Mycroft has been inside Sherlock's head for as long as he can remember. He is a constant, like the air or the sun. More constant than the sun, as he was also there on rainy days when the sun didn't shine.

Sherlock remembers Mycroft wrapping his hand around Sherlock's much smaller one and helping him draw a square on a piece of paper. Inside he drew two dots labeling them with their names.

"That's the two of us inside this box," Mycroft said. "The rest of the world is outside."

Then he drew the world around them naming the paths, the trees, the clouds. Defining the world for Sherlock until everything he looked at had a label floating beside in hand-scrawled graphite. A puff of white on a stalk labeled dandelion. A curved line and an equation to describe a dove's flight.

He taught him about people too. He explained how to tell that the checker at the grocery was stealing from the till, and that the woman who laughed so loudly at her boyfriend's jokes was sleeping with his best friend. Evidence was in the details if only one cared to observe it. It could be found in a mis-buttoned shirt or a light-colored hair on the back of a tie.

Sherlock learned how to observe. He learned how to be clever. He wasn't as clever as Mycroft, but it didn't matter because his brother was inside the square with him interpreting the chaotic universe that so often baffled him. Mycroft made things right.

But then Mycroft grew up, and he went away.

Mummy said that it was only natural for a young man to go off own his own, and Mycroft was eager to prove himself in a world that he hoped one day to rule. He went away leaving Sherlock alone with a gap inside that he didn't know to fill.

Without his brother beside him, Sherlock missed things. He blurted out observations that he didn't know were not obvious, saying things that others considered tactless and making enemies for himself until they labeled him a sociopath. They didn't understand how difficult it was to see in a world that had lost color and definition without his brother there to help him understand it. The labels were still there, but the nuance of emotions were gone. He had depended on his brother too much and the empty space in his mind crumpled up like a wad of waste paper that had failed to land in the bin.

When things seemed as if they could get no worse, he ran away from school. He made a perilous journey to town and knocked on the door of his brother's flat. The sight of his brother brought a smile to his face, but it was not returned.

"Not now, Sherlock!" Mycroft said. "Go away." And he shut the door in Sherlock's face, but not before Sherlock observed an unfamiliar coat draped across a chair and two glasses of half-drunk wine resting on a low table.

It wasn't jealousy, not exactly, that caused his despair. It was a feeling more like loss. Like the absence of a step where one was expected. How could he help but fall. He had lost an anchor, a teacher, a judge, a part of himself. It had always been we, and now it was only he. Parts of his mind collapsed in on itself making some things incomprehensible, a world of drawings, full of logic, but missing color. He found that he was only able to glimpse the colors beyond violet when he was high on cocaine.

He filled the gaps with trivia, building walls to protect himself that collapsed almost as quickly as they were made. And then one day, he woke up in a hospital bed to find Mycroft sitting beside him holding his hand.

For a moment, there was color again and everything was fine. Then Sherlock slapped his hand away.

"I don't need you anymore." Sherlock said. "You're outside now."

Mycroft was aghast. He didn't recognize the boy who had hung on his words and dogged his footsteps. He didn't understand this bitter youth. "I'm sorry," he said. "I didn't know it would hurt you." He rose to his feet and walked out of the room. Sherlock read, out of the corner of his eye, the guilt writ large across his brother's forehead.

Sherlock was angry, not just because Mycroft had gone away, but because his brother had never told him that he wasn't really a part of Sherlock. Sherlock had never learned to walk alone. Whenever he had started to fall, Mycroft had been there to catch him. He had never learned to struggle but he would do it now. It was only natural for a young man to go off on his own.

He built himself a mind palace to order the world, and he learned to live among people, refusing Mycroft's ever more desperate calls and demands. Mycroft sent others to watch him, men and women who pretended to be his friends, but who were as obvious as if Mycroft's name was written beside their faces. The truth was that despite all of his work, that little pencil-drawn square was still waiting to be filled.

He had barely known John a month before he realized that he had filled that empty place inside of him. John interpreted for Sherlock helping him understand the feelings of others while helping others not to fear him. It was astonishing how good a team they made. Sherlock got on his knees and drew the box firmly around the two of them and Sherlock felt whole again until Moriarty came to break everything apart.

He couldn't bear to start over, so he swallowed his pride and called on his brother. Mycroft was happy to help. He gave him everything that he needed to destroy Moriarty's organization and save John's life, but when he returned, John had found someone else.

It wasn't the two of them against the world anymore, there were now three. He pushed back the furniture in his mind palace wondering if they could all fit. But he had felt fear, and without thinking he reached out to his brother.

"Just like old times," Mycroft said when he called at John's wedding reception. "John and Mary, domestic bliss." And Sherlock had worried, because without John beside him, he felt like only half a man.

Sherlock tried to stand on shaky feet alone, for John's sake. He tried to make the marriage work, but before long John was in danger yet again, and the only solution that Sherlock could see was to put a bullet in a man's brain to protect a woman that Sherlock should never have brought inside.

Sitting in front of a metal table, Sherlock read about his fate. It was either a long life in prison, or a short one as a spy. Like a child, he let his brother decide. It had been shame, for the most part, that had kept Sherlock away from his brother. Shame that he continued to care about others. Shame that he couldn't stand alone, and Mycroft could. His hand shook as he signed his name to the consent form for the trip that would most likely cost him his life.

He was surprised, then to find another hand wrapped around his own, as his brother's fingers led Sherlock's hand up and around to draw a square on the page and place two dots inside.

Sherlock turned and looked into eyes damp with compassion. It was then that he knew that a part of him was inside of Mycroft's mind as well. A child who watched his best decisions, and his worst ones. Sherlock looked at the square and knew what it meant as if his brother had written it across the sky.

"I will always protect you. The two of us are inside this box. The rest of the world is outside."