On a hunch—a gloomy, bitter hunch born of his belief that all other law-enforcers were incompetent—Sam went straight back to the prison and made Warder Fletchley open the trapdoor in Ellini's cell. He didn't need the blast of cold air and the smell of damp which followed to tell him that it led straight into the caves.

Of course it led straight into the caves. These moronic warders had covered Ellini's face for five years and given her a cell that an infant could break out of. They wouldn't miss the opportunity to make life even easier for her by ensuring that the trapdoor in her cell led to a place where she could disappear completely—into another world, if Fabienne Desault's book was to be believed.

She had given Sam a copy of her local study on Black Annis and Charlotte Grey, saying that she expected it back in mint condition. Sam was currently bending down the corners of the pages to annoy her.

Warder Fletchley was bewildered by his return, and even more bewildered by his irritation. He seemed to think it was the most natural thing in the world that the trapdoor should lead to the caves. As Sam and Jack descended the stone steps into the cool, still, underground air, he tottered after them, trying to explain his logic.

"Well, of course the secret door leads into the caves, sir! It'd be no good having a secret door that just came up in the vegetable patch, would it? If there's a riot goin' on, the Warders 'ave got to be able to escape to somewhere they feel safe. And they know Charlotte Grey'll be on their side, see, 'cause all the Warders used to leave little dishes of fruit and cups of wine for her down here. 'Course, we're not that superstitious now. But I wouldn't say a word against her, all the same."

Jack, seeing the murderous look on Sam's face, ushered the Warder back to the trapdoor, and suggested he go back to his office and put his feet up.

"After all the trouble you caused in Agra, I would have thought any member of the 4th Bengal European Light Cavalry would be glad of an opportunity for a rest."

Sam raised the paraffin lamp he'd taken from Warder Fletchley's lodgings, and tried to get an idea of his surroundings, but it was useless. The beam of the lamp couldn't penetrate more than a few feet into the gloom. The darkness seemed to suck at it, making it shrink and dwindle, like a boiled sweet. He could see a few stalagmites and stalactites, and formations of milk-white stone that looked like mounds of whipped cream, but that was it. The dark silence stretched endlessly in front of them, in mockery of light and sound.

Since he couldn't see the caves, Sam directed the beam of light onto the pages of Fabienne Desault's book and read the description of Charlotte Grey—even though he'd read it five times in the carriage on the way over.

The first recorded sighting of Charlotte Grey occurred in Cherry Hinton in 1641, when the local Blacksmith, Geoffrey Grey, walked past the western entrance to the caves, and saw what looked like the ghost of his dead daughter, Charlotte, except she now had white hair, and black ribbons twisted artfully up the length of her arms.

Charlotte had gone into the caves ten years before for a game of hide and seek, and never come out again. Geoffrey Grey had led the search party himself but had come back empty-handed. Since the caves extended for twenty miles under the Gogmagog hills, and were filled with pits, potholes, and hidden underwater lakes, most of the village folk at the time had been forced to conclude that she'd fallen somewhere and broken her neck, or else drowned in one of the deep pools lying hidden in the darkness.

Geoffrey had been toiling on, in grim acceptance of her death, for ten years before this sighting. And, since he wasn't the kind of man to suffer from hallucinations, or make up stories, his sober account of an apparition which seemed just like his daughter, looking no older than the day she'd died, but with hair turned white, as though from the shock of some terrible event, caused quite a stir in the little village of Cherry Hinton.

The locals had always believed there were creatures living in the caves in any case—sometimes fairies, sometimes witches, sometimes the Devil himself. Indeed, legend had it that the cavemouth at Fulbourne, five miles away, had been an entrance to hell, which Saint Stephen was said to have miraculously dammed up in the eighth century.

Since their ears at the time had been filled with fire-and-brimstone sermons about the perils of sin, the villagers had little difficulty believing that Charlotte was a ghost escaped from hell, still smarting and smouldering from the fires.

She didn't speak to her father. Nor did she appear again for another fifty years. By that time, there was no one left alive to confirm her resemblance to the original Charlotte, but they remembered Mr Grey's account of a young woman with white-blonde hair and black ribbons wrapped around her arms. Over the next two hundred years, she was sighted at Fulbourne, Sawston, Stapleford, Babraham and Great Abington—always as a silent spectre with white-blonde hair and those strange black ribbons twined about her arms.

There was a picture on the next page: a black and white engraving, apparently made by an eyewitness, which showed a thin girl with white-blonde hair, dressed in a tattered, sleeveless black dress. The most interesting things about her appearance—the only things which made her seem even remotely mystical—were the black ribbons round her arms. They covered her fingertips completely, like black bandages; then they wound up her fingers, crossed over her palms, and twisted themselves round her arms like climbing ivy, almost reaching up to her shoulders. They looked like very long, very ragged, black gloves.

"She looks good," said Jack, who'd been reading the book over Sam's shoulder. "A bit skinny for my taste—" He grimaced, and added, "as far as I know—but impressively sinister. Is she supposed to be a kind of cave-siren? You know, luring innocent travellers to their deaths in the darkness?"

"It's a bit of a grey area," said Sam. He flicked over to the next page and read aloud: "'Because she always looked the same, and because her help was by no means certain, she began to be associated with justice in the minds of the villagers. A rumour spread that, if you were a good man, who had faith in her mercy, you had nothing to fear from her. She made sure the unholy got lost, and the righteous found their way safely home again. This, of course, reflects the levelling nature of the caves themselves. In the pitch-darkness under the hills, you only have your character and your faith. Beauty and fashion, playacting and pretending, do not matter in the dark. The dark leaves you bereft of everything but your inner self, so you had better be confident that your inner self is worthy.'"

Sam stopped. There was so much of Fabienne's aggressive manner in that last sentence that it would only depress him to continue. He was feeling as though the world was out to get him as it was. But this was where he did his best thinking, when he was surrounded by idiots, who were trying to prevent him from getting at the truth.

Jack squinted at the picture. "She doesn't look much like Ellini, though."

"No," said Sam impatiently. "But Ellini knew her. And so did Helen Thorne's killer. So we find her."

"Even if she doesn't exist?"

"Especially if she doesn't exist."

"Ah," said Jack, swinging his arms in the cold air. "That's some kind of an Oxford riddle, isn't it?"

Sam ignored him and turned back to the book.

Could Charlotte Grey have been Helen Thorne's killer? Unlikely. He didn't know many killers who would stand over their dying victim, muttering their own name. And whoever strangled Helen Thorne had had long nails—possibly claws—whereas Charlotte Grey's nails were wrapped neatly in the black ribbons which then went on to twist up the length of her arms.

In fact... Sam leaned closer to the illustration, willing it to yield more detail. The ribbon was wrapped thickly around the ends of her fingers. Further up her arms, the black ribbon and white skin formed a kind of loose, helter-skelter pattern, but, at the ends of her fingers, the ribbon was wrapped like a bandage, without a sliver of skin showing through.

Had Charlotte Grey lost her fingernails, just like Ellini Syal? Was that another thing that connected them?

Sam started riffling through the pages of Fabienne's book. He'd read something about those black ribbons, hadn't he?

He came to rest on page five and waded through Fabienne's prose.

The thing that makes Charlotte Grey unusual in the annals of folklore is that details of her appearance are offered without any accompanying explanation. All accounts seem to agree on the black ribbons wrapped around her arms, but nobody has even suggested why they might be there. It is usual, in tales of witches or prohibitory monsters, for every detail to reinforce the point of the tale, or help the creature carry out its dastardly schemes. So, Jenny Greenteeth has hair like waterweed to help her lie concealed in the deep pools from which she snatches her victims, and Black Annis has long, sharp talons for scratching her prey to death, before she drinks their blood and hangs their skins up to dry outside her cave.

But Charlotte Grey did not wear those ribbons in life, nor would they serve to allure or terrify her victims. A great many arguments have been put forth to explain the white hair—of which my favourite is the tale that Death laid his skeletal hand upon her head, and her hair turned bone-white in sympathy. But the ribbons are an unexplained detail which storytellers nevertheless seem quite insistent upon. Whether they have any figurative significance is of course a different matter. Are they there to increase her patchwork and ragged appearance, in order to convey the idea that even the finest robes of state are superfluous in the deep darkness under the world? Do they suggest she is in mourning? Or are they simply present because they look so striking—a good visual detail to fix her image in the simple yokel's mind?

Sam sighed in irritation, thinking how easily she could have avoided the word 'yokel' in that last sentence.

Perhaps the killer had been under the impression that Helen Thorne was Charlotte Grey? That seemed more—but still not very—likely. It would be all very well if he'd shot her, or stabbed her from behind, but you had to get in close to strangle someone. The canal path wasn't very well-lit, but in the time it took to throttle someone, you would surely realize that something was wrong?

Unless you weren't going by sight at all.

Sam stood very still, and clutched hold of a stalagmite, to try and keep this idea from slipping away.

Supposing the killer navigated by a different sense? By smell? All of Helen Thorne's personal possessions had been left undamaged, except that perfume bottle. The killer had been angry enough to thump it into the ground.

"So," said Jack, still swinging his arms to entertain himself. "Are we going to be walking through here all night, then?"

Sam squeezed the stalagmite in annoyance. "Something wrong with that?"

"No. I'm just saying that there are twenty miles of these caves, and we're combing them for someone who probably doesn't exist. Could be time-consuming."

And you want to get back to Ellini Syal, don't you? Sam thought. But he didn't say it out loud.

Still, something about the sheer reasonableness of Jack's other objections rattled him, and he felt the need to lash out.

"That Robin Crake man didn't seem to like you very much, did he?"

Jack shrugged gloomily and kicked at the loose rubble on the floor. "He's probably done even worse things to me than the ones I can remember. For example, I don't recall him ever stabbing me with that knife, but, when I picked it up, my fingers could remember. Like my flesh could remember the pain even if my brain couldn't. I felt the same way with Ellini, except it was harder to place."

He hesitated, and then went on, with an embarrassed smile, "You want to hear something bizarre? She makes me crave cigarettes. I can't stop looking at her, but all the time I'm doing it, I just think about cigarettes. What do you suppose that means?"

"That Alice is a cruel, sadistic harpy?" Sam suggested.

"Yeah. But everything can mean that if you think about it hard enough."