The first time they meet he's thirty, she's twenty-three. He's as high as a kite, and her fingers are gentle through his hair as he surrenders to a restless slumber; he knows who sent her but he doesn't care, it doesn't matter as long as she's not scolding him like his brother would.
Somewhere through the haze in his brain he thinks that if he could stay like this forever, then he wouldn't need to use anymore. He doesn't tell her, he's too stubborn and she's too smart to play mother to her boss' wayward little brother; and if Mycroft does guess, at least he's decent enough not to point it out loud.
xxx
He keeps a weather eye on her along the years, watches her as she flirts with countless men; she smirks as she rejects them all, a Mona Lisa with a BlackBerry and a licence to kill.
It's only after she turns down John Watson that he starts to wonder. She likes men, that much is apparent, and yet he's not sure she's ever had anyone; he hasn't had anyone either and most people think it's weird, but perhaps it's not after all.
They're sniggering at Mycroft over a glass of wine when she toes off her high heels and climbs into his lap. He's about to stop her when she places a reassuring hand on his shoulder; that's when he finally sees the truth, and allows himself to relax into her touch.
She tastes like the wine they've been drinking, rich and heady with a faint note of raspberry. He threads his fingers through her hair, and feels her smile against his mouth.
xxx
Years go by, and many things happen. St Bart's. Serbia. Appledore. The East Wind coming.
He knows she's always there in the background, fighting for England and the greater good, and the thought is enough to give him some measure of strength. He never tells John, for he wouldn't understand any of it; Mary, on the other hand, she can tell when he's fibbing.
"Go to her, you twerp," she prompts him as soon as he steps out of the plane; her husband frowns, but Sherlock only smiles at her in gratitude.
Andrea doesn't say a word when he shows up on her doorstep; she merely stands on her tiptoes, wraps her arms around his neck, and he knows he's come home at last.