Edit: Screw you, random anonymous commentor! I'll use they/them pronouns for the Doctor if I fuckin want to! To anyone else thinking the same: don't even try it, you won't get anywhere.
Donna had picked up a lot from the Doctor in the time they'd been travelling together. A heightened desire to buy more long coats, for example. Several pairs of fuzzy socks, because the TARDIS got cold sometimes. An appreciation for the universe for which she would always be grateful, though she wasn't going to tell them that.
She'd also picked up the nightmares. Not their fault, of course; their adventures could get stressful, sometimes disturbing, and though she wouldn't trade them for the world it did weigh on her from time to time. And thus, she'd acquired their love of wandering the TARDIS when she couldn't sleep, exploring the near-endless supply of rooms.
Tonight she had managed to find her way to a winding hallway she was pretty sure she'd never encountered before. Most of the doors were closed and didn't want to open, but she'd already passed a ballet studio, a back entrance to the library she hadn't known existed, and a room containing nothing more than a single water fountain. Now she was nearing the end of the hall—an uncommon discovery on the TARDIS, as the corridors normally just circled endlessly—and at the end of the hall was a door.
It was just like every other standard metal door on the ship, except that there was a piece of paper hung next to it, like a sign outside a child's bedroom. The writing didn't belong to any language Donna could decipher, but it resembled the strange and intricate circular designs that the Doctor liked to write on the sticky notes all over the console room, and she supposed this room must belong to them. The door was open, casting a soft, warm light into the dim hallway, and music played from somewhere inside; though it was nothing like she was used to, lilting and dissonant and evidently alien, Donna found she enjoyed it.
As she approached the end of the hall, coming to lean against the doorframe, she saw that the room was some sort of garage, albeit not like any garage she'd seen. Half-built machines, big and small, were scattered around the open space to her right; currently, an engine as tall as her occupied most of it. To her left were three benches, on which resided an array of shop tools and stray parts, glue bottles and clamps and packages of screws, and against the wall behind them were dozens of mismatched parts cabinets, each little tray filled with components. It was exactly the sort of chaotically organized space she would've expected from the Doctor.
At the back of the room, the Martian themself sat hunched over a desk, trying to maneuver something into place with a pair of tweezers. A little old radio sat beside them. The bright desk lamp illuminating their workspace provided the only light, and transformed the high-ceilinged room into a surprisingly cozy place; the illusion was compounded by the worn old sofa in the corner next to their desk. The warm light and the oddly soothing music had Donna yawning, wondering whether she should return to her room.
But this was a side of them she almost never got to see. They looked more calmly, peacefully happy than they ever did on their adventures, and she found she wanted to take it in for a bit. They had on an expression of soft interest, clearly engaged in whatever project they'd found. When they stopped fiddling to examine the parts, they had a habit of tapping the pliers against their desk to the rhythm of the song; when they picked them up again, they kept on jiggling their foot in time. They seemed to find the song quite moving, in fact, swaying slightly with the rise and fall of the music. She even thought they might be humming along, and she smiled fondly.
Then she squinted. She'd heard them sing before—albeit jokingly—and she recognized their voice. When she listened she could easily pick it out from the nasally instrumental tones. But they were, very clearly, singing two distinct parts at once; they held a grating chord for a moment before resolving it all on their own, their voice splitting to soar and fall along separate paths. She listened for a long time, absorbed in trying to figure out how they were doing it. And, if she were honest, marvelling. Just a bit. It was beautiful.
The Doctor must've spotted her out of the corner of their eye (or perhaps they'd heard her? smelled her? she was never quite sure) because they abruptly stopped humming and glanced over their shoulder before refocusing on their work. "What're you doing up?" they asked.
Caught listening in, Donna pushed herself off the doorframe and wandered over. "How were you doing that?" she asked, a little awed, leaning against their desk as she tilted her head to examine them.
She watched, amused, as a faint blush spread over their cheeks. "Doing what?" they inquired, delicately setting down the pliers and the circuit-like thing they'd been holding.
"The singing, of course."
"How… was I singing?"
She huffed. "How were you singing two things at once?"
"Ah." The Doctor nodded, lips pursed, and spun in their chair to face her. They twiddled their thumbs together. "Didn't realize you were listening to that," they mumbled.
"Oh, come off it," said Donna. She couldn't even be bothered to tease them. "You're brilliant and you know it."
They glanced up at her with a sly twinkle in their eye that told her she was exactly right. She shook her head, but couldn't help a hint of a smile playing across her lips.
"Well, you know, it–" they gestured ineffectually– "it's because of my respiratory system. Can't support all that extra stuff and retain the same structure." She raised her eyebrows, hoping for a deeper explanation, and they shrugged, apparently flustered by her interest. "The sounds don't… they don't translate well either. Your brains are just too different. You hear an extra undertone, perhaps two, but that's not how it's meant to be." They cast a rather sad look towards the radio. "I was trying to replicate it. Y'know, hoped to– to show you. Hasn't… worked just yet."
"Oh." Donna reached out, squeezed their shoulder briefly in comfort. She could only guess at how important it must be to them. "I'm sorry."
"Ah, didn't really expect it to turn out," the Doctor dismissed. They offered a smile in an attempt to hide their evident disappointment.
"Is there any other way I could hear it?" she asked. "Properly. Like you do."
They pursed their lips, leaning back in their chair. "There's… well, there is telepathy," they said after a moment. "Might work, might not. To be honest, I'm not sure."
Donna knew they were a telepath; they'd mentioned it before, in passing. She'd always found it rather intriguing, the idea of being able to feel what they felt, see things the way they saw them—at times like this more than ever, for occasionally she would realize just how different they were from her, and though it could get a little unsettling she couldn't help being curious. And getting to hear music from their homeworld as it was meant to be heard… that added a whole new layer of intimacy. She would've liked to hear it, she really would've. She wanted to share that with them, the daft Spaceman. But they were avoiding her gaze, fiddling intently with their hands, and they had that nervous little furrow in their brow that meant they'd (quite accidentally) gone out of their comfort zone. She reached down and took one of their hands in hers.
"You're not really ready for that, are you?" she asked softly.
They glanced up at her briefly, then bit their lip. It was all the answer she needed.
"It's okay," she said, and meant it. "It still sounds good like this."
They gave a small smile, and remained quiet for a moment, still looking down at their lap. "Maybe… one day," they murmured. "Maybe."
Donna's heart clenched in fondness. She cupped their cheek in one hand, and leaned down to kiss their forehead. "You let me know if you want to," she whispered. "'Kay?"
As she let them go, they drew a deep breath, in and out, and nodded. "Thanks," they mumbled.
"My pleasure, Martian Boy."
For a minute, neither of them moved. Then something flickered across the Doctor's expression; they frowned, and glanced ineffectually around the room.
"Blimey, it's late, isn't it?" they asked. They looked her up and down. "You're in your pyjamas," they pointed out.
Donna redirected her gaze to their little project, feeling herself blush. "Not that late."
"What are you doing up?"
They were studying her with the exact same concern she turned on them, when they stayed awake too long. She sighed.
"I had a nightmare," she admitted. "Well… a couple, I suppose. I– I didn't really feel like going back to sleep." Suddenly, she understood their customary reluctance to talk about these things.
"Oh." They gave her a pained look. "Oh, I'm sorry, Donna."
She shrugged. "'S fine." Even as she said it, she felt that little spark of fear in her chest again, and her eyes stung; she really didn't want to have another nightmare. She didn't know how they put up with them.
The Doctor saw it, of course. Carefully, as if unsure whether they were overstepping their bounds, they stood up and placed a hand on her arm, then drew her into a hug. Their arms wrapped around her tightly, holding her secure, and she breathed a shaky sigh, hugging them close to her in return. Their hearts beat strong and steady under her ear, and though they didn't feel any warmer than the room, they certainly did feel safe.
When they pulled back, a long minute later, they kept their hands on her upper arms.
"D'you want to sleep here?" they suggested. They glanced at the old sofa next to their desk, and shrugged. "It's nothing fancy, but it works."
Relief washed over her, and she nodded. "Yeah, I mean, if… that's alright."
They tilted their head, looking at her fondly, as if to say that was a foolish question. Then they ushered her over to the sofa and got her set up, fluffing pillows and unfolding blankets. As soon as she lay down and tugged the blanket up to her chin, she felt the exhaustion returning full-strength, and closed her eyes. The Doctor, never one to short her, pressed a gentle kiss to her temple, before squeezing her shoulder and heading back over to their desk. A warm, melting sensation filled her chest, and she smiled.
"Hey, Spaceman?" she mumbled.
"Yes, Donna?"
"Could you keep singing? Please?"
"Course," they murmured, and she thought she could hear a smile in their voice. There was a pause. Then they started up a quiet, melodious tune, humming contentedly as they worked.
It took her a moment to realize that it was a very slow rendition of "Frosty the Snowman". She shook her head.