At least he knew what to expect when he hopped from his bed, his jim-jams tighter in the hips and shorter in the legs, and his hair now long and tumbling down his back. He gave his back a scratch and grumbled about having a large arse.
His fist pounded on the Master's door to his room---that was, of course, his cell on the TARDIS.
"I suppose you think you're terribly funny, don't you?" The Doctor's voice was terribly high-pitched in this form. He couldn't see the other man through the small, barred window on his door.
Laughter from the other side, however, proved that yes, yes, the Master did.
The Doctor rolled his eyes and headed over to Donna's room. He gave a loud knock on the door. No response. Another knock.
"Fine! Gimme a minute! Unlike you Martian lovebirds, some of us humans wanna sleep once in a bloody while!" And cue the tall (taller than the Doctor now, and that was disconcerting) redhead opening her bedroom door.
"Who the hell're you?"
The Doctor sighed. Loudly. "I'm the Doctor."
"No you're not!" Donna's jaw had pretty much hit the floor, "You're a…you're all girlie-girl!"
"I am most certainly not a girlie girl." The Doctor put his hands on his hips. "I am temporarily changed into a woman. By the---"
"Is this one of those regenerational-thingies? Cause I can't handle you bein' a girl, Doctor. S'too damn weird."
The Doctor sighed. "Regenerations are permanent, Donna. This is temporary. Like I just said---"
"Did I miss the big glowy-light-thing? Martha said there were a lot of lights."
"Lights are for regenerations---look, Donna, do you have clothes?"
"Clothes? 'Course I've got clothes, what do you think I'm wearing?" The woman had to have been being purposefully difficult, because there was no way that a human being could be that difficult without trying.
"For me to wear while I'm in this form. Unless you'd rather I try to stuff myself into my current clothes, I'd rather I had something that fits and…" He examined his new breasts through the jim-jam top. "Supports, I think. These don't need too much support, most of the issue appears to be in the hips…"
"Right! Right, I'll get you something, just quit fondling yourself!" Donna scurried back into her room and over to the closet. As the Doctor suspected, she wasn't sleeping, rather she was looking at future posts on tmz.com. Whatever the appeal to Bradgelina was, the Doctor would never know.
"So, how did this happen if it wasn't a regener-whatzit?" Donna asked from within the closet.
"The Master," the Doctor replied, dropping onto Donna's bed.
"Your boyfriend locked up in that cell?"
"He is not my boyfriend, why do you insist on calling him that?"
"You're just always talking about him, it's worse than it was with Rose!" Donna poked her head out of the closet. "And you two have that big mysterious past---it's just achin' for someone to write a lot of---what do they call it?---slash about it."
"I swear, Donna, if you even think about writing anything regarding the Master and I, I will take you to the deepest corner of Volag-Noc and leave you there."
Donna held her hands up in surrender (one of which was holding a pink frilly brassiere). "Fine, fine. How'd he do it, then?"
The Doctor crossed his arms and leaned back. "Tackhrin extract. Used to use it a good bit back at the Academy as a practical joke. Temporarily turns a Gallifreyan into the opposite sex for 72 hours. Little to no side effects, but you absolutely have to let it run its course, there isn't a cure. So, one has to deal with a few days of embarrassment during classes. And clearly the Master has resorted to immature jabs since his failure on the Valiant. Wonder how he made me ingest it. Dinner, probably."
"So, what? You eat a bit of this Tackhrin thing and you turn into the opposite sex for three days?" Donna asked. "How often can you do it? I know I'd pick three specific days a month to."
"No, no, it's more dangerous and painful in a human," the Doctor said, "And you lot get this weird side effect where you have to mate to actually have the cycle run its course. It's pretty strange, but…all species are a little different, I suppose."
"And it's rare, I suppose?"
"Especially since the War. I wonder where the Master found some. Must've been when I let him out on Terahrash."
Donna looked genuinely disappointed, but she emerged with some undergarments, a pink shirt and a pair of jeans. The Doctor examined the jeans.
"No, no. I thought your hips would be the right size, they're a bit too big. I'll have to check in Martha's room, see if she has any."
Donna looked positively furious, and she pointed at her door. "Out!"
The Doctor blinked, "What? What?"
And he hopped from the bed, scooping up the clothes, and was out the door right before she slammed it.
More laughter came from the Master's room.
The Doctor kicked the other man's door. "I hate you." It was weak, and probably not entirely true. Well, in this instant it was fairly true, at least.
The Master's face appeared in the small square window and he grinned so widely the Doctor was quite sure it would split the man's face.
"Yes, Doctor, but don't you look absolutely stunning as you do it? Are those pink knickers? Oh, that'll just fuel my fantasies for months, Doctor. Months!"
Donna's door opened and she stuck her head out and her finger in the direction of the Doctor, "You see! What I’m saying!"
The door closed again with a slam, and the Doctor kicked the Master's door.
"I hate you!" He screeched back in his tiny female voice.
The Master laughed, "It's always the women."
Muse: The Doctor (Ten)
Fandom: Doctor Who
Word Count: 1,022
Special thanks to onlysayinghello for creating Tarkhin in this absolutely amazing prompt.