The mist that came up off the Regent's Park lake was a dark old spirit, the Thames fog's pale little brother. It rose up when the air was chill, especially thin autumn mornings like this one, waiting for the short dark hours of November or February when it would have the run of the city all day. It forged out from the lake-bed in search of the weird and unsettling, and never had to search for long: Regent's Park was a bazaar of curiosities. The fog wrapped around the park's inhabitants, shrouded them all in wet woollen grey: the lost and lonely, the hiding, the forgotten, the only-sleeping and the something-worse. It clung to the enclosures of the zoological garden, invisible somewhere to Arthur's left, where strange animal noises rose disembodied through the secret-filled mist. Arthur had checked his pocketbook, ruled out a cab, and set out alone to fight the dread that had held him in thrall for two days. The thick, damp air had a taste of its own, and he shuddered at the thought that it was getting inside him.

It was a day for walking fast and keeping to oneself, which suited Arthur perfectly. Last night's vivid red sky had belied the morning's bone-invading chill, and he could feel the winter coming on. It stirred in Arthur the same old melancholy that haunted him every autumn. The summer had gone as fast as a trap springing shut, unappreciated when it was here and promising winter's grim terrors in its death. Arthur had a disconcerting feeling that he wouldn't be leaving London soon enough.

His fingers slipped unconsciously into his jacket pocket and worried at the letter in the envelope within. It had been waiting on the table when he woke, propped among the breakfast things his neighbour Selden had abandoned there. Arthur sipped the cooling tea from the pot, not so hot but round and wet and sweet, and beamed unselfconsciously. His brother's lazy, affable handwriting demanded to know what scandal had befallen Arthur at sea that he couldn't return home and share the gossip with his own family, and whether they should post an obituary notice in the Scotsman. It had made Arthur's stomach clench with homesickness. Fiona and Kate had each got a shade more beautiful and were asking after him. His father needed new glasses and thought Arthur should consider ophthalmology when he went back to his studies. His brothers were off to shoot in the Highlands without him, and Mr Clark at the Picture Post said thank you for the adventure story he'd submitted before he left for Greenland and that, with a few revisions, he'd like to serialise it. The time when he'd written stories to enliven the daily round seemed like a dream now. He hadn't written a line since arriving in London. The words wouldn't come. He used to write to a meticulous plan, deciding at the outset on a neatly-executed resolution. When it was real screaming and fainting and waking up in pantries and not having the faintest idea what was going on, Arthur rather felt he could do without adventure.

If the letter had been the only post, Arthur wouldn't have been trudging through the fog an hour later. But he'd had to glance at the folded telegram sheet wedged between the teapot and cup, even though Selden's name and address were printed on the outside. A glance had turned into a peer, and then he'd been reading the telegram with saucer-eyes. HEARD NEWS RE INGLIS STOP CAN'T SAY SURPRISED STOP MEETING ISABELLE AT ONE SHAN'T BE ABLE TO DINE STOP J STOP.

Arthur had swallowed his heart back down from his mouth and bought a morning paper at the tobacconist. He'd scoured it cover to cover, but found still no word about Inglis. The more time passed, the more Arthur was convinced that he was simply going slowly mad. He had set out across the park deep in confusion. Selden's correspondent 'J' – whichever of the nasty gossips he was - had sounded completely unmoved in his telegram. It didn't read like the grave communication of a discovered brutal death. On the contrary, it had elevated the terseness of the medium to an artform. The newspaper's silence was just as incomprehensible. If the bloody business was out, how could no one care? Perhaps, Arthur thought, this sort of thing was par for the course in London these days. Perhaps they'd read so much about mysterious disappearances and brutal murder that there was no popular appetite for it anymore. They cheerfully read and wrote thrilling serial stories about the macabre, and didn't give a fig when the real thing was on their doorsteps. Arthur had frowned down at the headlines. Why the British Association's visit to Swansea was greater cause than society murder for remark, he wasn't sure, but Londoners were very strange people.

Arthur fingered his brother's letter longingly as he made it out of the fog-smothered park and into the prim streets near Hardenhuish Square. What choice had the telegram left him with? If all of this was coming out, he could hardly hop on a train and avoid the investigation, the recriminations, and – what he hoped for most – the explanation that must follow. He wanted to be a writer himself, not a callous, sensational one, but the sort who produced works of substance and meaning. Out of professionalism, if nothing else, he couldn't drop out when the pyrotechnics had faded and everything was turning black. He had to know the end of it.

The door of Underwood Villa was opened by a slender maid with a soft Welsh accent. "Miss Addams is not receiving visitors at the moment," she told him rather kindly. Was that good or bad? She was scandalised by the news and had gone into hiding, or cared as little as the press and saw no need to act?

"And Mr Addams?" asked Arthur, trying to remember if he had ever seen Cecily's father.

"He's away, sir." She regarded him placidly and immovably.

"I wonder – it's rather an important call," said Arthur hesitantly.

The housemaid moved subtly, blocking his attempt to peer past her into the darkness of the hallway. "Perhaps you'd care to leave a message, sir?"

Arthur bit his tongue, then forced himself to go on: "It's, er, actually imperative that I see Miss Addams now – perhaps you could tell her it's Mr Doyle?"

"She is not receiving anyone at all, I'm afraid sir."

Even as the maid spoke Arthur heard other voices in the hall – one clear and even, one deep and punctuated with a low, rolling laugh. He frowned slightly, but the maid's sweet, blank expression didn't shift.

"Was there a message, then, sir?" she asked again.

Arthur sighed. "Perhaps you'd just tell her I called."

"Very well sir," smiled the maid. "Good day, sir." She nodded to him with a perfect pre-assembled smile, and closed the door.

"Where are we going?" William groped blindly with one hand, the other held fast in Drusilla's, a strip of frayed-edged red silk from God-knew-where tied over his eyes.

"Down into my burrow," Dru told him, tugging him onwards. "Down, down, down," she mumbled, talking to herself now, or to someone else who wasn't there.

William lifted the edge of the blindfold and squinted out. Dru wheeled around instantly: "No peeking! Wicked boy."

"How did you know –"

"Course I knew," tutted Drusilla.

"We're just inside the rail shed, aren't we?"

"Not all as it seems," Drusilla chastised him, tapping his nose with a pointed finger. He blinked behind the blindfold. The sun must have been only minutes from dawning by now, but Darla and Angelus hadn't returned. He and Dru had spent the night laying waste to the bed. That was properly a daytime activity, really, but it had been a special occasion.

"Drusilla?" She'd let go his hand. He hadn't quite the words for how it felt when she was gone: it didn't seem colder, there was no loss of her breathing or her pulse, but he could still tell. He frowned. "Where have you gone?"

Just silence.

"Dru, love?" He pulled the blindfold up onto his forehead and looked around. It was the inside of the rail shed, of course – there was the rumpled, ruined bed, there was the chest with her dolls arranged on top in order of how well they'd behaved. Nothing in the least out-out-of-the-ordinary. And no Dru. He took a step forward – into nothingness.

William fell.

Drusilla's hysterical giggling echoed around him where he sprawled painfully on something alternately hard and sharp. "Ow!" he complained, then caught Dru's infectious laughter.

"Naughty, wicked Spike," she told him. "Told you not to peek. Should have tied your hands."

"That's right, I'm your wicked Spike," he agreed, feeling neither wicked nor anything much but a dead, defeated poet as he lay on his back on the floor. "Where are we?"

"Down where the demons lurk," she told him. He hauled himself to his feet, shaking his shoulders and setting his face into the expression of a naughty, wicked Spike. "Down in the belly of the earth."

They were in a cavern, quite dark except for the shaft of light lancing down from the rectangular hole through which he'd fallen. The ground underfoot crunched, and fragments of chipped stone fell from his back as he straightened himself. Spike took a step into the darkness, and felt something else under his feet: wooden slats, metal rails.

"The underground railway?"

Drusilla just tilted her head from side to side. "Big, yellow-eyed, screaming snake demons. This one's abandoned its lair."

"It goes right under the shed?"

"Lurking beneath us. Sometimes it screams," Dru told him. "But nothing screams down here any more."

William squinted into the gloom. When he was alive, the underground had given him unpleasant shivers and he had used it little. Truth be told, he kept mostly to his own part of town: London was a city he could only conceive of in small chunks, and roaming about too much in the dizzying expanse of it all left him disoriented and confused. He took the underground only if it was absolutely necessary, hating the jostle and claustrophobia and dirt down on the subterranean platforms. The trains were grim, too. Everyone crammed cheek-by-jowl, pickpockets and thieves lightening your pockets for you as the compartment jolted and you clung to the straps and bars to stay upright. But someone influential seemed to be in favour of the whole business, and they'd been building new lines linking in more and more of the city. There was even a tunnel under the Thames now, as if any right-thinking person would want to go south of the river.

"How did you get down here?" he asked Drusilla.

"Ladder and steps. Easy." She indicated one laid out neatly near the trapdoor through which he'd fallen.

"Why did you take it away?" he asked, wounded.

"Wanted to give you a surprise," Dru answered brightly. William propped the ladder back up so it rested in the mouth of the trapdoor hole, amazed that Dru had managed to do it so quietly inches from where he'd stood, blaming the blindfold for confusing all of his senses, and climbed back up into the shed. He fetched a lamp and returned, carrying it carefully down the ladder, and held it up to examine the underground space.

"I found it when we moved in. I hide here sometimes, quiet as a mouse, and listen to grandmum and daddy. They never know I'm here. I come back up and tell them what they've been doing, and they get all uncomfortable, say I've had a vision!"

William regarded his coyly smiling girl with astonishment. "You're a bad girl," he admonished, grinning. "Your bad girl," Dru told him, scrunching up her nose just like a mouse.

The lantern's light showed track stretching away far ahead in one direction, but ending a few metres from the trapdoor in the other. There, the rails ceased and a low wall blocked off any further investigation. The air smelled stale and dusty, with no sign of recent human presence. A few pieces of rail and split sleepers lay abandoned by the track, and at uneven intervals there were gaps in the rail. "The trains don't come down here," called William to Drusilla. "It must have been an abandoned project."

"Too far out for the demons," agreed Dru. "They like places with more to feed on. Nothing to eat here. Lots of rats, further on. Daddy made me eat rats once. Lots of 'em."

"Bastard," muttered William, stumping back up the rail to the trapdoor. "So, your sweet sadistic sire doesn't know about the tunnel? Never seen you come down?" Dru shook her head and smiled, chewing on a fingernail. William grinned. "Nice to know. I think I like it down here."

As he spoke, the ceiling of the tunnel suddenly began to reverberate to a series of rapid thumps and bangs emanating from the shed above. William and Drusilla looked up, and Dru started to beam as they identified the noise of the door banging shut. "See? Quiet little mice. Don't frighten the elephants," she whispered.

"Trapdoor's open, they're bound to see it," William worried.

"You shouldn't have gotten so drunk." Darla's voice was clearly audible as she paced over their heads. "That last one's blood must've been seventy percent proof!"

"Ah, but it's when I'm drunk I feel like serenadin' you," drawled Angelus. "My mother and father, next door they do lie, kissin' and embracin', why not you and I? Ow!" The sound of something hard connecting with something that crumpled. William stifled a snort, and applause for the punch as critical commentary on Angelus's musical abilities.

"Am I going to have to knock you unconscious, or are you nearly there yourself?"

"Where're Dru and that troublemakin' little sod?"

Underground, William bristled.

"Catching fire, maybe. Then we won't have to worry about taking them with us."

Taking them with us?

A crash, as though something had been knocked down by a flailing limb. "That's my children you're talking about! Course they're coming with us." More staggering steps, and Angelus's muffled voice turning into a leer. "And now, I'll come to thee..."

Further sounds of heavy footsteps, then Darla laughing derisively. "Darling boy! With that much gin in you, you can forget that idea."

"Are you saying I'm not man enough for the job? I'll have you know..." A heavy, unmistakeable thud: William had heard enough bodies hit the floor lately to recognise that one.

"Sleep it off, sweetheart." Darla's voice was drowsy, talking to herself. "I'm gonna make you pack tonight." She began humming, then the bedsprings creaked and she settled into silence.

Drusilla, her forefinger still pressed over her lips, beamed crookedly at William through the darkness. "Quiet as ghosties!" she told him, peering ahead into the blackness as though one might come and join them. William hugged his arms around his body despite himself. Dru had been right: they could sit quiet and not be discovered. It wasn't fear of the others' wrath that made him feel the cold down in the tunnel. It was something in Darla's words – taking them with us, packing tonight – that tugged disconcertingly at his mind. He hadn't planned on going anywhere. Certainly not anywhere with this shambolic crew. And definitely not when he still had so much business to settle with London.