Twilight Sparkle and the Planet of the Pantsless Pony People.

Twilight wondered why her counterpart had a dungeon.

Admittedly, it was a nice dungeon, as those sort of things went. It was surprisingly warm and dry, with silken curtains bunched against the stone walls as decoration. The underground chamber was devoid of any prods, pokers, gibbets, garrets, burning coals or iron maidens. Instead, the few furnishings seemed to have a … different purpose.

By the time she woke up, Twilight had been stripped of the few decorations Rarity had given her, and strapped to a padded, X-shaped table, staring up at the ceiling. Leather cuffs at her wrists and ankles held Twilight in place, and a few more experimental wiggles revealed further bindings along her wings, and even one at the base of her tail. Completing the binding was a rubbery ball-gag strapped around her muzzle. Twilight found it surprisingly comfortable, once she got past the initial shock of 'holy crap I am tied to a table without any pants on.'

Twilight tugged at her bonds, but to no avail. It was one thing to roam around nude in a society where it apparently was the norm, but something else entirely to be strapped to a table like something to be … examined. And with her limbs splayed out to the corners of the table, there was absolutely nothing Twilight could do to cover herself. That thought alone was enough to make Twilight's heart beat faster. She shivered, despite the rising warmth she felt in her ears. And … elsewhere.

“What are you?” The other Twilight leaned over the table, brow furrowed. A serpentine green and purple lizard coiled over her shoulders, peering down at Twilight with a quizzical, almost feline air. “You're obviously not a changeling, I know that much. I've already hit you with enough dispellation cantrips to cut through even one of Chrysalis' illusions. You're not single minded enough for to be a sentient reflection … not that I've looked into any Mirror Pools lately, but still. And you can't be a time traveler, because if you're me from the past, I'd remember this. And if you're me from the future, you'd know better than to dump a princess' worth of magic into the Maypole festival. It'll take them days to cut down all that brush.”

“Hrry?” Twilight tried an apology through her gag.

The other Twilight rubbed at her chin, thoughtful. “Okay, here's what we're going to do. I'm going to remove that gag so we can have a rational conversation. But if you so much as mutter a prestidigitation charm, I will zap you with an etheric overload and then put you back into stasis until I can figure out what to do with you. Do you understand?”

Twilight nodded, getting the gist of it.

At that, the other Twilight reached up with surprisingly gentle hands, and undid the buckles to the gag. She snapped her fingers, and the small green dragon slithered down the length of her arm to hop down to the floor.

“Spike, I'm going to have a conversation with … uh, me. Go upstairs and lock the door-- do not open unless I give the signal we talked about. Understand?”

The little dragon nodded, and scurried up the stairs. A few moments later, a door swung shut with a heavy thud, followed closely by the clank of a deadbolt sliding into place.

Twilight worked her jaw a few times, and ran her tongue experimentally over her (sorta flat, sorta equine) teeth to make sure everything was in order. She looked up at her doppelganger's expectant eyes (and decidedly not at her bare, generously-proportioned chest), and finally found the courage to talk.

“I'm … you. I'm Twilight Sparkle. Just from another dimension.”

The other Twilight frowned. “That's impossible. There's no such thing as other dimensions.”

“And I thought there was no such thing as a dimension full of naked pony people, but that shows what I know.”

“Wait.” The other Twilight frowned. “What do you mean 'pony people?' We're just … people.”

“Well, where I come from, things are a lot different. In my dimension, the dominant species evolved from primates, instead of equines.”

“Primates? That is different.” The unbound purple pony person started pacing back and forth, while her bound counterpart pretended not to look at her bum. “But … that theoretically makes things more plausible. It's such an outlier on the scale of possibility you wouldn't have to worry about things like temporal entanglement. But!” She turned on a hoof, and held up a single, elegant finger. “Your theory has a flaw.”

“Oh?”

“Let's say I admit that an alternate, primate-dominant reality is possible. But do you expect me to believe that there's another pony-- er, another person named Twilight Sparkle?”

“Yes?” Twilight craned her neck up a bit, looking down the length of her own body. “I haven't figured it out entirely myself. My going theory is that you can apply principles like The Conversation of Energy to magic-- a Conversation of Destiny, if you will. Or maybe a Conversion of … Souls, even? You're not the first alternate me I've met, to be honest.”

“There's a third dimension?”

Twilight nodded. “Like this one, only everyone-- everypony? –is quadruped. And, uh, less … randy. Theoretically. I haven't really had the chance to ask much. Still, I wouldn't be surprised if there were more dimensions beyond that.”

“That's even crazier than the primate dimension!”

“I know, right? But that dimension's Twilight crossed over to my dimension-- only when she got there, she wasn't a pony, she was a person- er, sorry, human. Primate. Whatever. She looked just like me.”

“And now you look just like me.”

“Exactly. Which means--”

“We share the same dimensional imaging pattern!” The other Twilight rushed to one side of her dungeon and pulled a chalkboard out from behind what appeared to be an iron maiden (sans-spikes). Twilight nodded approvingly, somehow unsurprised her counterpart had such a thing in such a place. It's what she would do, after all. You never knew when inspiration might strike. Twilight tried to keep track as the other version of herself hashed out rough equations on the chalkboard. She put her whole body into the writing, which caused her purple tail to swish from side to side, revealing the soft, curved globes of her posterior.

She has a nice butt, Twilight mused.

Which means I have a nice butt, Twilight added on.

Well then.

“Uh.” Twilight forced herself to look at the equations, which were almost as interesting and enticing as her counterpart's rump. “You're on the right track, but don't forget to factor in paradox resonance.”

“Of course! How could I forget?”

“Don't be so hard on yourself. It took me a week before I realized I had missed it in my initial calculations.” Twilight said.

“We really do think alike, don't we?” The other Twilight set the chalk down, and turned back towards the bondage table. Which, in turn, gave Twilight the oddly sexy vision of … herself. It reminded her of looking at her nude reflection when she'd first arrived in the pony-person dimension. Only … more natural. More confident.

“Well, yes. It's not that we're the same person, it's that we're the different instances of the same person, I guess?” Twilight offered.

“Which is why we look the same.” The other Twilight said with a familiar lightbulb-over-the-head sense of epiphany.

“And why we have the same inquisitive, intellectual personality.”

“And the same kinks!” The other Twilight said.

“What.” Twilight said, suddenly remembering that she was still naked, still strapped to a table, and still dealing with her equally unclothed (but decidedly less bound) dimensional counterpart.

“The same kinks! I mean, I like a little bit of … restraint from time to time, so it stands to reason that you would … right?”

“What.”

“Okay, so, uh. Confession time, I guess.” The other Twilight rubbed at the back of her neck, and her cheeks tinted in a blush. “Ponyville doesn't have a jail, so I figured I'd just use what I had in the castle already.” Gentle fingertips grazed the inside of Twilight's wrist, leaving faintly tingly sensations in their wake. “After a long day of Princess stuff, it can be really nice to just hand the reins over to someone else. Literally. Mmmh.” The other Twilight shivered slightly, and her purple wings flared out a little.

“I, uh, don't have sex with my friends.” Twilight Sparkle murmured.

Her counterpart blinked, puzzled. “Then who do you have sex with?”

“I … don't?”

“Oh.” Other Twilight said. “I guess our dimensions really are different.”

“You're telling me.” Twilight stared up at the ceiling. “Everything here is just so … uninhibited. No clothes. No shame. No boundaries. It's a heck of a culture shock, really.”

“I bet.” The other Twilight bit at her lower lip, eyes gleaming with concern. “You didn't do anything you didn't want to, did you?”

“No!” Twilight blurted. “It's okay! I'm okay! I mean, for a bunch of sex-crazed nudists, you guys are actually pretty nice. Even if Rarity gets a little handsy sometimes.”

“Yeah. She does.” The other Twilight said with a smile and a pleasant sigh. “I mean! Uh. She means well? Especially if she thought you were me. But I can see how that would be a little awkward--” Her face fell as a realization struck her. “But not as awkward as trying to have an actual conversation while tied to a table. Let me get you out of that.”

“Um.” Twilight's heart beat faster. “You don't have to do that.”

“No, no, I do. I really should have let you go once I figured out you weren't a threat.”

“Maybe we should do an experiment, first?”

“Experiment?” The other Twilight's fingers stopped short of the wrist cuff.

“You … you mentioned enjoying this kind of thing, right? Well, uh. Maybe … we could figure out just how … similar we are.” Twilight squirmed as a fluttery sensation rippled through her belly-- and lower. “It … it makes sense, because if there's anyone here I can trust--”

“It's yourself.” The other Twilight said, slowly. Her eyes traveled down the length of Twilight's bared and bound body, taking in every detail with a more-than-scientific interest. She placed her hand on Twilight's stomach and gently ran it upwards, between her breasts, ruffling the short purple hair of her coat. That hand finally came up to cup Twilight's muzzle, tilting it upwards slightly. “That … is a very tempting idea. Are you sure you want to do this?”

“Absolutely.” Twilight's mouth went dry, but she managed an eager, blushing grin despite that.

“Okay.” Twilight's counterpart leaned in for a brief, honestly awkward little kiss, lips barely brushing. “But if I start doing anything you don't want to do, or anything that hurts, just say the word 'coconut' and I'll stop. Okay?”

“Okay.” Twilight licked her lips. “I … I've just never done anything like this before. With anybody. So, uh, I apologize in advance?”

“You don't have anything to apologize for.” The other Twilight bent down over the table for a brief, gentle kiss. One or the other moaned-- Twilight couldn't quite tell so swept up she was in the moment.

Twilight's counterpart worked her way downwards with fleeting, teasing kisses. She took one of Twilight's stiff purple nipples into her mouth, and Twilight let out a surprised, delighted squeal. “Oh!”

The other Twilight giggled, and pulled away from Twilight's breast. “Mmm. Relax. We haven't even gotten started yet.” She ran her hands greedily over Twilight's exposed form, somehow seeking out all the right places. Those hands traced lower, circling briefly around Twilight's belly button before reaching the apex of her legs.

“What have we here?” Gentle fingers ran over Twilight's pussy, parting the soft, moist lips. “You're enjoying yourself, aren't you?”

“I … I … yes.” Twilight blushed, heart beating faster as she felt a fire kindle between her thighs. She tugged at her bonds again, even though she knew they wouldn't give way. The steadfast restraints just added to the experience, reminders that she was completely, absolutely at someone else's mercy.

Her own mercy, technically.

Twilight squeezed her eyes shut, all the better to savor the sensation of those fingers. Fingers that knew just where to go. “There's so much I could do to you.” The other Twilight cooed, dragging her fingertips up and down the length of Twilight's sex. “I've got toys, I've got candles, I've got rope … but I think that's gonna be a little too much for a first time.”

“Uh.” Twilight looked down the length of her body. “I'm … open minded?”

“I bet you are.” The other Twilight winked, and then pushed a finger in.

Twilight breathed harder, faster, as she felt the digit curl within her, stroking her slick inner walls. She writhed in her bonds, and pushed her hips needily upwards.

“But for now, this'll do. I know what I like … so this should make this pretty easy, hm?” Twilight's doppelganger cooed, even as she sank down to a kneel between forcibly spread legs.

Twilight blinked, and looked down the length of her purple body. “What're you-- oh!” She trailed off into a loud, wanton moan at the unfamiliar (yet entirely too enjoyable) sensation of lips dragging across her … well, other lips. Where Twilight's pussy had been wet already, the filling feeling of a dextrous, skilled tongue pushing between her lips was enough to get her practically dripping. Each pass of the tongue, each push of the finger only got Twilight's heart beating faster and faster, to the point where she could do little but stare up at the dungeon's ceiling in shocked, open-mouthed ecstasy. Twilight fumbled words on her lips, but they only came out as pained, needy moans, echoing off the stone walls. Yet even without coherent speech, the other Twilight knew exactly what to do, where to go, how to best overwhelm Twilight with pleasure.

By the time the wet, light tip of a tongue flicked over Twilight's swollen clit, she was already past the point of no return. She came. Hard. Repeatedly. She bucked against her bonds, shivering with each delightful, terrible pulse of pleasure radiating out from between her legs.

After a few agonizingly transcendent moments, Twilight collapsed back onto the rack, occasionally shivering as aftershocks of pleasure traveled through her body. Some time later, she didn't know how long, she opened her eyes to see … herself bending over her, smiling.

“Are you okay?” The other Twilight said.

Twilight nodded. “M-more than okay, really. It's just that was. Um. Intense. Wow.”

“I kind of cheated.” The other Twilight's hand returned to Twilight's breast, squeezing playfully, affectionately. “I mean, I just thought of what I would've liked, and … well. It worked, apparently.”

“I'd say so. Wow.” Twilight stared up at the ceiling again, and panted. “But … we're not spoiling ourselves, are we? I mean, if we're that, uh … similar, and so we know each other that well … how could anyone else measure up?”

“Don't worry.” The other Twilight pinched one of Twilight's still-sensitive nipples, drawing a giggly squeal from the bound girl. “If you ask nicely, I'm sure my friends will be happy to prove you wrong. You should see what Fluttershy likes to do with her wings.”

Despite (or perhaps because of) the situation, Twilight felt a blush rise to her cheeks. “Oh my.”

“But that can wait 'til later.” The other Twilight smiled, and leaned down to kiss Twilight gently on the nose, locking her eyes on hers. “For now, I just have one question.”

“What's that?”

“Wanna switch?”