"Well, that was only... mildly horrible," the Doctor said to himself as the students filed out. Luckily the physics themselves weren't too difficult--basically the subject was the same as back home, just without the added wrinkle of established, accepted magic. But he was never professor material. His desire impart knowledge upon others was sort of at odds with his ability to actually do so with any degree of effectiveness. He could ramble for ages on any given subject, but only a few select ponies could ever parse anything meaningful out of it--gods bless Derpy--and trying to slow himself down for a group of ordinary students? Maddening. Still, it was only an hour. He was able to survive an hour. The students did, too, from the looks of things. "At least that's over and now I can get down to..."
"Hello, Doctor," a sudden voice said from the classroom doorway, causing him to jump. He turned to see that the owner was a young girl who was clad in a leather jacket and who also had distinct, striking red hair with yellow streaks throughout. Something about her was definitely familiar. Very familiar. On the tip of his tongue familiar. She was in the class he just taught, wasn't she? But it was more than that. It was... Wait, did she just call him...?
"'Doctor?'" he said, putting on his most convincing (i.e. not very) act. "Now why would you call me 'Doctor?' 'Professor', maybe. Or even, ugh, 'Mister Turner.' But 'Doctor?' I know I'm brilliant but last I checked I didn't have the credentials."
"Oh, sure you do," the newcomer said with a smirk. "You are still carrying around that psychic paper, aren't you?" The Doctor's jaw dropped so suddenly he was surprised it didn't hit the floor, and the young woman laughed. "You don't recognize me, do you? I suppose that's understandable. New species, new face, and I was barely out of fillyhood the last time you saw me at the castle."
The Doctor's eyes narrowed as he scrutinized the girl in front of him. So she was an Equestrian native, eh? That red hair was really, really familiar, and she said he'd seen her at the castle? That sort of narrowed things down... And suddenly, he realized exactly who he was talking to. "Wait. Wait wait wait wait wait. Wait wait waitity wait wait. You're Sunset Shimmer? My Sunset Shimmer?"
"Well, I'm not your anything, but, yeah, I'm... woah!" Shimmer was suddenly yanked off her feet as the Doctor enveloped her in a grand old hug, the kind that pulled her off her feet and spun her around the room.
"Oh, it's great to see you!" the Doctor managed to say through the laughter.
"I, ah. I can tell," Shimmer said, her eyes spinning. "Could you... could you put me down... down please?"
"Oh, sorry. It's just... one day you were there, the next you weren't. Completely vanished, from the history books and everything, after June third, 988 C.R. And Little Tia wouldn't tell me anything. Not one word about it, even after a decade of pestering. Always changing the subject, always asking me to 'just drop it.' Right. Like that ever works. You know how snoopy I can be, but there wasn't any kind of evidence for me to find, either, and whatever happened was a real doozy. Perhaps even bigger than that--maybe a slobberknocker--because the TARDIS just plain and simply refuses to take me there to see for myself. Lots of very powerful magics released that she doesn't like. So thanks to my two favorite girls, your fate's an even bigger secret than the secret behind her whole 'raising the sun' thing, and you know how much Little Tia enjoys hanging that one over my head. I mean, it's just... completely impossible, even taking magic into consideration. Planets orbit celestial bodies. That's just how it works but..."
Shimmer glanced away briefly, and the Doctor saw the shame in her expression, which caused him to drop the suddenly off-topic rant. "So she never talked about me? Somehow I'm not surprised."
"What...? No. No! You she talks about all the time. Well, not all the time. Sometimes. Mostly on lonely sort of nights where... never you mind. I think that's one of those privacy sort of things I'm still working on recognizing and respecting. The nights aren't so bad anymore, anyway, what with Luna's return and all. What she doesn't speak of is that specific day. The one where you disappeared. It's similar to how she used to shut out her battle with Nightmare Moon, now that I think on it. The difference being I was there for that one. The aftermath at least. The point here, and believe it or not I do have one, is that she loved you. And whatever it was that happened that day you disappeared didn't change that."
The sigh Sunset Shimmer made was surprising to the Doctor. "That... almost makes it worse."
"Oh? Care to share? Or is this one of those privacy things?"
She shrugged with another sigh. "Yes? No? I don't know. It's not a day that I'm proud of. I sorta started a... well it wasn't a coup exactly, at least I didn't think of it that way at the time. Your ship's right though. There was a lot of casting that day. Real dangerous stuff. Looking back on it, it was the childish tantrum of a young foal who just wasn't getting the lessons Princess Celestia was so desperately trying to impart."
"Aha!" the Doctor exclaimed. "I thought it was something like that." At the look on Sunset's face, he backpedaled. "Oh, sorry. It's just... well there's only a few things that make Tia clam up like that and one of them is, er, well... I'll... I'll stop talking, now."
"Please do," Sunset said with a playful smirk that didn't quite hide her pain. "I messed things up. I mean, I really messed things up. But I spent years, literal years, blaming everyone but me for my troubles. And then, when I finally figured out the secret to the portal--that secret being that it only opens at certain times, these times being years apart, mind--and managed to make my way home, what did I find? A purple little pony taking residence up in my room."
"Twilight Sparkle," the Doctor said with a thoughtful, almost blank expression on his face.
"Yeah. Twilight Sparkle. My replacement." Even Sunset seemed surprised at the venom in that last word, but she continued on. "I mean, I was gone--what?--half a decade, and she'd gone and replaced me? It usually takes centuries for her to take up new students. You know that better than anyone. And she replaced me in less than five years." Sunset closed her eyes and released a long breath to calm herself. "Of course, that's not exactly fair, is it? Huh, I didn't even know I was still so bitter about it 'till just now. I guess I've just... avoided thinking of it. Twilight and I're actually really good friends now, y'know."
"Actually I didn't." The Doctor couldn't help but grin a little.
"Right, but at the time, I well, snapped, basically. Started down a very long, very dark path, one that would have led to my own destruction and possibly the destruction of two worlds except..."
The Doctor waited a good four seconds for her to continue before he said, "Except...?"
The smile that was now on Sunset's face was small but surprisingly bright. "Well, except that Princess Twilight came over to this side to stop me. And when she did, I became friends with... heh... the girls." Here she reached into a pocket of her jacket, withdrew her smartphone, and held it up for the Doctor to see the screensaver.
The Doctor's first inclination was to marvel at the technology. Of course, it wasn't as if he hadn't seen similar, but it was certainly far away from what Equestria was capable of at the moment. But he then actually looked at the picture and couldn't help but chuckle. In the middle was Shimmer, a wide but slightly embarrassed smile on her face, and cramming into frame from all sides were five young girls that were unmistakable even as bipeds. There was just no mistaking those pink curls, or that freckled face under the Stetson hat, and there was certainly few who would forget that disheveled head of rainbow locks. The versions of these girls he knew back home were known as the Bearers of the Elements of Harmony. Here, they were apparently just a group of high school girls. And from the sound of things they were still capable of great things.
Sunset chuckled. "Yeah, I thought you'd recognize them. I bet back home the history books have a lot to say about them, don't they?"
The Doctor joined in on the laughter. "Oh, you have no idea. Those five ponies, and Princess Sparkle, are among the most important of ponies in this current age. I daresay they do more for Equestria than Starswirl the Bearded did in his day."
"I'm not surprised," she said with a little nod. "They're certainly something special on this side of the mirror. So. Enough about me. What brings you here?"
"Oh, nothing much. Especially compared to my usual disasters. Just a non-malicious little pest that happens to carry a rather annoying psychic disease."
"Psychic?"
"Makes ponies irrationally angry for about twelve hours. Probably people, too. Sooo... kinda need to find it quickly."
Sunset Shimmer's face paled slightly. "So what are you doing waiting around here then?"
"Well, I can't very well go snooping around without my companion and screwdriver, can I?" the Doctor said as he walked out into the hallway. "I'm betting both'll be back here aaaaany minute now." Sunset was about to ask what he meant by that when the Doctor was very nearly knocked over by a grey blur that launched itself into his chest.
"Hi, Doctor!" the suddenly arrived Derpy said as she gave the Doctor a big hug. "Didja miss me?"
"Always," the Doctor said with a smile. "So what did you find?"
"It's in the pipes. I've tracked it all over the school and it doesn't look like it's coming out any time soon."
"Wait a second," Shimmer said with a smirk. "She's your companion?"
"That's right," the Doctor responded, a jovial smirk of his own on his lips. "Is there something you find funny about that?"
"Well yeah, kinda." Turning to Derpy, Sunset added, "No offense."
"None taken."
Clearing his throat, the Doctor got down to business. "Now then, with the Nnn'Tok in the pipes, things are going to get a little dingy, but nothing I didn't already think of. We just need to AKK!"
This time the Doctor was knocked off his feet, from behind. "Ooops, my bad," a familiar voice said from atop his back.
"Quite all right, Der...py?" the Doctor said as he noticed something. Looking down on him, grins on their faces, were Sunset Shimmer and... Derpy. "Ohohoho no. Don't tell me you...?" He didn't need to finish his question as Shimmer helped up the newcomer off his back, and he saw that it was, indeed another Derpy. "Derpy, I thought we talked about this."
"What," his Derpy said with a shrug. "It was an accident."
"Besides," the other Derpy said in response. "We all know about the pony world anyway. What's the big deal?"
"And that's besides the point," the Doctor grumbled lamely as he got back to his feet.
Sunset grinned as she brought out her phone. "Well, if you're bringing in extra help anyway, I'm bringing in the girls."
The Doctor actually paled a little. "Don't even think about..."
"Too late. Already texted them. C'mon, we'll meet them out at the soccer fields."
"When did I lose control of this operation?" the Doctor groused as he was led away by the aggravating little redhead.
"Losing control implies you had control," Derpy--his Derpy, he was pretty sure--said in a lecturing tone.
"That was a rhetorical question," the Doctor groused.
"Yeah, and?"
As the group disappeared around a corner, another figure suddenly appeared standing in the hall as if he'd always been there. He was tall, dressed in an old brown suit that was nice once, but was now patched with a number of mismatched additions. His hair was long and grey, as was the beard he was stroking. From beneath an equally old and patched fedora hat, a pair of red eyes narrowed as a yellow-toothed smile appeared on the man's face. "Hoooo, ho ho ho ho. This place never fails to entertain."
With that, he snapped his fingers, and in a flash of light, completely disappeared.