A/N: In the interest of full disclosure, this story is told in alternating chapters. All the odd chapters deal with the present, the even chapters with the past. Just so there's no confusion.
/Now./
It was the moment when Mycroft punched Sherlock in the mouth, in front of half of New Scotland Yard, that John realized Sherlock had crossed the line. What line that was, exactly, he didn't know, but from the other side of the crime scene he'd seen them bickering, all normal as far as anyone present was concerned, then Sherlock had turned around and looked at Lestrade. His eyes narrowed, flicking up and down the DI's body, and then turned back to Mycroft.
The next moment he was on the ground spitting up blood and checking his teeth.
John made it across the street in time to see Mycroft slide into his car and disappear into the night.
"What the hell was that all about?" he asked, helping the Consulting Detective to his feet.
Sherlock glared at him, yanked his arm free of Johns hold, and walked away.
"Lovely," John said to the air, watching him go. "That's…that's just lovely. Really." Why did everything have to be so bloody difficult with him? Throwing his hands up, he turned and walked back to the crime scene to collect his things. As he passed through the doorway, Sargent Donovan grabbed him by the arm.
"He never told you, did he?" she asked.
"Told me what?"
She was quiet for a long time, standing and staring at him, deciding what action to take. "Look, I have to finish up here. Wait for me. We need to talk."
"About?"
"Lots of things."
Two hours later he found himself sitting at Sally's kitchen table, sipping a cup of Earl Grey and idly scratching her Yorkie behind its ears. "So are you going to explain what happened tonight?" he asked.
"I'm going to try. Ideally you should talk to Sherlock, but I won't send you into that battle without some information first," she sat down across from him at the dining table, a cup of tea in one hand, a shoe box in the other.
John was, to put it mildly, intrigued.
Setting the tea aside, she pulled the lid off the shoe box and pulled out a handful of photographs. Selecting one from the stack, she slid it across the table towards him.
He picked it up gently and looked at it. Sally stood next to a young woman, their arms thrown around each other, both grinning into the camera. Sally looked like a younger version of herself, though there were small differences. Her hair was longer, and straight, and she had this air about her. A sweeter, more innocent air. The woman next to her was uncommonly pretty. Her hair was a long strawberry blond mane, pulled off to one side in a fishtail braid. Her green eyes danced with happiness and mirth that John envied. They couldn't have been more than twenty five when it was taken.
"Who is she?" he asked.
"That's Merry," Sally said softly. "We ran the town together. Thick as thieves. My mother called us Thelma and Louise. I loved her more than anyone before or since. She was…my best friend. My other half. My soul sister."
"Was?"
"She died."
"Oh. I'm sorry," he said. "But...what does this have to do with Sherlock? Or Mycroft?"
Sighing, Sally reached inside the box once more.