Warnings for a little bit of angst.
I realise I haven't actually written TRF yet, but I think we all know what happens. In this 'verse, Sherlock leaves more than just his ex army doctor behind.

Age: 12

Sherlock knew it would be easy to sneak back into 221b. John hadn't changed the locks in the time Sherlock had been 'dead'. He also hadn't fixed the bullet holes in the wall, and the yellow smiley face he'd spray painted in his throes of boredom years ago continued to look down on the room. Billy the skull now wears the old deerstalker, which Sherlock frowns at, until he sees a small framed photo of himself, John and Hamish taken shortly after they first met.

He almost doesn't hear the front door open and close, only realising he isn't alone when he hears soft footfalls on the stairs.

"Is that you John?" An almost familiar voice calls.

Sherlock turns to see Hamish frozen in the doorway, now twelve years old. Sherlock feels his throat tighten at the sight of Hamish; he'd gotten so much bigger in the time he'd been away. What had been a scrawny nine year old, had become a strong twelve year old, already filling out and losing the childishness of his face. Sherlock suddenly realises how much he's missed. He missed celebrating his son's birthdays, finishing primary school, getting his SAT scores, sending him off to secondary school. He can't help but notice he's still small for his age, but has perfect posture and piercing eyes that remind Sherlock of his own.

"Dad? Are you a ghost?" Hamish's satchel falls from his shoulder, landing with a quiet thump.

Sherlock smiles sadly, Hamish's voice had started getting deeper, something else Sherlock had missed.

"Hamish, I—" he starts. He's interrupted by the twelve year old slapping his cheek and shouting.

"I hate you. How could you?" Tears begin to roll down his cheeks, "how could you leave us? We were happy, dad. You, John and me. Was that not enough for you?"

"I can explain," Sherlock says calmly, rubbing his cheek.

"Don't bother," Hamish growls, "I don't want to see you again." The front door slams and Hamish turns on his heel and climbs the stairs as fast as he can, snatching up his bag and continuing to shout, "John, you might want to get rid of that piece of rubbish in the living room. I doubt you'll want it either."

"What are you talking about, Mish?" John calls back, slowly making his way up to the flat, "you best not have made a mess while I was out."

Hamish snorts loudly and throws his bedroom door shut, making sure the noise echoes through the flat.

"What's got into him," John mumbles to himself, pocketing his keys and trudging up the final few steps. At the sight of Sherlock, John lurches forward, dropping his bag of shopping by the door and curling his hands into fists.

"Please, no," Sherlock holds his hands up in surrender, "Hamish already slapped me."

To Sherlock's surprise, John stop and smiles sweetly, unfurling his hands and taking Sherlock's face in his hands, "is it really you, Sherlock? You're alive?"

Sherlock nods quickly, hoping for a less extreme reaction.

"Right," John says, before standing on his tiptoes and head-butting Sherlock's nose, making him stumble backwards.

"John?"

"You complete and utter cock," he snarls.

"John, I—"

"No, Sherlock. You have no right to just waltz back in here. You didn't even let Hamish, your own son, know you were alive?" He paces in front of Sherlock, "he's been so good. He dealt with your death so well and now you go and do this. The last time I saw him cry was after your funeral, and now he's up in his room sobbing."

Sherlock focuses his eyes on the floor, ignoring the throbbing in his nose, "I can explain."

"No. Leave. I can't even look at you right now," he sighs, "I can't decide if I want to hug you or punch you, so you'd best go before I decide. If you're going to ask to be part of our lives again, which I know you're about to, you're going to have to let Hamish adjust; he's the one who spent the last three years believing his father had died. I'm his legal carer now, and I don't want you anywhere near him at the moment."

Sherlock's shoulders slump, "I still have my phone. It's the same number. Text me when you're ready."

"If," John folds his arms, "if we decide we want you back."

Sherlock's raises his eyes once, taking in John's stern expression, and nods, starting to walk towards the stairs.

John waits until he hears the latch drop before going up to Hamish's room.

He knocks on the door gently, "hey, you okay? Can I come in?"

He hears a sniffle and a quiet, "yeah."

John opens the door and sees Hamish sat cross-legged on his bed, the sleeves of his jumper damp from where he'd quickly wiped his eyes.

"Is he gone?"

"Yeah," John sits beside him, wrapping an arm around the small boy's shoulders, "I told him that he isn't allowed back here for now."

Hamish leans into John, "I don't want him to come back. We buried him. How could he do this to us?"

John makes a soothing sound and pulls Hamish into a hug, which the boy melts into, finally letting himself cry properly into John's jumper. John gently strokes Hamish's hair and waits for him to calm down. He presses his nose against Hamish's temple, encouraging deep breaths.

"I have to ask you, Hamish. Do you want to see him again? You don't have to say yes, and you can take your time to answer," John soothes.

Hamish sniffs, "I do want to see him. I'm happy he's not dead, but I'm angry and I'm afraid I might hit him again. Or kick him, whichever part of him I can reach."

John laughs, "get in line kiddo."

"Can I watch when you hit him this time?"

John squeezes Hamish's shoulder again, "like I'd let you miss that."

"Are you going to see him?" Hamish asks after a moment.

"I have to," John sighs, "I'd rather not, but he said he had a reason for leaving us and I'd like to know. I'll probably meet him somewhere neutral, in a café or somewhere, if you want to come."

"I want to know too. I'd like that."

John smiles, "we'll sort it out. What he did was horrible, and I may not be able to control the direction of my fist next time I see him, but no one gets second chances like this, Mish. "

"I guess," Hamish shrugs.

"I'll talk to him," John rubs Hamish's shoulder, the small boy leaning in close again and wrapping his arms around John's waist.

Part one of Sherlock's return, to be continued in part two; The Reunion.