'Hello, Sherlock.'

Sherlock turned. He was standing in the ruins of what once was a city, but was now a crumbling shell of its former magnificence: the tower blocks, their roofs scratching the sky, were open and hollow, gashes yawning in the flaking stone; the pavement was worn and split open, revealing pipes snaking like rusted veins beneath the surface, and weeds had burst through the fretwork of cracks that had spread across the road. Overturned cars were strewn amidst the rubble of collapsed buildings, shattered windows staring like unseeing eyes at the wreckage. Ash was falling silently through the air like snow, coating the ground with a thin, white layer that crunched underfoot.

'Who are you?' said a voice. It took Sherlock a moment to realise that it was his own.

'I am your nemesis,' said the man standing opposite him. He was dressed in a grey suit and tie, the shirt beneath crisp and white as the drifting ash. Despite the clouds of dust that rose with every step; despite the rust that coated the car he was leaning against, his clothes were miraculously clean and unblemished.

Sherlock stared across the street, feeling the ash settle in his hair and on his shoulders, watching as the man took a step closer to him. He walked with an odd, swaying gait, his hands thrust deep into his pockets, a smile playing on his mouth as he sauntered casually towards Sherlock.

'People say that you can't have enemies,' the man said, his dark eyes locking with Sherlock's. 'Apparently it doesn't happen any more.' He grinned, and his teeth glinted. 'But I know for a fact that I'm yours. Isn't that right, Sherlock?'

Sherlock didn't take his eyes off the man as he stopped a few feet from him. There was something about that face - those dark eyes - that he couldn't read.

'You can't do it, can you?' the man said, seeming to notice the slight flick of Sherlock's eyes up and down his body. 'You can't make any deductions about me.' He chuckled, bringing his hand up to his mouth like a child and biting the knuckles. 'It's because I'm different, Sherlock.'

Above them, black clouds were writhing, spreading like ink on a wet cloth across the sky.

The man took another step forward so that his face was close to Sherlock's, the black eyes still boring into the other's.

'I am your downfall,' he whispered.

Sherlock felt coldness seep into his chest like some noxious gas, spreading throughout his body so that every limb seemed leaden.

'Who are you?' he breathed.

The man grinned and his black eyes seemed to grow, the dark tunnels at the centres expanding so that his face was engulfed with shadow. The cityscape was disappearing, being eaten away by this terrible, crepuscular nothingness.

'Goodbye, Sherlock,' said the man's voice. 'They're calling for you. Can't you hear them?'

Sherlock staggered backwards, lashing out at the man, hearing his snide laughter echoing in the darkness, and -

'Sherlock!'

He opened his eyes.

There was a man staring at him, both his hands gripping Sherlock's wrists in a vicelike grip, his brow furrowed and anxious.

'What? Where am I?' Sherlock asked, sitting up.

The man looked more concerned than ever. 'We're in a taxi, going home. Don't you remember?'

Sherlock pressed his fists into his eyes and rubbed hard. The man seemed familiar to him, yet he could not quite remember his name.

Apparently he seemed unwell, for the man said, 'Are you alright?'

'Fine,' Sherlock said, still staring confusedly at him.

'Can you remember your name?' the man asked, still holding his wrists, though more gently now.

Sherlock blinked twice. 'Sherlock Holmes.'

'Well, at least you remember that much,' the man muttered, relaxing slightly. 'What about my name?'

Sherlock flinched. He could feel it on the tip of his tongue, waiting to be spoken. 'John - John Watson.'

'Everything seems in order,' John said, settling back into his seat with a sigh. 'I've never seen you sleep in a car before, let alone a taxi. Just tired?'

'No,' Sherlock said. He still felt dazed, though he was quickly regaining his efficient manner.

John blinked, frowned and turned his head to gaze briefly out of the window. Then he turned back, looking seriously at Sherlock. 'Thank you, by the way.'

Sherlock looked at him, surprised. 'What for?'

'For saving my ass from that bonfire, that's what for!' John said.

'Oh, yes,' Sherlock said, the memory flooding back. 'It was the only thing I could have done. Have you been checked over?'

'Lestrade brought an ambulance and they made sure I was alright. You were the one who told him to.'

'I did?'

'Ye-es, you did.'

'Unsurprising,' Sherlock said brusquely, turning his head slightly to watch the city blur past. He felt odd; his brain seemed to be moving very slowly, and there was a cold, sluggish weight seeping across his chest, just like in the dream.

The minutes ticked past. Sherlock felt his head droop, yet he didn't feel remotely tired. He could tell John was watching him and kept his body firmly upright. He didn't want to worry him unnecessarily.

After a while, John stopped the taxi a few streets away from Baker Street and they exited the vehicle. As they began the walk home, Sherlock realised that his symptoms were getting worse. He was finding it more and more difficult to stay upright, having to surreptitiously hold onto railings and walls, and the cold weight was spreading down his arms and legs. His vision was becoming blurred, and sounds boomed unnaturally loud.

When they reached a quieter, darker street not far off Baker Street, John stopped to call Mary and inform her that he was still alright. She had agreed to allow him to return to Baker Street for the night, as it was more convenient for a hospital in an emergency, and was going to visit the following morning.

As John talked on the phone, Sherlock leaned heavily against the railings, feeling numbness begin to fester in his chest. Something was wrong. These weren't the normal symptoms of exhaustion or illness.

A horrible suspicion dawned on him. As though in a trance, he drew the sleeve of his coat up his arm and undid the cuff of his shirt.

There was a small, red mark on his forearm. The veins around it were burning red through his skin, which seemed grey and sickly.

He had been poisoned.

'Sherlock?'

John had finished his phone call, and was moving towards him. 'Are you -'

'John,' Sherlock said. His voice seemed distant and far away. 'I think I've been drugged.'

John stared at him for a few moments. Then he took two quick steps forwards, seized Sherlock's arm and stared at the mark. Horror began to dawn on his face.

After a second, he looked up and said sharply, 'Are you experiencing any symptoms? Dizziness? Nausea? Blurred vision?'

'All of them,' Sherlock mumbled. He couldn't see John any more for the smears of colour that were swirling across his vision.

'Sherlock, I need you to stay awake. Can you do that for me?'

Sherlock gripped the railings tightly and tried to focus on John, but there was a roaring in his ears and darkness was leaking into his eyes.

'Sherlock? I need you to stay awake. Can you hear me?'

'Can't feel my hands,' Sherlock muttered.

He felt arms around him; felt him being supported into a sitting position on the cold ground. Pain was burning in his temples, making his brain slow and stupid.

'I need to call Lestrade, Sherlock. You have to stay awake for me. I'm right here.'

Sherlock grunted an affirmation, and after a moment he heard John talking urgently into his phone.

'Looks like he was drugged. Started about twenty minutes ago. He forgot where he was. Yes, I - no. Yes, alright.'

Sherlock felt a hand on his arm again and knew that John had crouched down beside him again. 'Lestrade's on his way, Sherlock. They're going to make sure you're alright, and then they're going to take you to the hospital so you can -'

'No!' Sherlock's eyes flew open, and he grabbed John's arm blindly. 'I'm not going there. Not going.'

'But Sherlock, you have to -'

'Baker Street,' he said, attempting to bring his eyes into focus. 'Antidotes there. Not hospital. Please.'

The roaring had become louder, blocking out all other noise so that he couldn't hear the cars on the distant road, or John's voice, or the wail of the police siren as Lestrade sped towards them on the busy streets of London. Sherlock wondered how long he had been sitting on this cold pavement; how long the drugs had been in his bloodstream.

'Lestrade's here,' he heard John say through the deafening roar. 'You're going to be alright now.'

Blackness was shrouding his eyes. He couldn't see; couldn't hear. His whole body felt cold and numb.

His head dropped onto his chest, and he felt himself falling into darkness. The last thing he heard before unconsciousness dragged him down was John's voice, echoing in the crushing blackness.

'SHERLOCK!'