Inside his mind is a Palace.
Inside the Palace are many rooms.
Within the largest room is the Throne.
Upon the Throne sits the King.
From time to time, the Throne Room changes. Sometimes it gets redecorated, sometimes it moves to a new "largest room" entirely, and sometimes—a very rare sometimes—he demolishes the room himself and builds it anew. But when the Throne Room changes, so, usually, does the King.
He begins to build the Mind Palace when he is a child of five. It is more of a Mind Flat at that point, albeit a relatively spacious one. The living room is the Throne Room, and it is furnished with oak promises and velvet stories and bright blue wallpaper made of whispered secrets and deductions.
And on the carved-wood Throne sits twelve-year-old Mycroft, the elder brother he adores beyond compare.
And then Mycroft lies, and he gets older, and he goes to school far away, and Sherlock tries his hardest to tear apart the Throne Room.
In time, he forgives Mycroft, but never again does his brother sit upon the Throne.
When he is seven, the Mind Palace is not a Palace or a Flat but a Fleet. He builds a pirate fleet in his head and lets it sail on an ocean of language, of tempestuous words in four different tongues. He has two cutters, two trysails, three raffees, a schooner, and a merchant brig. The brig is much slower than the other ships, but Sherlock needs it because it holds more, and he needs somewhere to store the things he ought to know but doesn't particularly want to.
This time, the Throne Room is located in the hold of the Death, the largest of the raffees and Sherlock's flagship. The hold is dark and just a little spooky, stocked with barrels overflowing with fact-jewels. Gold pieces stamped with the names and profiles of pirate captains glint in the swinging candlelight, spilling out of treasure chests made of ship terms.
A pirate King with wicked-looking facial scars smiles down at Sherlock from a Throne of stolen riches. Sherlock smiles back.
After the Fleet comes the Mansion, which he mostly deletes afterwards, and after that comes the Museum.
He likes the Museum. There are so many interesting things there, not least of which is the Throne Room.
This time, the Throne Room is located by the exhibit on the human body, and the Throne itself is made of bones.
Upon the Throne sits a brain, which Sherlock sometimes takes in his hands and examines. It's brilliant.
Little by little, the Museum shifts, grows, builds upon itself, until the day comes when it is no longer a Museum but a true Palace. Sherlock doesn't mind it, really; there is so much more space here, and he thinks that "Mind Palace" sounds much better than "Mind Museum".
The Throne Room shifts minutely with each new addition, but the King does not change for many long years after.
When Sherlock is twenty-two, he discovers cocaine.
The Throne is relocated to a brightly-lit room with walls of observation-pills and insult-tablets, and for the first time, a Queen sits upon the Throne.
She is white, so blindingly white it hurts to look at her with her nails like razors and her teeth like needles but Sherlock doesn't care, he doesn't care because she's so good to him, god she's so good.
She's so good he wants to die in her arms.
Mycroft attempts to usurp the White Queen, but it doesn't work. He had his chance to reign, long ago.
Sherlock is twenty-seven when he forces himself to delete the doors to the old Throne Room and shove it down to the deepest sub-basement dungeons.
The new Throne Room is comfortable, if a little dark in the corners, and is wallpapered with yellow chemistry and grey news articles and indigo history. The Throne he painstakingly crafts out of steel resolve and elastic obsession, and decorates it with nicotine patches and musical notes.
It isn't perfect, but it's a match for his new Queen, with her bloodstained dress and her motive-coloured nail lacquer and her crown of beautiful horrors.
She is the Work, and when she settles onto the Throne, Sherlock decides that this is the last one. He will never have another monarch aside from her.
And for six years, he doesn't.
He meets John Watson and stores the information in the little room where he keeps facts about people he doesn't particularly dislike.
After John moves into Baker Street—after John tells him he's fantastic, after John runs across London with him, after John shoots a man for him, after John laughs and frowns and tells him he's an idiot—Sherlock builds a little room inside his Mind Palace and allows John's facts to move, too.
John's Mind Palace room is bright and dark at the same time. The ceiling is decorated with glow-in-the-dark solar system decals against a black abyss of nightmare ramblings. His morals are tacked up on a cork bulletin board side-by-side photographs of a Browning and the bullet hole in the shoulder of a not-very-nice cabbie. His facial expressions are hung up between his jumpers and his favourite swear words.
It's a cosy room, really, and sometimes Sherlock can even bring himself to sleep between the Army-canvas bedsheets, his head on a soft pillow of complaints and praises.
There is the Pool; the aftermath; the return to normalcy. There are cases and clients.
The Throne Room expands to accommodate the work-related things he wants to remember, and the Queen watches over it all with benevolent malevolence.
Then there is Irene, who gets her own room. It is smaller than John's, but that's more than most people get with Sherlock. She is The Woman. She is interesting and intelligent and...
And then she dies.
Except she doesn't.
He follows John—he has to follow John, he always follows John, especially when John is idiotic enough to get into an unmarked car that is most definitely not one of Mycroft's—and she's there, and he hears something about couples and being gay and then there's his text alert noise, and he leaves.
And later, much later, after he takes his revenge on the American for hurting Mrs Hudson, after John tries to talk to him, after he has holed himself up in his room, he goes back into his Mind Palace and sits in Irene's space and wonders if he ought to tear it down or not.
He decides not.
It's only when he's trying to find the connection between Liberty and In and hound and Baskerville that he realises something is off in his Mind Palace.
But he's on a case, and that matters most. He can investigate the anomaly later.
"Later" turns out to be when he is sitting on the floor of the lab at Bart's. The lab where they first met.
He goes into his Mind Palace and stalks down halls, through closets, examining anything that strikes his fancy as he makes his way to the Throne Room.
Which has somehow, without his realising, changed, morphed, shifted—
No, it's merged.
The wallpaper is the same yellow-grey-indigo, and the shadows in the corners are still there, though now they stretch to the ceiling, which is nightmare-black and starry. Memory-photographs and case-clippings alike are tacked to the cork board affixed to one wall. Jumpers knitted out of autopsy reports hang in a battered wardrobe of broken promises and stolen files.
It's as if John's room has melded into the Throne Room, leaving Sherlock with this strange... amalgamation.
It frightens him more than anything. More than the Hound. More than this final confrontation.
He doesn't want to look at the Throne itself, doesn't want to see who sits there, who rules over this nowhereland inside his head because he'd sworn to himself six, seven years ago that he'd only have one Queen, and he can't have this happen. He just can't. Not now.
But he is Sherlock Holmes, and he doesn't handle suspense well.
And when he looks, what he sees takes his breath away.
He stands on the ledge, stares down at John, at the end of this (whatever this is). Maybe, when it's all over, he'll come back and they'll be just as they were.
He doubts it.
He takes a breath, retreats into his Mind Palace, and jumps.
Inside his mind is a Palace.
Inside the Palace are many rooms.
Within the largest room are the two Thrones.
The first Throne is made of steel resolve and elastic obsession, and its Maker has decorated it with nicotine patches and musical notes.
Upon this Throne sits a Queen with a bloodstained dress and a crown of beautiful horrors. Her nails are lacquered the colour of motives and her hair is dyed the colour of deceit. One eye is guilt, the other innocence, and her lips are parted in a smile like adrenaline.
She is the Work. She is the Head.
The second Throne is more homely. It is made of honeycomb structure and rowan safety, and its Maker has adorned it with bullet casings and medals and tea.
Upon this Throne sits a King in a worn-out jumper and jeans and a crown of amber honey. He is compact, but terribly quick and strong. His hair is the gleaming brown-gold-silver of bravery, and his smile is comforting as a nest of blankets. One eye is temper, the other patience, and they are both a lovely bright blue.
He is John. He is the Heart.
And Sherlock sits at their feet, one hand in each of theirs, and loves them both.