Sam found he did his best thinking in the morgue these days. For one thing, it meant he could avoid the infuriating stupidity of the living. The dead were not much smarter, but at least they didn't hurt anyone.

Down here, it was hard to even catch a glimpse of the living. The high, ivy-covered windows of the morgue were at street-level, which meant he could only see the occasional pair of boots wandering by – although they wandered so disconsolately that he couldn't help being reminded of the riot the night before.

It had been very gentle in terms of casualties. Nobody had been killed at all. Gleeson was recovering in the Radcliffe Infirmary, and the doctors there were in raptures about the surgical precision of the wound. The bullet had passed clean through, they said, without hitting any arteries or bones or vital organs.

They said he had been very lucky, but what they meant was that Jack Cade had been very brilliant. People were always meaning this to Sam, especially since last night. Even the birds in the trees seemed to be singing his praises, and casting doubt on Sam's ability to stop him.

But the riot hadn't been gentle in terms of University property. College buildings that had been standing for five hundred years were now in ruins. Several important exhibits at the Ashmolean and the Natural History Museum had been scorched. Amongst the casualties had been a Stradivarius violin, a complete skeleton of an Iguanodon, and an embroidered mantle that had once belonged to the Red Indian chief Powhattan, father of Pocahontas.

And, earlier that day, a solemn deputation of Professors – one of them cradling the broken Stradivarius as though it was a wounded child – had turned up at the town hall to make a formal complaint about their loss. They weren't angry, just bereft. They had trooped in looking plaintively confused, as though they didn't understand why anyone would do such a thing. Sam had heard these men talk blithely about murders and massacres over their after-dinner port and cigars, but, when they contemplated a broken Stradivarius, they couldn't keep the whimper out of their voices.

"Well, for heavens' sake, give them back the little mother," one of them had said, as though he thought Sam was keeping her concealed under his coat.

And he pitied them, in a way. But, at the same time, he hated being on their side.

So he retreated to the morgue whenever he could, where he could be away from the viciousness and the childlike innocence of Oxford's inhabitants. It wasn't that he was conflicted. He knew now – if he had ever been in any doubt – that Jack was dangerous, and had to be stopped. He was just... disturbed to see the parallels between them. To know how much Jack had been hurt.

It would have been all right if he was insane; in his more deranged moments, Sam didn't pity him in the slightest. But he was just sane enough – or sane just often enough – to realize what he'd lost, what he'd done, and what had been done to him.

That was one of the reasons why Sam hadn't been able to arrest him last night. He wasn't sure whose side he was on anymore.

But that was stupid. He was on the side of not being an idiot, and no-one had behaved more idiotically than Jack.

Still, Sam had seen Gleeson's dark blood spreading out in the gutter and known that he couldn't risk anyone else getting hurt. He had to find a way of arresting Jack that involved a minimum of violence. He had to take him by surprise, somehow. Trick him.

And, since he knew exactly where and when he was going to strike next – since he had Jack's insolent, scribbled notes to help anticipate his actions – all he could do was try to turn the mortuary into a fortress. Or failing that, a trap.

The most important precaution – and the one he'd been most proud of – had been to place Mrs Darwin in a room that couldn't be reached except through the morgue, where Miss Syal's body was still lying on a trestle-table, under its white sheet. If Jack wanted to hurt Alice, he would have to face the corpse of the woman whose death had driven him insane.

Sam felt as though this was cruelty akin to the capture of Mrs No-name, and that it might almost make them even.

The other precautions had been more mundane. The doors and windows were secure. The officers were men he was sure of, although he had instructed them not to engage Jack directly, not to threaten him with a gun, and – most importantly – not to try reasoning with him.

He hadn't thought to ensure that there were still some men he trusted back at the station. That was a problem of numbers really. He just didn't trust enough men.

There was a gentle cough behind him, and he turned to see Dr Petrescu standing under the white-tiled archway that led to the post-mortem room, looking pale but composed.

Of course. He had come down to formally identify Miss Syal's body. It was unusual to do that after the post-mortem examination, but Sam had been putting it off because he was worried about the doctor's mental state.

Dr Petrescu was behaving in every way like a man trying to put his affairs in order before the end. Yesterday morning, he had sent a note to the housemaid at the Faculty, asking her to return all his borrowed books.

Sam didn't want to make him even more morbid by confronting him with the sight of a dead body. But he was running out of time. It was the height of summer. And even though it was cool in the morgue and they had plenty of ice, the bodies would have to be buried soon.

So he led Dr Petrescu into the room where the four bodies were laid out on their trestle-tables, each one under a white sheet.

He knew something was wrong as soon as he saw the sheet over Miss Syal's body. The shape underneath it wasn't right. It was too short – too pointed in the wrong places. It didn't have the recognizable curves and contours of a human being.

He lifted back the sheet dreading what he would see, but no amount of paranoid dread could have prepared him for it. 

It was a log, with leafy shoots sticking out of it like the bristles of some erratic beard. The bark had an ancient, grizzled texture – and, for a moment, the ivy-tinted sunlight coming in through the morgue's windows almost made Sam feel as though he was in a woodland glade. Perhaps all the bodies in here had turned into mossy tree-stumps. And the ivy would reach in, and shoots would grow up through the cracks in the flagstones, and the whole city would be reclaimed by the wood. Sam wouldn't have protested in the slightest.

His first thought was that Jack had stolen the body, but the Jack he'd seen last night wouldn't have had the composure to lift up the sheet, let alone carry his stone-cold, stone-dead lover out of the morgue.

Could he have ordered somebody else to do it? Bribed or blackmailed one of the officers at the mortuary? But Sam couldn't imagine him wanting anyone else to touch her either. It would be too disrespectful – too impersonal – for a man who loved her so much that he was prepared to burn the city to the ground to avenge her.

Besides, it had been Ellini Syal under the sheet when Sam had locked up the morgue yesterday evening. He'd slept with the key clenched in his fist all night, which hadn't been very restful.

"Wha-?" he said, spreading his hands helplessly as they stared down at the log. "What does that mean?"

This was one mystery too far. He felt too weak and appalled to even hazard any guesses. Besides, it was obviously magic, which meant that no amount of reasoning would do him any good. He didn't need Dr Petrescu's next words to convince him of that.

"You know, this will probably raise more questions than it answers, Inspector, but people used to say that, if the fairies wanted to take a human captive, they would leave a stock – a magically transfigured log – in his or her place."

The doctor sounded very far away. Perhaps he would have sounded far away even if Sam hadn't been frozen in astonishment. He was drawing into himself, wasn't he? Carefully winding in all his wandering thoughts, in preparation for the end. Even though this was – what? Grounds for hope? An enticing mystery? – it was far too late. It couldn't convince Dr Petrescu that the world wasn't horrible, or that he wasn't going to die.

Sam's shoulders sagged. He didn't want any more magic. He'd already been forced to chase after winged-gargoyle creatures with fourteenth-century chains. What was the point in being a police officer – what was the point in the whole science of detection – if dead bodies could turn into logs overnight?

But no, this was right, wasn't it? It fitted in with what he already knew. The body had been a copy – and an imperfect one, at that, because it had had fingernails, whereas Miss Syal didn't. The copy had died instantaneously, whereas Miss Syal had been injured but alive when Mrs Darwin had gone into the University Church.

So where was the real Miss Syal? Dead somewhere else? Or-?

"Are you saying she could still be alive?" he said, turning back to Dr Petrescu.

"Nobody should say that," said Dr Petrescu, removing his glasses and polishing them on his sleeve. "It would be too cruel if we were proved wrong."

Sam stared down at the log – at the mesmerising patterns on the bark – and wondered where he could find out about magic. It was a banned substance in Oxford, except for the kind that couldn't be helped – the prophesying, fang-growing, or walking through walls that some new-breeds did without thinking. Still, it seeped under the cracks, despite the rumours that practising magic made your hair and your teeth fall out.

He would have to find someone to look into it, if he managed to live through the night.

Still, there was one good thing about this baffling new development. He finally knew something that Jack didn't. He wasn't sure what it meant, or what good it would do him, but it was the only advantage he had, so he needed to guard it as carefully as he was guarding Mrs Darwin and Dr Petrescu.

He turned back to the doctor and said, "Don't mention this to anyone. At least, not until we know what it means."

Dr Petrescu gave him a rather pitying look over the top of his spectacles. "I wouldn't trust it to mean anything good, Inspector. That's not how life works."