Sam couldn't stop being a policeman, however hard he tried. It wasn't that he took the law with him like an encyclopedia in his head, and noticed every little infringement. He just perceived things – even with his eyes shut and his lungs blissfully cloyed with opium fumes. And, once he'd perceived them, he couldn't help worrying at them, to try and work out what they meant.

He had come to the opium den with every intention of being a patron rather than a policeman. In fact, he had come as a reader of Coleridge and De Quincey. He wanted oblivion, just like every other addict, but this desire had been legitimized in his mind by the drug's literary associations. 

Another stupid Oxford prejudice, and he hated himself for it. He could have drunk himself to death on gin, but what great art had ever come from gin? Come to that, he could have applied to the Metropolitan police force, and walked straight into a job with officer rank, but what great artist had ever failed to wallow in his own misery?

Yes, he had come to opium hoping for soaring visions of Kubla Khan and Xanadu. But Sam's cynicism wasn't so easily unchained. When he was stupefied with opium, his thoughts just wandered in dark, resentful circles. He plotted revenge on Jack, and then remembered that it wasn't really Jack he hated. Then he plotted revenge on Oxford, and the image of it burning in his mind was as brilliant as any vision of stately pleasure palaces or caverns measureless to man.

And, always, in his jacket-pocket, he could feel the weight of Lily's last letter – not much weight, of course, because it was only folded paper. But recently, it had taken on the feel of an anchor, the lynch-pin around which the whole world turned. He thought about it every few seconds, even in his opium-addled dreams.

He hated himself too much to open it, although this didn't make a lot of sense, because it was perfectly possible that the letter contained dire but eloquent insults and recriminations. Jack had seemed to think it would. He had said that people weren't at their best when they were about to die, and he ought to know.

But it had Sam's name on it. She had never written directly to him before. Even a bitter letter from her would be... well, more than he deserved, anyway.

Apart from the shaded lamps and the exotic clientele, the opium den could not have been less like Coleridge's poetry. It had a vaguely institutionalized feel, like a dirty hospital – probably because it consisted of a long row of beds, with hangings that kept the opium fumes from dissipating. When you lay back to dream your opium dreams, you stared up into a dark carapace of smoke.

You got four or five to a bed, each man laid across it horizontally, as though they had just sunk back from a sitting position. Sam had taken his place among them, determined to forget himself, but he had come face-to-face with his eternal, internal policeman, because he had been unable to keep his eyes off his fellow patrons. And now the half-conscious suspicions he'd been entertaining about one of them were hardening into certainties – certainties he would probably have to do something about.

He had seen a pencil-sketch of Mathilde Marron, but he never would have recognized her from that. This woman had taken some care to be unrecognizable. Her hair was black – still a little clumpy from the dye she'd used – rather than the white-blonde of the other slave-girls. And she had obviously decided that gloves over her torn-out fingernails would have been suspicious. Instead, she had carefully shaped and painted and glued on the fake kind that Violet Pike had worn – the ones that were probably made from painted eggshell, and must have hurt like hell to remove.

But, more than this, she had a way of shaping her features, just with her expression. The shadows collected in odd corners, giving her a completely different kind of face – a hard, sharp, London face that you didn't dare look at too closely in case it took offence and headbutted you.

No, he never would have recognized her from the likeness he'd been given. It was the way she held the opium pipe, with the clay bowl pressed so close against her skin that it sizzled. And yet she never flinched, and there was no burn-mark when she reluctantly passed the pipe to the next man.

When she lay back, with an expression of trouble-edged bliss on her face, Sam very quietly lit a match and held it against her arm. Nobody noticed. Nobody noticed anyone in an opium den. You learned to be deaf and blind to the rest of humanity, because they were pressed in against you so close – sometimes crying, sometimes moaning, sometimes gurgling – that you would have gone mad otherwise.

But the really telling thing was that Mathilde herself didn't notice. The flame lapped calmly against her skin until it guttered and went out. And Sam thought: fireproof.

He had found her. Now he just had to muster up the energy to care.

In the end, it was the same policeman's instinct that drew him to her. He just wanted to know more. She was lying on her back under a cloud of opium smoke while her fellow slave-girls were in peril of their lives, and he wanted to know why. So very slowly, he dragged himself up onto his elbows and leaned over her.

"Mathilde Marron?"

He was impressed by the way she handled it. Looming out of the smoke, with his massive shoulders, he must have looked just like one of the gargoyles. But her eyes didn't widen in fright. She just turned over with a sigh and said, "No, mate. Sarah Siddon. Pleased to meet you."

In fact, if her eyes hadn't darted to her fake fingernails to make sure they were still in place, he might honestly have thought he'd made a mistake.

Sarah Siddon, he thought. Still an alliterative name, just like Mathilde Marron. People couldn't help a touch of vanity when they invented pseudonyms.

"Are you sure?" he asked, nodding at the clay pipe that was still sizzling against her arm. "The woman I'm looking for is fireproof, and there can't be too many of those."

She narrowed her eyes and focused on him for the first time. "Who are you with?"

"Nobody," said Sam, realizing how depressingly true this was. "I'm a policeman from Oxford."

"From Oxford," she repeated, in a flat, gloomy voice.

"I'm not here to hurt you, Miss--"

"You'd 'ave a job if you were."

"I just want to talk."

She sat up suddenly and curled a hand around his upper arm, as though she meant to escort him from the premises. "No, I'll talk. You will listen. Meet me out on the balcony in five minutes. And you won't say a word until I've made my case clear, d'you understand?"

***

Sam counted out five minutes in his head – probably too fast – and then shuffled onto the balcony that overlooked the wretched courtyard where the opium den nestled. He hadn't looked at it properly on the way through, but now he could see dark, ragged bundles hunched up in every doorway, trying to take shelter from the rain. Some of them were shivering, but others were terrifyingly still. They wouldn't thank him for shaking them to make sure they were alive, he knew that. But he wondered whether it was still possible to have soaring dreams of Kubla Khan with the rain soaking through your clothes and making you cold to your bones. Perhaps it was if you'd had enough, but these people didn't look as though they could afford very high doses.

He could see Mathilde more clearly out here too, away from the fumes and the dim light of the opium lamps. The face looking back at him was one of formidable intelligence.

"Why aren't you with the other slave-girls?" he said, forgetting her prohibition on talking. "Aren't you sort of... their leader? I mean, co-leader with Miss Syal--"

Mathilde wasn't diplomatic enough to hold up a hand. She let him know she wanted him to stop talking by slapping him in the face.

"The girls are gathering in Oxford, yes?" she said brightly. "But, if I goes back to Oxford, I'll hear what 'appened to 'er. Best-case scenario, I'll hear that she's shacked up with lover-boy again. Worst-case scenario – and the one she was determined on, mind – I'll 'ear that she's dead. In this place, for as long as I'm smoking, she's alive an' possibly single. If I could leave my girls to preserve my ignorance, imagine what I'd do to you if you spoke one word out of turn. Got it?"

Sam was silent for a moment, feeling sympathetic but also somehow smug. It was the most humiliating thing in the world to give up on your purpose because you'd been hurt.

And then he wondered whether what he was doing was any less pathetic. Suddenly – and for the first time in weeks – his animosity towards every other living thing faded.

"If I were to tell you everything I knew," he said, watching as she readied her hand for another slap, "it still wouldn't tell you anything."

"Even that tells me too much."

"Fine. We won't talk about Ellini, then. It's Jack Cade I need your help with. He's gone mad--"

Mathilde raised her hands to her ears with a frustrated screech. "Shut up – shut up! Do you think I'm stupid? Think I can't infer one thing from another? Why would 'e go mad if she wasn't dead?"

Sam raised an eyebrow. "Would you like me to answer that, or are you still determined to preserve your ignorance?"

"Well, you might as well tell me now! How could it be worse than what I'm imagining?"

Sam shrugged and rearranged his hands on the rail, enjoying her suspense. "He thinks she's dead. I don't know what to think. The corpse they brought to my morgue looked just like her, but it had fingernails. And--" He cleared his throat. "And after a day or two, it turned into a tree-stump."

He had expected another slap for this – or at least a withering look – but Mathilde Marron burst out laughing. "A stock!" she said exultantly, when she could breathe again. "A stock!"

"What's a stock?"

"It's a magical means of makin' a copy of a living person. You enchant moss oak or some other wood and call down an elemental to give it the semblance of life. Once the elemental's gone, it turns into a block of wood again."

Sam stared at her. "You mean someone made a copy of Miss Syal? Out of a tree-stump?"

"You aint seen what magic can do, peeler," said Mathilde. She seemed to be in an extremely good mood since the laughter. Her movements were quicker and more decisive, as though she had laughed away the opium fumes.

"And how do you know so much about magic?" he asked. 

"Gargoyles captured me because I read too much," she said, with a contemptuous shrug. "Just like Ellini."

"But you're--"

Mathilde raised her eyebrows. "What? Workin'-class? I learned to read and write at the ragged school, thank you very much, copper. And I aint stopped since. An autodidact is what I am."

She pronounced it with a glottal stop, but Sam was no less impressed. Besides, he had dismissed Lily just because she was a shop girl, and she'd written some of the most beautiful letters in the English language. And he had seen for himself that the other side of Coleridge and Kubla Khan was a bunch of ragged bundles shivering in doorways. Artistic snobbery could only take you so far if you had a policeman's brain.

Perhaps that was one shining, solitary reason not to hate himself. He had a policeman's brain, which meant that all Oxford thinking would evaporate sooner or later. Sometimes – as with Lily – it evaporated too late, but he would get at the truth eventually.

"So...who could have made a copy of her?" he persisted.

"She could 'ave done it 'erself," said Mathilde. She seemed to be fleshing out, away from the opium lamps. Her face was growing rounder by the second, waxing like the moon. "Females make the best magical practitioners – well, apart from Faustus, and 'e never gave a damn about gender, his own or anyone else's. She couldn't 'ave done it to fool the gargoyles, mind, even if she drenched it in sandalwood perfume. The master's too deranged to ever believe she was dead. And she wouldn't 'ave done it to fool you, because she knew you'd end up with a corpse 'oo 'ad fingernails. She must've been doing it to get away from this Jack Cade."

"I'm not surprised," said Sam grimly. "This is what I've been trying to tell you. He's gone mad – which would be fine except he's gone mad in charge of my city. He's probably instituted martial law by now. He's probably given all the seats on the city council to his thugs. You've got to help me stop him!"

And there it was. He'd actually said it. How long he'd been intending it, he didn't know. He certainly hadn't been aware of plotting Jack's downfall, or going back to the city he hated, when he'd been trying to kill himself with opium fumes. But he must have had some inkling, even then, or he couldn't have turned around this quickly.

It must have suddenly occurred to Mathilde to be cautious. Parts of her full, glowing face collapsed back into shadow. "What can I do?"

"Oh, it's more magic," said Sam, with an irritable wave of his hand. "Apparently, Miss Syal put an enchanted bracelet round his wrist that makes him impossible to kill."

"Curiouser and curiouser," said Mathilde. "An Achilles cuff? She wanted to get away from 'im, but she also made 'im invincible?"

"She wanted him to suffer. Believe me, he deserves it. But apparently, you're the only one who can defeat him, or take off the bracelet. He wants you to. He's been asking for you. I told you, he's losing his grip on reality."

She bit her lip and looked back towards the opium den. Little tendrils of smoke were curling out of the open doorway, as though beckoning to her. And for the first time, Sam wondered if she hadn't gone there to forget, but to remind herself. Perhaps the sizzling darkness and the thick air were so like the fire-mines that it was... comforting to her. You could get addicted to your prison very easily, if you didn't have much of a reason for leaving. Perhaps it was even happening with him and Oxford, right now.

"Think about it," he said. "I'll see you again. Where else in London can you get opium this cheap? And you'll have to go back to the slave-girls sooner or later. I know you because I knew Miss Syal. You're more than individuals, aren't you? You're Charlotte Greys, and you need the other Charlotte Greys to be complete. How are you going to get anything done with pieces of yourself scattered about the countryside?"