"John."
"Hmmm?" John didn't look up from his computer, where he was blogging about their latest case.
"John."

"What, Sherlock?" John still didn't look up.

"There's a thing." John finally looked up, and saw Sherlock staring unblinkingly at the kitten John had brought home the day before.
"I brought it home yesterday and you only just noticed? It's a kitten, Sherlock. Jesus. Have you deleted that too?"
"But...why is there a kitten?" John stood up and removed the forgotten cup of tea from Sherlock's hand. The kitten tried to rub itself against Sherlock's leg, purring, but Sherlock took a step back. "John, John it's trying to kill me!"
"No it's not. It wants to be friends. It's not even an it. Its a she, and her name is-"
"Murder." Sherlock cut John off suddenly. Now it was John's turn to look confused.
"What?"
"Her name is Murder. If you want me to put up with it, don't argue."
"Fine." John picked up Murder and started stroking her. Sherlock stalked off and curled up on the sofa looking miffed.
Mrs. Hudson walked in a few minutes later and found Sherlock still curled up facing the wall and John sitting in his chair with Murder curled up on his lap.
"Don't worry dear, he's just jealous. He thinks the cat is replacing him. Just give him time." Mrs. Hudson patted John's shoulder in a motherly fashion before bustling off to clean up their kitchen a bit. Sherlock rolled over and glared balefully at Murder. John sighed. How much time was it going to take? Quite frankly, Sherlock was being ridiculous.
A week later, John walked in on Sherlock trying to coax Murder out from under the sofa. He straightened when he saw John and muttered some excuse about looking for a pen. John just smiled.
After that, Sherlock was much more careful. Whenever John was around, he kept his distance from Murder. Mrs. Hudson said John just had to give Sherlock more attention to show him that no cat was ever going to replace him in John's heart.
"Bollocks. Sherlock just needs to learn to be a bit more sensible and a bit less stubborn."
"I think that's a bit much to ask, dear."
For a whole month after that, the mood in 221B was cold but slowly defrosting. Every now and then John would catch Sherlock stroking Murder tentatively
Finally, the ice melted and Sherlock finally started speaking civilly to John again. Then, one day, John came home from the grocery store and dropped his bags in shock when he saw the scene in 221B. Sherlock was lying on the sofa in his thinking pose, hands tucked under his chin, almost as though he was praying. That was completely unthinkable, though. Sherlock Holmes did not need to pray.
BbThis scene was completely normal. The shocking part was Murder, who was curled up on Sherlock's chest, sound asleep. Every now and then Sherlock absentmindedly reached down and stroked her gently, his eyes shut as well.

In reality, Sherlock was deep in his mind palace, having a spring cleaning. However, that irritatingly adorable kitten had somehow wormed its way into his head and kept following him around his mind palace. Stupid cat. Sherlock still denied the affection growing in his heart for the little black kitten, but her presence in his mind palace was a dead giveaway.

John smiled at the scene and, careful to make no noise, slid his phone out of his coat pocket and took a video of Sherlock stroking Murder. Grinning wickedly, he texted it to Lestrade, Mrs. Hudson, and Molly. Mrs. Hudson had been right. It had just taken time.

Murder shifted in her sleep, and Sherlock scratched under her chin, smiling softly.