This is based on the TAB flashback we saw.
Watched TFP last night, and it destroyed me. Really liked it though, but am sad that it's over.
It's where Mycroft finds him, in a dirty, ransacked building, lying on his side.
In a crisp, white, dress shirt, he's out of place - but nobody gives him a second glance, or even a first. They're too out of this world to notice anything other than their bliss or the fear of their next comedown.
He doesn't look at them in disdain, like the people at work - he can't, because when he looks at their shivering, moaning bodies, all he sees is his own brother. It's such a familiar picture that he nearly winces, reacting instinctively to the violent contortions of their faces. He almost reaches out to help them, moving before he can think, like he's done so many times before. It's so unlike him that his lip curls up with half-hearted distaste. He never has much energy for emotions when it comes to anyone else than his brother.
Sherlock had once asked him, if there was something wrong with them, for not caring like the others do. Sometimes, when he's alone on his dark walks to find his missing brother, he thinks he cares too much.
He climbs up the stairs carefully, placing a hand on the rough rail. The clean, polished dress shoes make a heavy tread on the decaying steps. His brother always preferred a place near a window - his shooting-up ritual, he think wryly. Perhaps he likes a glimpse of the sun before the needle, a foolish bit of indulgence. His little brother was always so much more human that he believed.
When he pushes open the door, he closes his eyes for a second, like he always does - Mycroft is not so simple as to deny that he's always terrified of what he might see. The room is dark, dirty, and foul-smelling, with little more than a small cot opposite of the window. The air is damp and chilly against his skin, but he can't notice it. Not with what's in front of him.
Sherlock is lying on his side, shivering violently against the stained fabric of the mattress. The window is boarded in inconsistent spots, throwing odd light across his body. Groans carry from his pale mouth to Mycroft, and he can only just stand there, watching his brother writhe in pain. There's little more that he can do than wait it out when he's like this - but he's always there. When his brother needs him, he could never ignore. It's a sentimental weakness he carries for no one but him.
He rolls the pristine sleeves up, a practiced motion - threatening and dominant in his office but only weary in this building. Measured steps to cross the room, and he stands over his brother, with tired shoulders, with sad eyes. Sherlock can't tell he's there, he never does.
Carefully, he takes a seat on the mattress near Sherlock's knees, after leaning his umbrella against the torn wallpaper.
"Mycroft, Mycroft!" The sound of feet running up the stairs, and a little boy bursts into his room. His eyes are bright with childish happiness, his jumper and hair all askew.
Mycroft lifts a tired hand to his forehead, rubbing his headache away - but already sensing a new one. His internship is a relatively new one, but both he and the employer are quite aware of the potential he shows. The work is not something he cannot handle, but distractions are something he does not wish to deal with now.
Sherlock's eyes are eager, but carefully, they turn a little wary at the obvious tension in his brother's expression. A little more timidly, he asks, "Would you knight me, Mycroft?"
"Knight you?" His face turns incredulous, and lifts from the paperwork to stare at him. He's sure he's never heard anything so ridiculous in his life, but children so often are. It's something he's tried to train out of his little brother, but without much luck.
Pleading now, "Would you, Mycroft? Please?"
Mycroft sighs, casting a weary eye around his tidy room before landing on an umbrella, leaning against the wall. It's a practical, conservative black, with a curved and polished wood handle. A favorite of his, so the grip is silken to the touch. "Fine." He gestures with his hand for Sherlock to come closer. "Come here."
Leaning in his uncomfortable chair, he reaches with his long limbs to pick up the umbrella, grasping it lightly in his hand. He throws a pointed glance at his brother, pointing from the little boy to the ground with the tip. "On your knees, then."
Obediently, Sherlock drops to his knees.
Tiredly, Mycroft begins, "Do you swear to protect your crown and country?" He's making up some silly script, but Sherlock seems to accept it, nodding solemnly up at him.
He exhales noisily through his nose, idly thinking of work that needs to be done. "Good," he ends briskly. The edge of the umbrella is resting on Sherlock's small shoulder, brushing past his messy curls. "Then, in the name of King Mycroft - "
"Queen," Sherlock whispers up at him, from his spot on the floor.
Mycroft blinks back at him. "Excuse me?"
"The Queen knights people, Mycroft," he says earnestly, eyes wide and innocent.
Mycroft gapes at his younger brother for a moment, before deciding to proceed. Anything, to wrap up this childish playacting. This time, he struggles with the words. "Then, in the name of Queen...Mycroft - ," Sherlock giggles quietly and he glares back, " - I now dub you Sir Sherlock Holmes." He moves the tip to the other shoulder lightly, and is rewarded with a blinding grin. Mycroft adds dryly, "Congratulations."
On the cot, his brother whimpers. Sherlock's black curls are slick against his forehead with cold sweat, and his body contorts as it's racked with pain. He reaches out a cooling hand to lay against his clammy forehead, and Sherlock calms for a moment, the spasms stilling.
A folded piece of paper is still clutched in his hand - like it always is, and Mycroft gently slides it out. He straightens it out with his palm, reading the neatly printed lines with a carefully neutral face. It's ghastly, it really is, but Mycroft never reacts when it really counts - when Sherlock's most vulnerable.
The list is tucked into his small notebook, filled with all of his past findings. It's a grim record of all the times Mycroft has failed him.