Greg honestly didn't know why he was surprised. Sherlock had done enough strange experiments, and he had entered 221B more than once to find a body part on the table. A scientist looking for a way to get into parallel universes wasn't that strange, he supposed.

Then he realized that there was a half-formed suspicion at the back of his mind, and he drew in a deep breath.

"Is it –" he stopped because it sounded incredibly even in his head. Sherlock and John in a parallel universe, Trevelyan perhaps having escaped into one as well...

It wasn't possible – was it? The DI remembered Sherlock's favourite saying.

Once you have eliminated the impossible, whatever remains, however improbable, must be the truth.

And Mycroft Holmes had spent the last hours eliminating the impossible.

Greg swallowed, trying to order his thoughts. Sherlock and John might be in a parallel universe. The one who'd sent them there might be with them, or he might have sought refuge in a different one; if the DI remembered correctly the few things he'd caught about this sort of thing over the years in books and movies, there was more than one parallel universe.

And if he was right –

"Is there – " he cleared his throat. "Is there a way to be sure? And – and to bring them back?"

"I have people working on it" Mycroft replied, staring at the papers.

And Greg understood.

Mycroft hadn't called him here to investigate; he behaved differently than he normally did. While he was still leading the search for his brother and was as diligent as always, he let his worry show.

Mycroft hadn't called him here to investigate.

Mycroft had called him here because he needed a friend.

He cleared his throat again, unsure of how to proceed. Their friendship, if one could call it that, consisted mostly of long silences and glasses of brandy, shared after a long day. When Mycroft called him because of Sherlock, he was always short and to the point.

Greg had no idea what he could do or say to comfort him.

He decided to do what they always did. Work.

He grabbed a file from the desk.

"We might as well try to figure out what makes this guy work".

He could have imagined it, but as Mycroft stepped next to him and took a file of his own, he thought he saw a grateful smile on his lips.

What they found wasn't encouraging. Greg didn't understand everything – there were a lot of mathematical equations and scientific articles that told him that Trevelyan had to be good at his job, but nothing more – and he concentrated on the scientist's personal notes. A half-sentence at the side of a page; words scribbled beneath a report about an experiment gone wrong. He had often learned more about suspects through small signs like these, evidence they didn't know they had left behind, that they hadn't considered important, than through interviews. Remarks like the ones he was looking at where often added only half-consciously or as an afterthought, in a hurry, and they revealed what type of person they were dealing with.

What he found made him certain that Sherlock and John had gone up against a dangerous man indeed.

There was an article written by a professor of a well-known university. Trevelyan had supplied an introduction in which he praised the man, but his notes told Greg that he thought the man was an "imbecile" who didn't "see the possibilities that might open up if we were to conduct – "

He put the file down when he realized what he was reading. The professor had been researching radioactive poisoning, and Trevelyan thought that it might be best to inject people with it and then try different cures.

He'd added to his own notes, in a different pen, "accident victims not advisable – history must not be compromised."

And he definitely didn't like the professor. "Imbecile" was the nicest word he'd used.

"He is – "

"Dangerous" Mycroft finished. Naturally, in the time Greg had read one file, he had gone through three.

"Why is he working for –"

"He is brilliant" the British Government interrupted him. "And his results have always been outstanding."

Greg rubbed his face with his hands. He did understand the reasoning, but he wouldn't agree with it. Sherlock was brilliant too, and he brought exceptional results. But if Greg hadn't known, hadn't felt that somewhere deep down a good man was buried he would never have offered him to help on the cases.

"He doesn't care much for others" he answered. "Might explain his wish to enter parallel universes. Perhaps he wants to interact with himself? That's what parallel universes are, right?"

Mycroft frowned.

"I do not think he is looking for company. There is no reason he shouldn't find it here."

"But what then?"

He knew that he wouldn't like Mycroft's answer. It was written plainly on the other man's face.

"According to the files I read, he believes that by influencing a parallel universe, one can also set things off-balance in one's own."

Sherlock and John were sitting in a cab with Bill and the doctor's counterpart on the way to St. Bart's – Greg had decided not to go with them and had left with a mumbled assurance that he'd "try to find the guy John was so keen on" – when Bill's phone rang.

"Mike. I forgot to tell him I wouldn't be coming over – " He picked up. Sherlock didn't turn his head, but John could only listen open-mouthed to the conversation.

"Hello. Yeah, sorry, I won't be coming over – something happened... No, we're safe. Yes, I promise. Going to St. Bart's, in fact. I will call you."

He laughed.

"Don't worry. Bye".

He hung up and John looked at Sherlock. The consulting detective didn't appear to have even noticed the phone call.

The doctor bit his lip.

Sherlock's and Mycroft's relationship was complicated, more so than his and Harry's, and that was saying a lot. He had never heard the consulting detective talking so care-free to his brother, so obviously comfortable with their conversation, and he never would.

The British Government and his best friend had got along better since he'd returned, but they would never have this, this easy camaraderie; Bill had laughed at something Mycroft had said at the end of their talk, most likely a joke about this world's John, and he had never seen Sherlock truly smile at his brother.

He wondered if he should say something, but he was certain Sherlock would have preferred to have this conversation in private, if at all.

Bill caught John's eye and smiled.

"He's a bit overprotective".

Sherlock snorted, and John couldn't help the chuckle that escaped him either. Some things never changed.

"I guess he told you to watch out?"

John's, the consulting detective's voice came as a surprise to the doctor, who had almost forgotten about his presence. He detected hostility in the question – not much, but enough to tell him that his counterpart and Mike didn't like each other.

He wasn't surprised. He'd rather not think about the time Harry came over for tea and Sherlock deduced her "promiscuous lifestyle".

"He's concerned, that is all".

For the first time, John could hear a warning not to go further in Bill's voice, and he hoped the other man would understand as well. Sherlock always did, even if he sometimes went on just to spite him.

John only made a non-committal noise in reply, and the doctor relaxed into his seat.

He was aware that Sherlock shot him a look, most likely not understanding why it was important to him that their counterparts didn't fight. John wasn't sure that he'd be able to explain it if he'd asked.

But – even if these were different versions of themselves, even if they were different people – he couldn't stand it. Couldn't stand the thought that they might fight, that one of them might move out. Because he had never contemplated moving out, not once, neither before nor after Sherlock died and returned. True, he hadn't been able to stay in the flat for a while after his "death" but he had come back, had come back to the smiley and the skull, the dusty chemistry equipment that Mrs. Hudson had not had the heart to give away, the violin, and had unconsciously waited for his best friend to return.

He wasn't interested in parallel universes where this wasn't normal.

The cab stopped and he looked out of the window at St. Bart's.

"We cannot go in all at once" John announced. "The cab driver obviously didn't care that we looked alike, but our acquaintances might see it differently".

Sherlock nodded.

"You go first, we'll follow through the back entrance."

They waited exactly three minutes in a comfortable silence, because John couldn't think of anything to say. To many thoughts were crowding his brain. But, after they had exited the cab, Sherlock gave his shoulder a brief squeeze, and John realized he understood.

As they walked towards the lab Bill had told him Jim worked in, thankfully not running into anyone, John took deep breaths.

He had thought that he was prepared to look into Jim Moriarty's face.

He wasn't.