Molly Hooper is very clever, very lonely, and very interested in dead bodies. She always has been, since she can remember. She likes corpses, likes the way they don't look at her as if she's stupid, likes the way they can't lie or say cruel things. The cadaver, she knows, is a great equalizer: some big strong men who pinch and wink and leer can't stand the sight of one, can't hack the smell or cut the flesh or probe the organs. They can't, but she can.
Always has been able to, since she was a child. Most fathers, when their daughters asked for a scalpel and a corpse for their twelfth birthday, would have panicked. Sent for a shrink, sent her away. But not her da, never. He bit back a gasp, told her she could have a scalpel when she'd been trained to use one, and took her to a local funeral home. Introduced her to the mortician and let her stay as long as she liked. She went home with a thick book on embalming methods, a new friend — the tall, dour, hunched embalmer having taken instantly to the tiny girl — and a new sense of certainty about who and what she was.
And so when she goes to university, she studies anatomy and chemistry and forensic science and a few other things. And when she graduates at the top of her class, she gets a job as a morgue attendant at Saint Bart's. And when she starts work, they warn her about the odd man who comes in sometimes — tell her to let him do what he wants, but he can't take the bodies out of the morgue, and he has to be supervised.
But when he comes in, angular and unearthly in his beauty, and proud and inhuman in his intelligence and his clarity and his cold clinical precision, Molly Hooper is suddenly and irrevocably lost. Because he's her, but taller and more brilliant. They're so alike, and he will never, ever see it, and she figures it out, eventually, but by then it is too late and she's letting him take corpses and work alone and trying so hard to look pretty and make him see her. And then he brings in that Army doctor and she sees it, understands it, and it's over for her, and something in her shatters forever and ever.
And then Jim, sweet, funny Jim from IT, with his calculated smiles and his perfect performance, and she knows he's hiding something but can't figure out what, so when he meets Sherlock and she watches him bumble and flirt, she thinks, "Oh, that's what it was," and when she finds out the truth, something else in her breaks again and Molly Hooper hides in her apartment for three weeks, crying and holding her cat and trying to decide, as she has for her entire life, if she'd be happier as a corpse or as a person who cuts into corpses.