This takes place in the HeadSpace universe - if you are unfamiliar with it, please read Out of My Head first. If you really don't want to read 27k words, you need to know that Sherlock is able to read minds, but the only people who know about this are John and Mycroft. He can also project thoughts to John.
They're thundering up the stairs, barely stifling the giggles over Sherlock yet again having shown Anderson up at NSY. But when they get to the landing in front of the flat, Sherlock stops short. John crashes into him, nearly tumbling down the stairs, which sets off another fit of laughter, until he realises why Sherlock stopped in the first place.
"Hello, Mycroft." He's made himself at home, sitting primly in Sherlock's chair, reading the newspaper. He inclines his head slightly, nodding at the pair standing in the doorway.
"Hello, Sherlock. John."
The look on Sherlock's face is one of sheer irritation, damn Mycroft for spoiling his good mood. John just looks resigned, because if there's one thing he's learned it's that stopping either of the Holmes brothers from making their point is an exercise in futility. May as well let him say whatever he's got to say. Sherlock merely grumbles in mute assent.
"Did you really think you could send my men in to clean up after you two without any sort of..." Mycroft pauses, clearly for emphasis because the man is never at a loss for words. "intervention on my behalf? Did you realise the potential risk involved in haring off alone like that?"
Realising this is going to be more fraternal head-butting than he currently needs or wants, John heads into the kitchen to make tea.
"I wasn't alone. I had John."
"You're being obtuse." Something could have happened to him, Sherlock. How would you have liked that?
"Stop that. And yes, I considered that, but in the end it was the best possible decision."
John raises an eyebrow, clearly confused by the part of the conversation he's missing, but lets it slide.
"Besides, your hypothetical fretting is just that. It's over, done with, and we're none the worse for wear. I'm sure you can just do that thing you do, shuffle some papers around, and nobody will ever know the truth." Sherlock makes a dismissive gesture with his hands, a mocking imitation of literal paper-pushing. "However, since you're here, why not stay for tea, catch up?"
John nearly drops the mug he's holding. Sherlock, what? Did you just invite your brother to stay for tea? Mycroft's having the same thoughts, but he's not bothering to vocalise them. He decides to stay put merely out of sheer curiosity.
John, please, let me have a bit of fun. We still have that chocolate cake Mrs. Hudson sent up in the fridge, don't we?
Oh god, Sherlock, what are you going to do?
I thought it was time I tried to project to someone other than you. Much as I hate to admit it, Mycroft seems like the most logical test subject.
As they're holding their silent conversation, John finishes making the tea and heads back into the sitting room, placing a cup on the table in front of Mycroft and settling down on the sofa himself. Sherlock gets up, grabbing his mug from John's hands.
"If you gentlemen" Sherlock looks directly at Mycroft when he says this, his voice dripping with disdain "will excuse me for a moment, I have an experiment to attend to in the bedroom."
"Not a problem, John has enough manners for the both of you, he can keep me company while I drink the tea you offered me."
What do you need me to do?
Just chat with him a bit, keep him occupied and distracted.
The things I do for you, Sherlock. John's aiming for irritated, but it comes off as amused and indulgent.
Sherlock just smiles an unnervingly cat-like smile and saunters down the hall towards their bedroom. He opens and shuts the door, hoping that will lull Mycroft into a false sense of security, but lingers in the hallway where he can keep an eye on them through the kitchen. He clears his mind, listening briefly to the gentle, confident undercurrent of John's thoughts and the vaguely irritating multiple strains of concentration in Mycroft's, before settling down to the task at hand.
Cake. Delicious puddings.
Nothing, not a twitch. Mycroft continues chatting with John about painfully insipid garbage like the weather and whether he misses locum work. Everyone in the flat knows he's just doing it to overstay his welcome, but for once Sherlock doesn't mind the intrusion.
Caaaaaake. You want it.
For a mere fraction of a second, Mycroft's eyes bulge, as if something has shocked him slightly.
Careful, Sherlock. I can hear you too.
Damn.
He concentrates hard, directing all his energy in his brother's direction.
Moist, gooey, delicious, chocolate...
This time the shift in Mycroft's body language is nearly imperceptible, but Sherlock's eagle eye notices he's drumming his fingers against the arm of the chair, something he rarely does unless he's ruffled. John's doing a marvellous job of keeping him ostensibly occupied, asking mindless questions about government functionaries.
Better, I don't hear you anymore.
Éclairs. Blancmanges. Doughnuts.
Sherlock can barely contain his mirth when he hears the steady flow of Mycroft's thoughts. I wonder if I could treat myself to a small slice of something when I head home... I really shouldn't, but... I've been doing so well with my diet, I deserve a treat.
Unable to resist, Sherlock keeps goading his brother. Yes, you should indulge. You deserve it. Delectable pastries, unctuous icing...
At this point, Sherlock emerges from the bedroom and starts rummaging around in the refrigerator, making a show of moving around a few boring items before pulling out the decadently rich confection and diving directly into it with a fork. He stalks back into the sitting room and throws himself down onto the sofa beside John. The look on Mycroft's face is absolutely miserable. At this, John loses whatever control he had over his own laughter, and he is immediately joined by Sherlock.
Sherlock, what is going on here?
"Oh, talk, would you?"
"Fine." He sniffs irritably. "Can you please explain this nonsense to me?"
"Well, my opinion as a medical professional..." John attempts to spit out between snickers. Sherlock manages to compose himself, rather gratuitously sucking some icing off the back of a fork.
"What John is trying to say, dear brother, is that my abilities seem to have progressed."
"So you put that suggestion inside my head?" Mycroft looks uncharacteristically nonplussed. "Sherlock, how long has this been going on? Have you done it to anyone else?" The ethical ramifications of this, Sherlock... I can't even begin to imagine.
Fuck your ethics, Mycroft. You only apply them when they're convenient.
Mycroft goes pale. Clearly the idea of Sherlock turning the tables and taunting him non-verbally like this is making him uncomfortable.
"Sherlock, I'm... sorry. It's unpleasant. Violating, even, to be forced to listen like that. I never knew."
"No, well, now you do." Sherlock's snappish, the familiar defensive wall creeping up around him again.
John's doing his best to keep quiet, keeping his mouth and his thoughts to himself. It's obvious the brothers have a lot to work through here. He makes as though to get up and leave, but feels Sherlock's steely fingers grasp his wrist. Stay, please.
What he said, about it violating you... do you feel that way when I do it?
Sherlock's face clouds over for a moment, leaving Mycroft to ponder things by himself, as he studies John. Never, John. I chose to let you in. I want to let you in. You never use this as an excuse to demean me, to make me feel subhuman.
Thank you, Sherlock. If you ever need some peace and quiet, let me know.
Honestly, John, when have I ever not let you know what I needed from you? Even before all this started?
"Would you two nauseating lovebirds save this for later? I may not be able to hear anything, but it's bloody obvious you're chit-chatting over there."
"Oh, do shut up, Mycroft, and eat your cake."
With a shrug that's too poised and elegant to be a true admission of defeat, the elder Holmes brother snatches up the plate and allows himself a moment of indulgence.
"I am doing this because I wanted it, Sherlock. Not because of whatever nonsense you've pulled."
John's got to giggling again, but Sherlock can hear the strains of "pompous git" and "serves him right" running through his head, his own smirk an unspoken agreement. Finally, he composes himself enough to shoot a clear, concise Behave in Sherlock's direction.
Pushing aside the plate, still with half a slice of cake on it, Mycroft's face turns suddenly grave.
"In all seriousness, Sherlock, I hope you don't plan to abuse or exploit this. I wonder if we should head back to that institute in Nor-"
"NO!" John's hand is suddenly solid and comforting around Sherlock's arm as he jumps up, shouting at his brother. Mycroft holds his hands up in a placating gesture.
"It was merely a suggestion, let us not speak of it again." Assuaged, Sherlock slumps back onto the sofa.
"Thank you. I assure you, I have no intention of abusing this, I was merely curious to see if I could do it to someone I'd not had more intimate relations with."
At this, Mycroft flushes slightly, as if the topic has made him slightly uncomfortable.
"Oh come now, it's not as if you didn't already know."
"It's still not something I enjoy dwelling on."
"Mmm, I beg to differ." John, quiet for so long, has chosen now of all times to pipe up, closing his eyes and flooding Sherlock's brain with pictures that quite decidedly do not involve Mycroft. Sherlock gets the hint, and stands up.
"Mycroft, I do believe it was time for you to be going. Shall I pack up the cake for you?"
Huffing, he gets out of his chair and glares down his prominent nose at John and Sherlock. "Thank you, no. I'll be seeing myself out."
For what seems like the hundredth time that day, the two of them are lost in a cascade of childish laughter.