Sherlock has asked John to meet him at the crime scene. He's too focused to notice what John's wearing when he arrives, he just asks him to have a closer look at the body.
On enquiry from Lestrade, John replies, "Oh, I have a lot of them from my days at Uni still in my wardrobe and all my normal jumpers are in the wash, because someone decided to do an experiment on them and spill chemicals all over them." Sherlock looks up from his phone sharply, "I apologised, didn't I? Anyway, it was vital for the case at the time." John sighs.
"No you didn't apologise, you just snapped that we were out of milk and flounced out of the room."
"Actually, I had forgotten how comfortable these shirts are, anyway." John stretches, the t-shirt pulling tighter against him. Due to his muscles, previously from Rugby and the army, now from running around London after this genius madman of a flatmate, the shirt clings to his form, making his seem younger and more lithe.
Sherlock wouldn't admit it out loud, but he likes the way it looks on John, and endeavours to find a way to get him to wear the t-shirts more often. Lestrade notices him staring, and later tells John at the pub. John doesn't believe him, so Lestrade makes a bet with him.
"Alright, how about a bet? Whenever you wear one of your slogan t-shirts, you have to get Sherlock to do what the t-shirt says. If you do get him to, I win that round. If he's doesn't, you win. How's about a fiver on each round?"
"Ok, you're on. This'll be easy, Sherlock will never do it." John laughs, shaking hands to seal the bet.
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When John gets home, he notices Sherlock's gaze lingers on him longer than normal. When he spots this, John pitches the idea to Sherlock, claiming it to be an experiment to find out how much Sherlock would do for his friend. Much to John's surprise, Sherlock agrees, stating that it would be best to start the experiment right away. John's t-shirt that day states to trust him because he's a Jedi Master.
Sherlock responds to the t-shirt, saying, "I already trust you, John. Should I call you Master Watson from now on though? What even is a 'Jedi Master'?"
John Laughs, "No, you don't. What do you mean, what's a Jedi Master? You know, Obi Wan Kenobi. From Star Wars."
Sherlock's blank look makes John's eyes widen in surprise, "Don't tell me you've never heard of Star Wars!" Sherlock shrugs face still blank.
"Right, that's settled. We're having a Star Wars marathon, tonight." John steers Sherlock towards the sofa, and heads over to the DVD player. They spend the rest of the evening huddled together on the sofa watching the complete original trilogy. John making sure Sherlock actually watches for once, instead of playing on his phone, and Sherlock refraining himself from making many remarks, noticing how John's face lit up the moment it started.
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The next t-shirt John wears, states, "Smile, It confuses people", which couldn't be more true in Sherlock's case. Over the course of the day, everyone who comes into contact with Sherlock is even more wary of him. Unfortunately for Sherlock, this is also a day that Mycroft decides to visit.
"Brother, how nice of you to join us." Sherlock remarks sarcastically, though smiling whilst doing so. Sitting in his armchair and torturing his violin.
"I just stopped by to enquire why you're smiling at everyone, it is very unsettling and I was wondering what had possessed you." Mycroft smirks, examining the tip of his umbrella.
"Nothing, possessed me," Sherlock almost spits, screeching a sharp high note on his violin.
"Well something must have. You never smile. What was i-? Ah! It seems my question has been answered." Mycroft sneers. "Though it does beg the question… Why are you taking the advice of an old t-shirt Doctor Watson is wearing?"
Said Doctor has just walked in and calmly states "It's for an experiment, now if you would kindly let yourself out, we are busy." Ignoring Mycroft and turning to Sherlock, he says "Lestrade's just called with a case. Triple locked-room murder, no obvious connection between the victims. Sounds like exactly what you need."
"Call him back and tell him to text the address, we're on our way. Mycroft, do piss off. John and I have a case to solve." Sherlock replies.
"So it seems. I have important meetings to uphold anyway." Turning, Mycroft walks out of the door, swinging his umbrella along beside him. He calls over his shoulder and he descends the stairs, "Remember Mummy's birthday on the 17th. You're not missing the dinner this year Sherlock! You know how it upsets her." With that last remark, he is out of the door. The last thing heard, is the sound of a car pulling smoothly away from the curb, only moments later.
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John's next slogan states that his inner child is in need of a hug, and he'll take one too. Sherlock hugs him on the spot, and then randomly comes up during the day and surprises John with hugs. During one of the sneak-attack hugs, John states that he likes this t-shirt the most, if this is what it is getting him.
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The next day Sherlock surprises John again by grabbing John and hugging him randomly, even though John isn't wearing the t-shirt. When questioned, Sherlock ignores him, saying he's thinking about the case and can't be distracted by inane, irrelevant questions.
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John's t-shirts start getting bolder and bolder in the weeks following the hugging. Sherlock follows the commands on the t-shirts every time, occasionally still giving John sneak hug-attacks from time to time, too.
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Suddenly John stops wearing any t-shirts with slogans on them. Sherlock doesn't question him, thinking that he'd forgotten, or likely that the bet with Lestrade was off because he kept losing money [Yes he knew about the bet, of course he knew! He's Sherlock Freaking Holmes!]
John hadn't forgotten, rather, he'd lost his nerve. He only had one t-shirt left that Sherlock hadn't seen, and was apprehensive about wearing it, even though he had bought it specifically for the "experiment". So he put it off for a long while.
Until one evening he was sitting in his normal armchair reading the paper and he looked up at Sherlock, who was standing by the window, half turned, so his profile was illuminated by the street-lamps outside. He was overwhelmed with the striking features of his flatmate, and he realised that he wanted –no needed– to know Sherlock's reaction to the last t-shirt left. All his worries were gone, instead replaced by this sudden need to tell this beautiful man how he felt… and what better way than to do it using his t-shirt?
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He went upstairs and changed into it, getting to his bedroom door when his nerves returned full force. He mentally shook himself and walked downstairs, hoping that he wasn't about to ruin their friendship irreparably. Steeling himself he walked through into the living room, letting go of a breath he didn't know he'd been holding in, when he noticed Sherlock had left the room.
"You've changed your shirt." A deep voice behind him startles him, and he spins around quickly, coming face to face with Sherlock who had silently walked out of the kitchen and stood behind John.
"Jesus, you're like a Jungle cat, sneaking up on me like that." John starts to laugh, but he is cut off by Sherlock's lips descending on his own. He freezes, not expecting Sherlock to swoop on him so suddenly, but then the spell breaks and he kisses back with fervour. The kiss is passionate, each of them battling for dominance with their tongue, until the need to breathe becomes too important. They break apart, panting.
Sherlock has both hands fisted in John's t-shirt, and John has one hand on Sherlock's nape, playing with the hair there and the other on his waist, holding him close. Sherlock smiles at John, the latter of which breath catches in his throat. They kiss again, this time more languid and loving. When they pull apart they are both smiling. Sherlock envelops John in a hug and John rests his head on Sherlock's chest, enjoying the warmth of being in Sherlock's arms.
"I like the slogan of this shirt even more than the hugging one. Though I'm not sure how accurate it is, considering you aren't actually Irish."
John silences him by kissing him again, and they are content to just stay like that forever. Forgetting the outside world, all that matters is the two of them. John's lost a lot of money on the bet with Lestrade, but right now, he finds he couldn't care less. He's finally together with Sherlock and happy at last.