Author's note: This prompt: De-Aging. Enjoy!

When Anthea called him, Sherlock's first thought was that his brother was dead.

There was an edge of panic the PA's normally carefully controlled voice. Sherlock couldn't imagine anything else than that one of his brother's many enemies had succeeded in killing him.

As it turned out, he had theorized without data, even though he had to admit it was unlikely that he would ever have come to the right conclusion.

Mycroft had been inspecting a secret laboratory. Sherlock had for quite some time suspected that one existed in London, but it was even better protected than Baskerville, and he had not yet had occasion to look for it during a case.

There was some confusion as to how it happened; according to Anthea, everyone present told a different tale; but the fact was that Mycroft had got stuck in a room where bio scientists had been trying to rejuvenate cells, when one of the machines they had used for their experiments had just been started; and that, when they managed to stop the machine and open the door, a young boy had come out, wondering where he was.

He said his name was Mycroft Holmes and that he was ten years old.

He wore the same clothes Mycroft had worn (and how he had not only been rejuvenated, but how his clothes had shrunk, was puzzling the scientists enormously) and he had answered every question put to him in a way that convinced Anthea that he was indeed Mycroft Holmes. When she had asked about his family, he had eagerly told her that he had a little brother who was three and who wanted to be a pirate, and that Sherlock was awfully clever, if not as smart as he. But he was still young, he could learn.

For a moment, Sherlock was silent.

He had forgotten about this brother. Or had he deleted it? He didn't remember. But once, there had been a young boy who was proud to be called a big brother, who told him about pirates when he asked him, who helped him with his first experiments.

And it seemed like this boy was back.

The memories floated into his consciousness; if he had deleted them, he hadn't done a very good job.

He left the flat immediately, waving Mrs. Hudson away impatiently when she came out to ask what was going on. He had run down the stairs, and not in his usual manner. Of course she had noticed.

He waited for two minutes before the limousine arrived. Anthea had sent it before she had called him. He entered and spent the drive looking out of the window. He wanted to remember the way. It could come in useful one day.

Anthea was waiting for him in front of the non-descript building. She looked worried. He had never seen her show emotions so openly before. It was discomforting.

"Where is he?" he asked tensely.

"With Doctor Mallen. He's the one responsible for the project" she explained.

"Have you told him?"

She shook her head. "He is confused, asks about your parents. And about you" she added after a pause.

"What did he say?"

"He doesn't remember why he's here..." She trailed off, looking at him as they walked down the corridor, obviously wondering if she should tell him or not.

"He wants to know if you are alright. He keeps repeating that you are his responsibility".

Back when this had been a privilege instead of a burden. Sherlock felt pain shoot through him, raw and strong.

He swallowed. "What happens now?"

She didn't answer, obviously waiting for him to elaborate.

"What happens if they can't reverse it?"

It was more than probable that they couldn't. The machine had started working on its own; Mycroft's clothing had shrunk when this was obviously not considered possible. No cases of successful rejuvenation of humans were known. If Sherlock didn't know, Anthea would. She hadn't told him, therefore this was the first time something like this had taken place. And if they didn't know what caused it – how could the reverse it?

She hadn't replied him yet, and he said, "I'll take him home".

"Sherlock" she began, for the first time addressing him by his first name; "This isn't – " her voice faltered – "This isn't the brother you know".

"But" he said firmly. "It's the brother I once knew."

She didn't have an answer to that. They walked on. The laboratory was big, and Sherlock wondered how many interesting experiments had to be going on; normally, he would have been delighted. But he could think about Mycroft. It was so difficult to imagine his brother as a child at their time of life. Not because he thought it impossible – Anthea had called him, it had to be true – but because too much had passed between them to reconcile them with the memories of two boys running around the garden; talking late into the night even though they were supposed to be in bed; an older brother teaching him the Science of Deduction. It had been so long since they had used the word "brother" without sarcasm, without the knowledge of what separated them.

Mycroft, ten-year-old Mycroft, didn't have that knowledge.

Anthea stopped in front of a door. Her face didn't give anything away.

Sherlock opened the door and entered.

The young boy who sat on an examination table looked like the brother he remembered, but he didn't act like it.

He was trembling, holding back sobs. The tears in his eyes almost made Sherlock take a few steps back.

"I don't understand. Why can't I go home? I wanna go home. Mummy and Dad are waiting. And I promised Sherlock I'd read Treasure Island to him".

How often he had done that. Whole nights had been spent with Mycroft pouring over that book, Sherlock looking at him with bright eyes –

"Who are you?"

There was nothing of the usual confidence in his tone that Sherlock always thought Mycroft had already had when they were children. He was scared.

"Will you take me home?"

"Not yet".

Mycroft's lips trembled even stronger, and Sherlock traversed the room in a few quick strides. He leaned down so he could look into his brother's eyes.

"Mycroft" he said slowly, "I know what I am about to tell you will seem incredible. But I am your brother".

He laughed. "No! Sherlock is only a baby now".

"Three" Sherlock corrected him, indignantly, before he realized he had spoken like the child he had been, not the man he was now.

He shook his head. There had to be some way to convince him –

"Mycroft, you know what I wanted to be when I grew up. You remember?"

He huffed proudly. "You mean Sherlock? I'm the only one who knows. It's supposed to be a secret, though. I won't tell you".

"Pirate" Sherlock said quietly. "I wanted to be a pirate". Mycroft looked stricken.

"Think, Mycroft. I begged you to read Treasure Island to me. We stayed up all night long, instead of sleeping, you teaching me the Science of Deduction".

Mycroft stared at him, unbelieving. It had been so easy to read him, then.

"Once you have eliminated the impossible" Sherlock continued, repeating their mother's favourite saying, "whatever remains, however improbable, must be the truth."

"Sherlock?" Mycroft stared at him. "But you are... old".

He would look old for a long boy. He would have chuckled, only that the situation was far from amusing.

"So..." Mycroft's eyes widened.

"Did I travel in time?"

It was the easiest explanation, and Sherlock decided to let him believe it.

"Something like that".

The shrewd look that he received in return showed that he didn't quite believe him, but he didn't care.

"Get back to work" he told the scientists. "I will take him home".

"But – " Doctor Mallen looked uncomfortable.

"No". He wasn't going to allow him to test Mycroft, to draw his blood, scan him, probably hurt him in the process. "You have the machine. Figure it out".

It was an order. Without another word, he gestured towards Mycroft, who slipped off the table, and they left the lab.

Anthea was waiting, shooting shy glances at her boss.

He gave her a curt nod, and it was so like his adult self that Sherlock couldn't help the smirk that appeared on his lips.

"221B" he told her. She nodded. A few minutes later, they sat in a limousine, the driver knowing better than to ask questions.

"Is this yours?" Mycroft asked.

"It's yours" Sherlock informed him.

He raised an eyebrow. "So I'm important".

Sherlock couldn't help the chuckle that escaped him. "Yes. You are".

Mycroft didn't ask any more questions. When they entered the house, Mrs. Hudson came out, her mouth opening when she saw Mycroft.

"Not now, Mrs. Hudson" Sherlock said firmly but not harshly and led his brother up.

He scrutinized the flat.

"You live here?"

"Yes."

"Where do I live?"

"In a mansion across town".

"Why?"

It took him a moment to understand what Mycroft meant. To him, who was so close to his brother, it would seem strange that they didn't live together.

The days when they barely spent a minute without each other when they weren't in school.

He didn't answer, and he could see Mycroft trying to deduce him. Eventually, he decided to ask a different question.

"What are you, exactly?" The fear he had shown in the laboratory was gone, and Sherlock thought that he must feel safe with his brother. It was strange thought.

"I'm a consulting detective."

Mycroft frowned.

"A what?"

"The only one in the world. I invented the job. When the police are out of their depths, the call me".

Mycroft's eyes widened.

"You invented your own job?"

Sherlock waited for the derogatory comment to be sure to follow, but instead his brother exclaimed, "That's great!"

He was left speechless.

Mycroft asked questions about his job, and he answered them automatically. When his brother moved into the kitchen, he followed him and found Mycroft looking through his fridge, ordering him to eat something because "You are still too thin". Sherlock cooked. He could, he simply often chose not to.

The thought of calling John or someone else passed his mind, but he hoped this situation wouldn't last much longer. There was no point in alarming his friends.

Mycroft felt uncomfortable in his suit – he was squirming and kept looking at it as if he couldn't believe what he was wearing – and eventually Sherlock gave him a t-shirt of his and boxers. Both were too long, but he could wear them.

He stared at the t-shirt, then at Sherlock and said, matter-of-factly, "You got tall".

He kept talking throughout dinner. He'd never been this talkative; perhaps something in the machine had –

No, Sherlock suddenly realized when Mycroft good-naturedly wanted to know if he regretted not becoming a pirate, although it was good "that he used the Science of Deduction", he was wrong.

Mycroft had been talkative, and protective without spying on him; he had read to him and taught him and played with him. This was Mycroft before puberty, before the rifts that had slowly opened up between them, before he had told him that he couldn't accompany him to university, before he'd reprimanded him for his drug use and then left him to it for years, before he had let him know that he found it childish that Sherlock had invented his own job, even though he had more or less done the same himself.

It was obvious that the thought that they had grown apart hadn't even occurred to him. He was talking amiably, his eyes shining, like he used to all those years ago.

He didn't show any signs of fatigue, but that didn't surprise Sherlock. They had never needed much sleep.

Mycroft wanted to know more about his job. Sherlock explained to him cases he had worked on. Of course he understood immediately.

He asked questions, clearly interested, and seemed disappointed when it became clear that he hadn't helped. Back then, he would have been disappointed.

Late into the night, he finally asked the question that Sherlock had failed to answer.

"Why are we not living together?"

"It is not normal for siblings to live together" Sherlock replied tensely. "It is expected that one shares one's life space with a significant other".

"But I'm not going to have a significant other" Mycroft argued. "I don't feel drawn to other people. You're the only one I can tolerate except for Mummy and Dad. And you live alone. So why – "

"It is complicated".

Mycroft frowned.

"That's what adults say when they don't want to answer".

"That's what adults say" Sherlock corrected him, "when they don't know how to answer".

Mycroft thought about that for a few minutes, then he asked, slowly, "Do we hate each other?"

"No". He could answer that with confidence, at least. Neither of them believed in hate. It obscured one's thoughts.

"Then what?"

"It is complicated" he said with an air of finality. Mycroft understood and didn't ask further.

He was silent until he saw Sherlock's violin, half concealed between a few files on the table, and exclaimed, "You learned? You were going to – "

He broke off, confused. It was a confusing situation, and Sherlock began to play. It was a tune this Mycroft had never heard, but would. He had composed it when he was ten – it was his first composition, Mycroft had sat beside him while he did.

All of this still lay before the boy who sat in his chair, listening to the music. He was blissfully unaware that, a few years from the last day he remembered, his brother would be taking drugs and he would be at university, building a wall between himself and others, both of them growing colder with every passing day.

He didn't know, and Sherlock played. He played for the boy he hadn't remembered until he had entered the lab.

Mycroft fell asleep during the "concert", as he called it, after Sherlock had played for hours. He hadn't seen his brother relaxed for so long it seemed unreal, but he picked him up and tugged him into his bed.

He didn't sleep, instead walking through his mind palace and revisiting memories he had believed showed that his brother had never cared for him when in truth he had simply altered them subconsciously so he wouldn't be hurt.

It was disconcerting. He had always prided himself on his mind palace.

Shortly after sunrise, Anthe called to inform him that the scientists believed they could reverse the effect, and Sherlock woke Mycroft up and made him put on his suit, even though he still felt uncomfortable in it.

They didn't say much as they drove back to the laboratory, but before Mycroft went past a still-confused Anthea, he turned around and said, "I'll see you soon, Sherlock".

He couldn't keep still while the machine was running. Once it stopped, he rushed into the room, finding his older – his older – brother sitting on the floor, looking slightly dishevelled, but as dignified as ever.

He stood up and moved towards the door, shaking a little. Sherlock instinctively reached out to touch him, and when Mycroft looked into his eyes, he froze.

He had assumed Mycroft wouldn't remember.

He did. There was no other way to interpret that look.

"I – " his brother began, paused for a moment, then continued, "Will you accompany me home? I am certain the doctors won't allow me to return to my office or stay alone".

He nodded.

They didn't say much to each either before they left the laboratory, not even after Mycroft was cleared to go and Anthea insisted that he should stay home for a week.

But on the way to the mansion, Mycroft asked a question.

"Can we remember together?"

Sherlock's simple answer was "Yes."

Author's note: I hope you liked it, please review.