St. Bartholomew's Hospital, London.
Summer 2011.

John Watson's eyes were fixed on the man standing atop the old building. His hands were shaking, but still clutched the phone tightly to his ear.

Sherlock wasn't speaking. He heard the sound of something hard colliding with something else.

He had dropped the phone. He wasn't listening.

He was going to jump. Oh God.

Sherlock was going to jump.

In desperation, John called out from the street below.

"Sherlock!"

What use it would be, he didn't know. But it was something. It was the last thing he could possibly think of doing.

The man on the roof spread out his arms. What should he do? What could he do? Sherlock leaned forward. His body reached far beyond the point of recovering its balance.

"Sherlock..."

He fell.

The world seemed to freeze. John's blood turned to ice as he watched the man tumbling down from the roof.

His feet wouldn't let him stay. His mind couldn't focus.

He was a doctor. There was still something he could do.

Sherlock wouldn't die. He couldn't. The man was too stubborn to let something like this kill him.

John surged forward, building his pace as he clung ever tighter to that one hope.

He turned the corner and passed the building that had stood between himself and the fall.

...

His eyes widened.

He couldn't believe it. He really couldn't believe it.

The scene before him was beyond impossible.

He covered his face with his hands and took a huge breath.

"I know you've done some crazy things, Sherlock," he muttered under his breath, "but this is mad, even for you."

"Brilliant plan of yours, Sherlock. I mean, Sherlocks," the Doctor said as he ushered his three new companions out of the TARDIS. "Absolutely top notch. Simple, yet elaborate. Totally my kind of thing. You see, Doctor Watson, it's all quite elementary." He paused to see if he could get any kind of reaction from the two Sherlocks, but they appeared oblivious to his reference. "Oh well... Basically, we need a way to work out what all this... this Sherlock Holmesiness is all about, don't we? I mean, why are there two of them? At different times, but apparently on the same timeline? Do you see?"

The four men emerged onto a street not entirely dissimilar to the one they had left behind in old London, but for the fact that here it was mid-afternoon and somewhere in the early twenty-first century. Watson's reaction to everything happening around him was fascinating to watch.

"But that isn't all," the Doctor continued. "Oh no. See, we need to be sure that this is the same time line, right? Don't we, Sherlocks?"

"Quite," said Holmes, nursing his jawline.

Sherlock clutched his stomach. "Do shut up," he grumbled.

"Are you following all this, Watson? Because I don't want to have to go over something so... elementary again," said the Doctor.

He paused.

"Nothing? Not even a giggle?" He frowned. "Oh, never mind. I thought it was funny. Now, where was I? Ah, yes. There are two factors, Watson. Two measureable factors we need to consider on this case. Time, and space."

Watson nodded, though his expression was entirely blank. "I... see," he said.

"Our minor conflict back at Baker Street was a method in which to determine the nature of our connection, albeit only to a small degree," said Holmes, his voice still a little hoarse from being kicked in the stomach. "By violently striking one another in different areas, we are able to determine, to a practical degree, our... unique relationship."

"And to what degree are you two connected, Holmes?" Watson looked at each of the three other men expectantly.

The Sherlocks muttered between one another in an attempt to find the suitable terminology.

"Psychosympathetic extrasensory perception," the Doctor gesticulated. "Or something else fancy and clever-sounding like that. Think of sympathy pains or telepathic links between twins. Only nothing like that at all, on account of these two being the same person."

Watson removed his hat and scratched his head. "I fear I might make myself sound quite the fool for asking, but... what exactly does that prove?"

"It proves two things," said Sherlock through gritted teeth. "For one, it proves that our connection extends beyond our names, places of residence, DNA structures and poor choice in intellectual companions. For all intents and purposes," Sherlock grimaced, "we can consider he and I to be exactly the same person."

"Which should, theoretically, be impossible," added Holmes.

The pair seemed thoroughly ruffled at the thought.

"And the second thing? You said that this proved two things," said Watson.

"It proves that both of me can hit very hard," Sherlock grumbled.

The Doctor chuckled. "Come on, chaps. That's only half the puzzle! We've still got lots of impossible stuff we need to deal with before we get this thing sorted out."

"I trust that our current course of action, then, has something to do with the other half. Perhaps something pertaining to that beacon you mentioned back at Baker Street," said Watson, pleased with himself for catching up to speed. He pondered for a moment before his eyes suddenly lit up. "My word! I think I'm following now. We're after the beacon you mentioned! This is London, but in the future. Am I correct?"

The Doctor and the Sherlocks exchanged glances ranging from amusement to utter disappointment. "I don't know why he even bothers listening," Sherlock muttered. "It's not like he's really trying to understand."

"Now, now," Holmes smiled. "Let us be patient with him."

"Well, you're almost right, Watson. But not at all at the same time," said the Doctor, pressing his hands together. "I think you might have missed the point."

Holmes' hand touched down on Watson's shoulder. "You see, old boy, we are here to determine if this is, indeed, the London that you and I know. The Doctor has already briefly explained this, but there are two factors at play here that must be weighed up in order to understand the gravity of the situation we are in, and to know exactly how to approach it. Those two factors are time and space. Given that both I and the other Sherlock here profess to be from different periods in time, it would be quite easy to assume that this world is the future incarnation of our own. However... Are you familiar with the multiverse theory?" He produced a small book, seemingly from nowhere, and pressed it into Watson's hands. "I cannot profess to be an expert on the matter, but I have idly perused this publication, and so have familiarised myself with the basics. Essentially, the theory can be summarised thusly - "

"For every event that takes place in our universe, there are an infinite number of ways in which that event could have potentially turned out," Sherlock interrupted. "The theory suggests that each of these potential outcomes occur at the exactly the same time, branching out into its own separate reality, creating an infinite number of universes running parallel to our own."

Watson's lips pursed. "Indeed," he said slowly, with knitted brows.

"Technically, it shouldn't be possible for the TARDIS to cross the barrier between alternate universes any more, but that's one explanation that could explain two Sherlocks," said the Doctor. "Now, keep all that parallel universe stuff in your mind. If the beacon is still present in Baker Street in this twenty-first century, that proves that both Sherlocks live in the same reality, which leaves us with the question - how are there two identical Sherlocks in one universe?"

"However," Holmes concluded," the alternative also leaves a pressing question unanswered. If the beacon is not present, then we can assume that this is indeed a parallel universe, which the Doctor here has already stated to be impossible to reach with the limitations the TARDIS possesses."

"Either way, it's a case that needs solving," said Sherlock. "Does that help?"

Still clearly confused, Watson nodded. "… I shall not delay our progression with any further questions. Let us hurry to Baker Street, then," he mumbled, tucking Holmes' book into his jacket.

"Capital suggestion, Watson."

The four of them continued snaking a path through the streets of London in relative silence, but for the occasional "fascinating" and "most singular" uttered by Holmes as he occasionally admired the twenty-first century trivialities that they passed, such as passing cars or people checking their mobile phones.

However, as they grew closer to Baker Street, the quiet became considerably more heavy. It was apparent that Sherlock was getting rattled at something, as his pace seemed to quicken, his hands twitching and nervously seeking each other out as he moved forwards.

As their destination finally came into view, the three other men could see Sherlock freeze in place. He turned on his heels and sniffed the air, his eyes darting everywhere as he surveyed the street in minute detail. He mumbled something under his breath, took a few steps forward, craned his neck upwards to scan the windows of the buildings around them.

He turned about furiously to face the Doctor.

"You brought us back to the same date," he growled. "Why did you bring us back to the same date?"

"Very observant," said the Doctor, grinning. "How'd you work that out?"

Sherlock leapt forward and grabbed the Doctor's shoulders. "Why?"

The Doctor shrugged. "The coordinates were still recorded in the data core of the TARDIS. It made sense to use it as a reference point."

"You idiot. You absolute idiot!"

"Have I missed something?" said the Doctor. "I thought it would be the easiest way to compare - "

"Everyone is in danger. Everyone is going to die. If anyone sees me here, alive..." Sherlock stopped and drew a long, shuddering breath. His eyes widened in horror. "John," he murmured. His hands fell from the Doctor's shoulders and, in a single movement, he swept himself right around to face the direction they had come from. He pointed in the direction of 221B and shot a horrible, venomous look at the three other men. "Mrs Hudson," he hissed. "Protect her."

With that, he disappeared back down the street, his coat billowing behind him.

The Doctor stared at the space where Sherlock had been standing only moments before.

"What does he mean?" Watson puzzled. "Why should everyone be in danger?"

Holmes produced his pipe from his jacket pocket, lit it, and put it to his lips. He frowned. "I believe we may have stumbled into a case that the other Sherlock has not yet closed," he said. "A most personal case."

"But why should Mrs Hudson be in peril? And... John..." Watson shivered. "Is he referring to another John Watson? Is... is this London's John Watson in danger as well?"

Holmes nodded. "It would be safe to assume so."

It took a few seconds for everything to click.

Watson staggered backwards. "Holmes... When you and... and the other Sherlock Holmes struck one another, you both felt the pain of both blows. You felt the pain that the other Sherlock suffered."

"Yes," said Holmes, darkly. "I did."

"In that case... if we are to assume that I possess the same connections with this John Watson as you do with this Sherlock..." He faltered.

"I think it would be best to ask that man," said Holmes. He pointed his pipe at the Doctor, who was still staring, dumbfounded, into empty space. "He has considerably more experience with situations such as these."

Watson stepped up to the Doctor, wringing his hands. "Well, Doctor?"

The Doctor's mouth opened a fraction. Slowly, his eyes came to focus on Watson.

He flipped the sonic screwdriver into Watson's hands.

"Mrs Hudson needs to be protected at all cost. Go in there. Don't let anyone touch her. And keep the screwdriver with you. You just point it and think. Press that there," he pointed at the device, "and listen for a thumping noise." The Doctor patted the man firmly on the shoulder and turned him in place to face 221B.

"But... Doctor..."

"You," the Doctor swiveled and jabbed a finger at Holmes. "Come with me." He bounded forwards and hooked Holmes under the arm, and pulled both himself and the detective in the direction Sherlock had taken.

"Doctor," Watson pleaded. "What will happen to me?"

The Doctor slowed and let go of Holmes' arm. He shook his head at Watson. "Nothing will happen to you," he said. "Nothing. You'll be fine. You go in there and protect Mrs Hudson. You will be fine," he repeated. He smiled, but his eyes didn't appear to correspond with his expression. "I promise."

The Doctor and Holmes fled back down the street Sherlock had also taken, leaving Watson alone.

Watson looked at the sonic screwdriver in his hand.

He closed his eyes.

"This is madness," he said.

He held up the screwdriver and pressed the little switch. The tip glowed green, just as it had in the Doctor's hand.

His heart jittering in his chest, he made his way towards the flat. He stepped up to the door and exhaled shakily through his nostrils.

"Absolute madness," he said.

He knocked the door and waited.