Author's Note: Inspired by an anonymous ask on Tumblr, who asked for a Doctor/companion AU.

There were some days, some occasions, when he believed he hated the sight of her. On those days, he was that much colder, that much harsher, than the Doctor would ever have allowed him to be. On those occasions, the times when he deduced her human form and felt his stomach flip as he watched her flood from the room, tears in her eyes, he'd chastise himself. He'd retreat into his mind palace, warming himself with the memories she had given him, of time and space, reminding him of why they had become as stuck as they were. Why they had fallen into this rut.

"This will change my entire DNA – I won't be a Time Lord anymore, I'll be human. But there's a catch." She paused, her fingers lingering against the surface of the console. Taking a breath, she stepped forward and reached up, touching at the line of his jaw, as always seeing the fear he often tried to hide so uselessly.

"You have to look after me," she said quietly, the order behind it clear. Her brown eyes were sincere and wide and pleading. "Please, promise me. I don't know who I'll be, and I don't know how long this will have to go on for, but you have to look after me."

She touched at his jaw, as always seeing the fear he often tried to hide so uselessly.

"Look at me," she murmured. "And tell me you'll look after me."

He could've kissed her then, close as they were, their breaths mingling and their foreheads tipped together, and when he only pressed a kiss to her cheek, his fingers covering the hand she still held against his jaw, he hated himself.

"Always, Doctor."

Something about his words must have been notably pitiful, for the Doctor did something she rarely ever did: she drew her arms around his neck and burrowed her neck into his torso. He held on so tightly, he feared they both might break.

Her human name, the name of "Molly Hooper", was bland and plain and one that was burned onto his memory. He doubted he would ever forget it. When he saw her for the first time, walking into St. Bart's and eager to start her new day at work, it had been so strange, so uncomfortable to see this woman—this alien who fought monsters and saved whole civilisations—reduced to such mediocrity. So, obviously, he switched to default. With his hands shoved into his pockets, his fingers wrapping around the key he always kept on his person now, he stiffly demanded to see a body.

Yet small traces of his Doctor remained. Her smile, for one. The quiet steadiness of her voice, another. Most of all, there was her pure wonder at the world. That curiosity, that boundless curiosity which had started off their adventures together, was alive and kicking inside the mind of Molly Hooper. Yet it was only when she'd made the stumbling attempt to ask him out for coffee that he realised where her curiosity had landed her: in love. With him, with Sherlock Holmes, consulting detective. Taken aback by it, he had automatically shut her down with some scathing remark and walked off. After his meeting with John Watson—his Doctor's inability to make coffee had also, unfortunately, remained—he had phoned Mycroft.

"I need to see it," he snapped into the phone, his fingers twitching against his palm.

"Very well," his brother said coolly, unquestioning. "A car will be with you in ten minutes."

The car drove him to the outskirts of London, towards some dilapidated warehouse, where a young girl, wearing a long, oddly coloured scarf, was waiting. Nodding once at Sherlock in greeting as he stepped out of the car, she unlocked the door to the warehouse.

"It's still working," she said warmly, a vague sense of comfort filling him on hearing her words. Maybe that was why she had spoken them. As his Doctor would've said, she had a kind face. "Still humming away."

The girl pushed open the door and he stepped inside, pulling up his coat collar. There it was, the blue box with the chameleon circuit she had never bothered to fix, despite her many muttered reminders to herself for her to do so. (Fed up of said mutterings, he had once, in a fit of annoyance, introduced her to the concept of Post-it notes, and for a while, the walls of the TARDIS had been awash with them.) He took the key from his pocket. The walls of the TARDIS, the familiar blue, flecked as it was with chips of grey, were still warm, the hum of its heart vibrating through the wood. Sighing, he pushed the key into the lock and pushed open the door.

The TARDIS, the place which he had made his home when a strange woman had offered him the universe, had not changed. The lights on the console still flickered on and off, waiting for their mistress to come home. Sherlock advanced up the steps to the console two at a time and made for the monitor. A smile crept onto his lips, various bleeps and noises sounding when he drew up the video he had watched perhaps a thousand times already.

There, on the screen, was the Doctor, his Doctor, her hair loose and in her black suit, her collar open and her blue tie loose against her neck.

"Right, Sherlock, you don't know I'm recording this, but that's – that's good. As you know, I plan to change soon, into a human, and well – I thought you'd need some instructions." She beamed, proud of herself for being so incredibly helpful.

"So, here we go. Number one, I can't hurt anyone. Please, don't let me hurt anyone, even if you don't like them. Two, phone your brother, and clue him in on the scheme so he can arrange everything. I'll be out for a fair few hours after I change, and I don't want you to have the responsibility of waking me up. Have him hide the TARDIS – don't worry, I'll let it run on emergency power – but you have to keep the key. For God's sake, don't let Mycroft have the key. The man is the British Government; you can't trust him with it."

She paused, took a breath and continued. "Three – and this will probably be the most difficult one for you – don't get involved in anything big or dangerous, okay? The Master will be tracking you. The change will give me and the TARDIS a cloaking device which will pretty much make us invisible – I've programmed it that way – but it doesn't extend to you, so you have to be careful. Get involved with human problems if you must, but if you see or encounter anything alien, leave it alone. I want you safe. Now, four." Again, she paused, her smile fading. Just as he had seen before in the footage, her eyes shifted, dimming with hurt and worry and fear. And just like all the other times, he lowered his head and listened, unable to see his Doctor so vulnerable, and so—ironically—human.

"Don't let me fall in love, Sherlock. I can't fall in love - I can't have any emotional attachments. If - if the Master finds me, he'll use them. He'll break them."

He froze the video, and sighed, running his hands over his face. Sentiment, love. A dangerous disadvantage. The one human thing he had to protect her from, the one thing he had always been plagued by... and he, selfish, blind idiot that he was, had let her succumb to it.