A/N: This was written for the prompt "I'm going to keep you safe" from renegadeartist on AO3. Go check them out!
The first time it happened, Donna thought she'd misheard. The telly had been on, some action movie playing at full blast, and though the Doctor was curled up to her side and she could've sworn she felt the vibrations through his chest, she eventually figured she must be wrong. The Doctor was an odd person, yes, but… surely he couldn't be that odd.
The second time, it had slipped her mind. He'd come to her rambling, talking in circles, frantic in his attempts to distract himself; while he slowly calmed, enough to lie down and let her touch him, she found her thoughts running wild with concern. He had already fallen asleep by the time she remembered to ask, still making that noise, and he needed all the rest he could get, the stupid alien.
Thus, it wasn't until the third time Donna noticed it that she actually brought it up.
She was reading in the library, on a soft old couch in front of one of the fireplaces, when the Doctor wandered in. One glance told her that it wasn't a social visit; he was dressed in a vest top and pyjama pants, rather than his customary suit, slippers on his feet and a blanket wrapped halfheartedly round his shoulders. His hair was a mess, and he had dark circles under his eyes. He wore an expression so forlorn that Donna lowered her book, sitting up straight as he approached.
"Spaceman?" She peered up at the downtrodden Time Lord, trying to get him to meet her gaze. "What's the matter?"
Silent and withdrawn, the Doctor sat heavily on the couch beside her. He tugged the blanket tighter around himself.
"Couldn't sleep," he said quietly. His eyes fixed on the fireplace, tracing the dancing paths of the flames.
Donna offered a sympathetic little smile, and reached out towards him. He caught the movement out of the corner of his eye and nodded, so she shuffled closer and wrapped him in a hug, laying her head on his shoulder. Some of the tension left his lanky body, drained from his features. His gaze softened.
"I'm sorry," murmured Donna.
"Ah, what can you do?" he said, flashing her a smile full of resignation. "No use being all maudlin about it."
That, she knew, was exactly what he would be doing right now. She shook her head, exasperated but fond, and let him go to move back to her side of the couch.
"C'mere," she said, waving him over. "Come lie down."
Any sadness in his expression dissolved into gratitude. He shifted to gently lie his head in her lap, drawing his legs up onto the couch. He arranged the blanket over himself, curling up with his arms tucked in against his chest, wriggled around one final time to get comfortable, then sighed with contentment.
"Thanks," he mumbled.
"Don't mention it." Donna patted his shoulder, then picked up her book again. She managed to get it in one hand so she could stroke her fingers through his ridiculous hair, just the way that always had him nodding off, and he gave an appreciative hum. "Get some rest, yeah?" Then she teased, "I'll keep you safe."
"Alright," he said softly, not even bothering to protest the sentiment, and she knew he was already well on his way. The warmth of the fire and his weight in her lap had her getting drowsy as well; she wondered whether she should attempt to extract herself and lie down, but she couldn't bring herself to disturb him now.
For a person who so resented sleep, the Doctor took to it quite fast when Donna was present. He didn't last long before he started to relax, let his eyes close, and it was then that Donna realized he was doing it again—making that funny noise. It was quiet, almost inaudible, a deep rumble that she felt more than heard. She broke into a grin when the realization hit her.
"Doctor?"
Hearing the restrained laughter in her voice, the Doctor opened one eye and turned to look up at her. He cocked an eyebrow.
Donna bit her lip. "Are you… purring?"
The droning noise tapered off as he frowned. "No," he said, "I am not purring."
"You are!" she insisted, sniggering at the indignation on his face. She could still feel the faint vibrations through the couch, and she placed a hand on his chest over his hearts to confirm.
"Well… it isn't purring, exactly," he mumbled, knowing he'd been caught out. A blush tinged his cheeks, making Donna smile all the wider. She went back to playing with his hair, and the rumbling grew inexorably stronger once more.
"Sure it isn't, Spaceman."
The Doctor huffed, but seemed unable to stop himself. It was rather hilarious, she thought, that his so-called superior biology had betrayed him so thoroughly. She'd always figured he enjoyed her company, and contact, especially when he was feeling down, but he never quite managed to express it in words. Frankly, she couldn't think of a better way to have her suspicions confirmed.
"You really do like this," she remarked, brushing her fingers through his hair to clear it out of his face.
His eyes fell closed, crinkles forming around the edges, a little smile spreading across his lips. "S'pose I do," he grumbled, a weak attempt at retaining some dignity.
"The, er, not-purring, then—it's good?"
He sighed. "Yes. It's sort of a… a social thing. Positive feedback and all."
"You don't do it very often," she pointed out.
His expression sank, ever so slightly. "No. I don't."
Donna was quiet for a moment, her hand stilling as she considered whether to drop it or not.
"Y'know," she started cautiously, "you can always… talk, if you need. To me. I'm always here for–"
"I know," he interrupted. Then, realizing he'd been a bit rude in his haste, he took a deep breath, in and out, and placed a hand on her knee. "Thank you," he said quietly. "I… well, I appreciate it."
"Anytime," she murmured. She resumed stroking his hair, and he resumed his not-purring, and before long she found herself getting sleepy as well, the quiet rumbling a remarkably effective sedative.
Oh, you dumbo, she thought, a fond smile tugging at the corners of her lips. She put her book down on the side table and laid her head on the armrest, in the crook of her elbow, so she could keep an eye on him as he nodded off. Never before had she quite registered the trust he put in her, at times like this, and never before had she realized what an impact such a simple gesture could have on him. For Pete's sake, he was purring over it!
But the hilarity of it aside, she loved him. And to know that she could make a difference to him warmed her singular human heart far more than the joking. Enough, in fact, to make her even more tired…
She fell asleep first, in the end, despite trying to keep watch; the Doctor knew it when her hand stopped moving, falling to rest in her lap. He didn't mind. He could hear her rhythmic breaths, the slow beat of her heart, reminding him that she was still there, that he wasn't alone and that for this tiny slice of time he didn't have to worry. It was rubbish, of course, that he needed such a reminder. But it was here, and it was nice, and he wasn't about to look a gift horse in the mouth—no matter that the horse was embarrassing him a bit.
He smiled—like the cat that got the cream, he thought wryly, and Donna was sure to catch onto that one soon enough.
He let himself drift off at last, watching the fire and wondering how strong an electrical shock he would need to paralyze his laryngeal muscles.