Here's the thing.
Sherlock Holmes doesn't really love.
Sure, he loves John, he loved Mary, he loves Rosie by default. (He loves her parents, she is one-half John and three-quarters Mary, he had no choice but to love her with all of his heart.) He loves Mycroft, he loves his parents, he loved Redbeard. (What is the mystery of Redbeard? What is Eurus keeping from him?)
(He doesn't know. He hates not knowing.)
But he's never loved someone in that way grown-ups would whisper about, the "way that a man loves a woman" or whatever such sentimental garbage his mother likes to praise in the poetry books hidden around her living room.
That's a mystery.
It's one he doesn't unravel.
He doesn't know how and he doesn't want to learn.
He likes being oblivious.
They step into the next room of Eurus' trials and see a coffin. An empty coffin. Sherlock immediately analyzes it: 5'4" woman; well acquainted with death—they may as well call each other friends; alone. Completely alone in the world. He knows who it is before Mycroft shows him the lid of the coffin leaning nonchalantly against the wall.
I LOVE YOU.
It's speaking to Sherlock, emblazoned across the coffin, defining this woman and her purpose in Sherlock's life. And there's only ever been one 5'4" alone woman who's unafraid of the thing that all warriors fear.
John asks if it's The Woman, and Sherlock absentmindedly shoots the idea down. It's not her, it's the other woman, the only other woman.
It's Molly Hooper.
Molly
Hooper
Is going to
Die.
His sister is going to kill his friend, his pathologist, the woman who loves him, the only one whose love he's never doubted.
The one who counts.
The one he trusts (the most).
And he knows the game, and the game is Break Sherlock, and what better way to break him than to set off explosives and destroy Molly Hooper?
(Somehow he knows this will break him.)
He understands what he has to do, and Eurus calls Molly. Molly's in her kitchen, that beautiful and hideous and wonderful sweater hanging loosely about her small frame. She glances at her phone the moment it rings, but doesn't pick up.
Molly makes tea.
She doesn't pick up she doesn't pick up she doesn't pick up why doesn't she pick up she always picks up it's him it's Sherlock it's the man she loves
Eurus laughs and redials. The phone rings once, twice (John says the words that Sherlock's thinking just bloody pick up), and Molly picks up the phone.
He wants to scream "Get out of there, get out of there, run, please Molly, please run until I can find you" but he can't. He can't do anything except this.
She asks him what he wants.
He wants nothing more than for her to be safe.
She asks if it's a game.
No. She's never been a game to him, nor an experiment, not in so many long years.
She doesn't understand.
He asks her to say the words. The words that will be her salvation and his release.
She begs him to stop.
He doesn't understand why she can't say the words. (He knows why. Because they're real. It's why he can't tell Mycroft or his parents, because the words are too real and they'll swallow them alive.)
She is in agony. She is his friend and his savior, his admirer and his stronghold. And she won't let him save her.
He begs her to say them. It's just three little words. (Three little words that beat back the darkness of the world. They're just a candle, but even a candle can push away the blackest of nights.)
She tells him to say the words first. She tells him to mean it. To say I LOVE YOU like he does love her, like she's the light of his world.
He
Can't
He
Doesn't
Know
How
He says the words.
He can tell she doesn't believe him.
But she is his light. John and Mary and Rosie and Molly. They're the reason he carries on. Their lives and their memory, their strength and their light.
He says it again. Quieter, this time. Softer. Like they're sacred. (They are, they're sacred words, they're words that can set the world ablaze.)
This time… he means them.
He waits and he waits he waits and it feels like an eternity and three seconds left and "I love you."
She says the words and he's saved Molly Hooper and he can't find the strength to stand anymore.
Eurus taunts him and she tells him what he should have realized. Molly was never in danger. Eurus knows what's important to him. Eurus wants to hurt him as much as she can. All those little emotions—Eurus and Molly set them free. Eurus lit the spark, made Molly say them, but Molly made him realize something he can't bring himself to contemplate.
Emotions are his downfall.
The door to the next challenge opens and Mycroft steps through, John walking behind him. Sherlock takes the lid of the coffin, Molly's coffin, and gently places in on top. He doesn't quite know what he's doing as he strokes the top, lets his fingers rest on I LOVE YOU.
Then he screams. He punches the coffin, takes the pieces of the reminder that Molly Hooper will die, and crushes them. The idea that Molly could have died today is a sword to his heart, a piece of ice that's been lodged in his soul for years melting away and leaving a void.
The screaming (was it his screaming? He doesn't know) eventually stops and he's sitting against the wall, not quite knowing how he got there. The coffin is in shreds and John stands before him.
This isn't torture. This isn't an experiment. This is the agony of a five-year-old taking a scalpel and driving it through his chest and struggling to find his heart until he's ripped to shreds. It's dissection and he's still breathing.
They finish the trials and find Eurus and bring her back to Sherrinford. They close her in and Sherlock plays the violin with her.
Three cases solved. Eurus is defeated, the little girl is saved, and Redbeard is found.
Sherlock feels empty.
He and John fix up 221B Baker Street until it looks as it did the day before Mycroft became the client. Rosie toddles around, grinning and tearing at the fresh wallpaper.
He needs her. He needs to tell her.
He takes a taxi to her apartment the next day.
She doesn't want to let him in but he stands at the door and waits.
Finally she opens the door and sits in her chair and listens.
Eurus—his memory—Redbeard—Eurus' trials.
Why he was forced to make her say the forbidden words.
He doesn't expect forgiveness for the years of agony she's been through since meeting him. He just wants to explain and to say—
(He wasn't lying.)
As he stands to leave her and let her live her life free of him and the pain he always brings her, she takes his hand in hers. Molly stares at him, gazes into his soul, it feels like.
Then slowly, so slowly, she reaches up and rests her hands on his cheeks. Slowly, so slowly, she rises to her tiptoes and kisses him.
He can't respond at first.
But then he understands.
She's forgiven him.
But then he realizes that the woman he probably (certainly) loves is kissing him. So he pulls her closer and wraps his arms around her tight enough to ascertain that he'll never leave her again.
He whispers the words when they pull away from the kiss. They'll always be tainted for him. He'll always remember the day they were forced to say them. But someday—
Someday
Maybe he'll say them, and remember only Molly's smile, and Molly's laugh, and Molly's blushes as she repeats the words that start to heal the tear in his heart.
He brings her back to Baker Street. She marvels at the redecorating, and she laughs when Rosie shows her the skull that she's somehow yanked down from the mantelpiece. She giggles when Sherlock sits in his chair and gently pulls her down to sit on his lap.
And she laughs, and John grins, and Rosie coos. Sherlock looks around at his family, and he loves them, and he's happy.
Once upon a time, Sherlock Holmes didn't know how to love.
Funny that it takes a threat of an explosion to show him how very much he can.