.

"Neither did I, much."

There was silence again.

"I'll never help you," Robin added, as though the question of killing him was still up in the air, and this might be a decisive argument.

"Yes," said Jack, poking at the grass with the toe of his boot. "I know."

"And that means you'll never help her."

"I know that too."

"So what are you going to do? Trust her? Trust me?"

"Well, obviously not the last one," said Jack. "I'll think of something. I always do."

Silence again. When Robin broke it, his voice was distant and deadpan – it might almost have been conciliatory. But they were too old and too angry, and there was nothing left to conciliate.

"Funny if she was running off with the pianist while we were outside fighting over her."

"Ha ha," said Jack tonelessly.