When John got home from doing the shopping, it was already dark. "Sherlock!" he called out as he entered the flat, but he didn't get an answer. He went through to the kitchen and dropped the bags on the floor next to the mass of scientific inquiry that was their table. "Sherlock!" he called again, glancing down the hall towards his flatmate's bedroom.
The door to the detective's room was standing open and John could see the curtains on Sherlock's window shift in a breeze. Obviously, the window was open which only meant one thing...
The doctor walked down the hall, into Sherlock's room and climbed through the window onto the fire escape. Once there, he climbed the ladder onto the roof where he found the detective sitting, staring up at the stars.
"Fancied a smoke, did you?" John asked as he sat down beside his friend. "I thought you were doing better."
"Don't be an idiot, John. Do you smell cigarette smoke?" Sherlock looked up at the night sky and the twinkling stars that were visible from the rooftop. "I wanted to see the stars, to be closer to them. It's impossible to see them here like you can at my parents' house. Still, they are beautiful," Sherlock said, his gaze remaining on the stars. "Each of them represents a life and each has a story. It's born, it burns, then eventually it dies, but if it burns bright enough, John, everyone will remember that star... Forever..."
"That was rather poetic for a self-proclaimed sociopath," the doctor said with a smile. "I wonder what the Yarders would think to hear you go on like that."
"You don't have to be insulting, John. I'm never poetic. And if you mention this to anyone..."
John laughed. "I won't. Your secret is safe with me. I'll tell no one about your poetic soul. And you do have one."
They sat in companionable silence for a while after that, just looking at the stars. John's eyes locked on the brightest one in the sky.
"If a star is burning bright, doesn't that mean it's burning hot and fast and will die all the sooner?" the doctor asked.
Sherlock shrugged. "I suppose so. I haven't really thought about it."
John turned to face his flatmate, feeling suddenly terrified. He grasped Sherlock's hand, much to his friends surprise. "Promise me something, Sherlock. Promise me you won't try to be the brightest star in the sky."
"John?"
"I wouldn't be able to go on if you burn yourself out too soon. We have to burn out together. Promise me."
"John, I don't understand."
The doctor let go Sherlock's hand and cupped his face, then he moved in and pressed their lips together. At first, the detective didn't respond, then his lips parted and granted John entrance. When they broke apart, Sherlock's lips formed a silent 'Oh' of understanding.
"Now you see. You can't leave me in darkness, love. We burn out together and we do it a very long time from now."
Dazed, Sherlock nodded, then pulled John close for a hug under the beautiful, twinkling stars.