There's a WHATquestria?!

"Your Graces," the royal guard bowed. "We now present Her Mercy, Twilight Sparkle, Princess of the Magic of Friendship, She Who Has Returned the Moon, Tamer of Chaos --"

"Yes yes, let her in!" Celestia called impatiently, waving a hoof.

Luna gave her a worried look as the heavy doors opened. "Dear sister, I am telling you, something was wrong with her letter..." she whispered.

"I've had a horrible day, Luna, let me enjoy the bliss of Twilight's company, please," Celestia sighed in irritation, then softened her voice and nuzzled her sister. "But I am grateful to have you at my side to ease it."

"I shall be ever faithful," Luna promised, nuzzling back, and the two of them turned their heads forward to watch Twilight approach. Luna slightly furrowed her brows; the bookworm's gait was a bit different than usual, more careful, with heavier steps.

"Twilight," Celestia greeted warmly, though her exhaustion was clear in her voice, "To what do we owe the pleasure of your visit?"

They were surprised to see Twilight bow deeply to them - something she should no longer have felt obliged to do, as a fellow Princess. Stranger still, she bowed with her butt high up in the air, arcing her back like a cat.

"Your Milky Majesty," Twilight began, immediately causing both of them to freeze right down to their tails, "As always, I am honored and humbled to stand before you and your superior undercarriage, as encompassing and warm as the sun above. Your most glorious of mammaries have shone radiant and full for a thousand ages, and may they remain plump and bounteous for a thousand more, a soft haven for --"

Thunk.

"-- all... creatures of light... C-Celestia?" Twilight blinked, looking up to see her collapsed in Luna's forehooves, her tiara having fallen to the side of their thrones.

Luna stared at her in shock for several seconds before blinking and becoming very angry, though she kept her voice low. "Twilight Sparkle," the moon princess blushed with furious eyes, gently stroking her sister's ear, "What is the meaning of thy vulgar address."

"O-oh dear," Twilight meeped. Sensing the incredible danger she was now in, she cowered with her ears low, wisely keeping her volume down also. "I am so sorry..! Th-that's how I greet my Celestia..!"

"'Your Celestia'..?!" Luna repeated, confused and outraged. "My sister?"

"I... I didn't feel this was appropriate to discuss in writing and should have opened with this at once, I'm so sorry. I'm a Twilight of another world who has switched with yours..."

"Oh really." Luna huffed. "And my precious sister, who does no wrong to anyone, who has been the faithful servant of Equestria for, as you say, a thousand ages, who is exhausted after a tiring day faithfully seeing to many unthankful, contemptuous wretches, who is deathly self-conscious of her stature and her form as you know good and hell well -- in what world would the mare she holds in the highest of regards speak of her so boldly?" she gently hissed.

Perhaps only mine, Twilight thought with dread.

Had Luna been alone, she would be yelling at full volume at her for sure. Twilight knew the only thing keeping her quiet was her concern for further disturbing Celestia. That is, Twilight knew that, just as in her own world, this Luna was ready to whoop Twilight's ass for her sister's honor.

Twilight fully understood and accepted her doom in one regard: as she could not wield the magic of this Twilight, she could not hope to match even Rarity or Trixie, let alone Luna. But, naive to magic beyond titty, she failed to understand her doom in the second: that no magic available to a Twilight of any world could hope to overcome one sister's defense of or vengeance for the other, any more than she could defeat the love between Shining and Cadance (the flat one.) She had trespassed on sacred ground, and though she did not comprehend it in detail, her soul told her what she needed to know: she must now appeal to the gods not to strike her down at once.

But she did not know how; all she could do was tremble in terror, swallow fearfully, and offer the truth.

"I-in my world, Your Grace, you are, you are also admired as such," Twilight bowed deeply, which unfortunately prevented her from watching Luna's eyes widen dangerously. "I-in my world, as hers are as big as the sun, yours are as big as the moon..."

"Do you dare now, after what has been said and done, to mock me, Twilight Sparkle..?" Luna's voice was now a deathly whisper, her eyes as cold as a lonely winter night.

"N-no, Your Grace," Twilight stammered. "I would never..! I, I understand we are a world apart - literally - but I have nothing but respect for the two of you, this... this is the way things are in Tittyquestria!"

Twilight sighed, looking around at her cell. Well, that was a bust, she complained to herself, then paused and frowned. Pun unintended. She winced and rubbed at her chest, coughing.

She considered all of the firsts that she'd just accomplished within a few minutes of time. The first time she'd ever caused a Princess to faint. The first time she'd ever been tackled by a Princess who actually wanted to hurt her (or at least she got that distinct impression.) The first time she'd ever been in prison. The first time an active Princess had ever been in prison other than when a usurper had taken control of said prison. Well, a Tittyquestrian one, anyway. She had no clue about this Equestria.

If she was in here long enough, and the other Twilight found her way back, the list of firsts would go into some pretty insane territory. But she'd already explained everything to this Twilight's friends, and they'd surely hear what happened and come help her out. Right?

Her ears pricked up at the door opening and three sets of heavy hoofsteps entering. Two stopped, and one came closer. Her heart sank as she recognized the sound of Celestia's steps. She blushed and looked away. How could I have been so stupid as to presume this Celestia was as comfortable with her assets as my Celestia, she scolded herself, fidgeting with her hooves. She must be devastated! I'm lucky she didn't send me to boob kindergarten.

She squeezed her eyes shut, angry with herself. You dumb idiot, get with the program, there's no such thing as boob kindergarten --

"Twilight," came Celestia's voice, low and trembling. The pain in Celestia's voice made Twilight's wings shrink.

"U-um," she had to let her know she was listening, but she couldn't bring herself to look at her or try to say anything coherent.

"I don't know what in the world has gotten into you," Celestia continued. It would be scolding if her voice wasn't so... heartbroken. "But I am absolutely... shocked... you would say such repugnant and ridiculous things to me, especially within the court. I'm sorry that Luna attacked you.... and I believe she has judged you harshly for my sake... but I should have listened to her concerns over the content of your letter, and... I'm honestly considering letting you stay here for a... for a while, to... to think about what you've done."

"P-Princess..."

"Can you explain yourself?" Celestia asked unsteadily. Even in her state, she was willing to give her a chance.

"I'm telling you the truth," Twilight swore, still not looking at her. "I'm sorry I misspoke and hurt you..." She sniffled, her eyes wet. "And I understand if you don't believe me. But I'll stay right here until you do."

"That is ridiculous. This is completely unlike you a-and entirely beneath you," Celestia choked. Twilight's heart shattered. It didn't matter if it was "her" Celestia or not. There was nothing a Celestia could say to hurt a Twilight more while less meaning to.

"C-C-Celestia --" She finally turned to look at her.

"No," Celestia cut her off more firmly - she wasn't looking at her, either, and her face was wet with tears. "Not, not another word. I will make sure you're cared for well. I will not let anyone neglect or further abuse you on my behalf. But... we... we will speak again later." She turned and began to walk away, slowly, sadly, struggling to hold in her sobs.

Great, Twilight rued, her tears flowing freely as the door opened and closed again. Let's add the first time I've made Celestia cry to the list. Even though they would eventually realize she was telling the truth, she couldn't blame them for how ridiculous it must have sounded, and she doubted that confirming her story would make them feel any better. It wouldn't help her feel any better, that's for sure. What will their Twilight think when she finds out what happened? Her chest tightened. Will my Celestia be disappointed in me too? She broke and started choking on her sobs, burying her face in her wings.

Ever since she'd come here, she'd done nothing but upset everyone. Even the girls who still had big boobs weren't so comfortable talking about it as in her world. But Twilight Sparkle was the dumbest smarty who'd ever lived, her brain was fast but her mouth was faster, and she was a bit of a dunce even in her own world. In this one? She was a fiend. A monster. Was their Twilight as foolish as her? Was any?

She glanced up at her horn. Ha, she smiled sadly. They forgot to block my magic. Either that or they consciously didn't bother. Either scenario would only happen because they trusted Twilight - their Twilight - and so the thought that she might be dangerous never crossed their minds.

Could she teleport out of prison? The local Twilight could, she was sure of it, and she'd be able to do it with her proper body. But even with Starlight's and Rarity's help, she couldn't rely on more than basic levitation to work. Would she teleport out of prison? No way. She never meant to disrespect the Princesses, and she wasn't about to start. She promised Celestia that she would stay here - and so she would. She sighed and resolved to serve her penance.

I hope it doesn't take too long to clear this up. She blinked through her tears. I don't want to hurt anyone for one second longer. Especially not Celestia.