Twilight glanced up at the clock on the wall to discover that another half an hour had passed. She groaned and looked back down at the page. It was the first page of the fourth chapter; she had read the other three already over the week. But as she reread the first two paragraphs again, young Twilight nearly pulled out her mane in frustration.

So she closed her copy of Introductions to Alchemy, Volume 1 and set it aside, leaning over the little desk to rub her temples with her hooves.

“I'm smarter than this,” she whimpered, closing her eyes to block out the dull blue light of the magical Illumination Stones hanging from the walls. She really should have asked for a room with a window, but the fact that Celestia had allowed her to rent out a room at the castle at all, albeit a room smaller than a broom closet, was more than Twilight could have asked for already.

Twilight sighed and pushed herself away from the desk. It was an old piece, and, like the bed and dresser they had somehow shoved through the door, was taken out of storage when the room was first set up for her. It was still a little too tall for her without the use of a stool.

Granted, a filly her age wouldn't normally be reading a book on alchemy, but she was only staying there for two weeks as part of her summer vacation and it would be a crime not to take advantage of the castle library while it was at her disposal. Yet her time was half up and she had expected to be at least halfway done with the first volume by now. Princess Celestia would be back from her trip the day after tomorrow, and then she'd see firsthoof how little progress Twilight had made in her studies.

Twilight carefully lowered herself to the floor, searching for a place to land her hoof that wasn't already occupied by books and scrolls. Speaking of which, she hadn't finished learning that scroll-sorting spell she'd wanted to use. Where were her notes on that again? Oh right, up on the desk. She'd get back to that later.

Maybe a walk will help clear my head, she decided, sifting her way through the mess towards the door. As soon as she was out of the room, the Illumination Stones turned off to preserve their magic, and for a while she could close the door on her studies.

An aimless ramble up and down the halls gave Twilight the time she needed to clear her mind. She stuck to the outer corridors of the castle, letting the natural midday sunlight that streamed through the windows illuminate the halls for her. But no sooner were her thoughts off of her book than she remembered her unfinished report on Shearmare's famous “march to the sea”. She still needed to find one more source before it was up to her standard.

Shearmare used a “scorched earth” policy, she thought, having the unicorns under her command literally burn the land in her wake. Who was it that said that? Underwood? No. Groove Moss? Was it Moss? How can I not remember who said that?! It had to be Moss. No. No, it wasn't her. Oh, Celestia is going to kill me.

Twilight stopped and let out a primal howl in frustration. Feeling no better than before, she turned around to return to her room, and realized that this wasn't the hallway she thought she was in. There were no windows along these halls, merely the blue Illumination Stones that ran all day to give the minimal light a pony needed to see. How could she have not noticed the change in lighting? When did she veer off course? Twilight's ears drooped as she realized she truly had no idea where she was.

“Well, that's no problem,” she told herself, feeling optimistic, “I can't have gone far. I'll just retrace my steps. I'll be back before I know it, and then right back to work.” Far from being comforting, the thought alone seemed to drain her of her remaining energy.

Few ponies ever saw this part of the castle; back hallways like this were generally off-limits to guests and used strictly by the staff. Twilight was no stranger to the staff members after her time at the castle, and she knew they'd help her if they ran into her. Not that she needed help – she wasn't lost, just a little turned around.

But the hallway ended in a T, giving her no option but to turn. Twilight looked left, and then right, but as hard as she peered neither hallway suggested they ended by returning to the familiar day-lit corridors. She can't have wandered this far off her path without noticing it, right? Maybe she turned the wrong way? No! She couldn't start doubting herself now. She took the left fork, almost certain that she had taken more right turns coming that way.

Couldn't doubt herself… that seemed to be all Twilight found herself saying nowadays. That and “work harder”, but they weren't mutually exclusive. She knew that she had what it took to earn the Princess' approval, she just wished she didn't have to live with her nose against the grindstone. As she walked, Twilight began grinding her teeth at the thought of what Celestia would say when she got back, at the disappointment in her voice, how the Princess would dismiss her from private tutoring, probably kick her out of school for being a below-average student…

*Creeeeeak*

Twilight jumped and came to a complete standstill. With a twinge of fear already settled in her heart, she slowly turned around to glance down the hallway. All was as before, except that one of the doors was wide open, exposing a gaping black maw of a doorway leading into the unknown.

Her mouth was dry. Twilight began backing away from the door, not sure what she expected. It was just an unlocked door that hadn't been closed all the way. So why was her stomach turning? Her ears twitched, but could detect no sound. Everything was fine. She should turn around and keep walking.

After a moment, Twilight moved one hoof closer to the doorway. And then she took another step. Biting her lip, she cautiously walked up to the hole and peeked inside.

"H-hello?” she whispered. There was no answer. Obviously. It was just an unlocked closet.

Why had she been surprised when nopony answered?

“Is anypony there?” she asked with a bit more confidence, “I don't want to lock the door on you.” Silence remained her only companion. “Okay, if nopony is in there…” Twilight rested her hoof on the door, fully intending to close it and move on.

By now her eyes had adjusted a bit more to the darkness of the room. It looked nearly empty save for a few boxes lining the walls; for a disused closet it was nearly double the size of her room. There was some tall object taking up space in the center that seemed… off, like it didn't belong there. It had to be at least as tall as Celestia herself. She couldn't tell what it was, but near the bottom corner it seemed to give off a soft glint.

“This is your last chance to get out,” she said. But now that she was positive there was nopony else there, she couldn't imagine who she was addressing. She'd have felt positively silly if she wasn't so sure she was being watched.

Slowly, she lowered her hoof from the doorframe and turned around. Levitating down the Illumination Stone from the wall was easy, though it was pretty warm from the magic it radiated. She entered the room boldly, letting the stone cast its dull blue glow around as far around her as possible. She walked forward until the object itself entered her little circle of light.

Whatever it was, somepony had draped a large tarp over it. Setting the stone down beside her, Twilight grabbed the fabric in her magic, giving only a moment's hesitation before tugging at it with all her might. It slid down easily, landing at her hooves with a plop.

The frame of the mirror was made of some kind of dark wood. It must have been several hundred years old, because the color was faded and it was beginning to splinter. A small spire stuck out from the top on either side of the triangle that marked the top of the frame. The glass ended in a pointed arc, scrawled above the edge of which was writing in a language Twilight couldn't recognize.

All this would have been interesting had Twilight been able to ignore what was right in front of her. Despite the age of the frame, the glass itself was clear and unmarked, like a freshly cleaned window. Her reflection stood there, wearing the same confused expression she was, as well as a small silver brace around her neck and a set of silver boots. If she had a crown, she'd be wearing an official princess outfit.

“What is all this?” her reflection asked.

Twilight jumped, and the filly in the mirror reeled back in shock. Both had eyes the size of saucers.

“Y-you can talk?” Twilight asked, pointing a hoof at the mirror. Instead of mimicking her like a good reflection should, the Twilight on the other side of the mirror covered her mouth with her own hoof, as though to assure herself she had not asked the question out loud without realizing it.

Twilight ventured further. “What are you?” she asked, taking a step closer to the mirror. It really did look like a window pane, she realized. A very thin one. And when her double had spoken, it was as if they were in the same room; her voice was not dimmed in the slightest by the glass separating them.

The other filly lowered her hoof and leaned in a bit, now just as curious. For a moment neither of them said anything, and Twilight allowed herself to examine the world through the mirror a bit. It seemed just as dark on that side, and the other her even had the same Illumination Stone that Twilight took off the wall. A small tarp was huddled at the double's hooves as well.

She went on to examine the ornaments her duplicate wore. The brace was silver, with a small amethyst like Princess Celestia had in hers. The boots looked pegasus-made, like Celestia's own. Twilight remembered being younger and, one time when Celestia took her boots off to go barehoof, trying those boots on. Aside from just being too big for her, they were very slippery, since they weren't designed to be worn on the ground. She had always wondered how Celestia learned to walk in them.

She had almost forgotten about the question she asked when her doppelganger answered.

“I'm Princess Twilight Sparkle.”

“Princess Twilight?” Twilight repeated in disbelief. She laughed, “That's ridiculous. I'm not a princess!”

“That's because you're my reflection,” Princess Twilight told her.

Twilight snorted. “You're the one in the mirror,” she pointed out.

The princess gave her a quizzical look. She looked around the frame of the mirror, then ducked out of sight as she checked behind it. Returning into view a moment later, she said triumphantly, “You're the one in the mirror. This is the real world, where I am.”

“That's ridiculous,” Twilight repeated, “We can't both be not real! I mean… we can't both be real, can we?”

“As far as I'm concerned, you aren't.” But then the princess frowned. “So what are you then?”

Twilight rolled her eyes and suggested, “Maybe you're me if I was a princess.”

“And that would make you the me that was never made a princess,” Princess Twilight finished, “That's weird. Why are you at the castle then? You are at the castle, right?”

“Yes,” Twilight said, “I'm Celestia's personal student. She's my teacher in magical studies. I'm here to-”

“Magical studies?” Princess Twilight interrupted, her eyes sparkling with wonder, “As in: studying magic?” Twilight nodded. “That's wonderful! I wish I had the time for that!”

“Well, what do you normally do?” Twilight asked.

“Oh, the usual: dinner parties, beauty treatments, parades, stuff like that. What are you working on now?”

Twilight was taken aback by the question. “I'm trying to learn the basics of alchemy,” she said. She refrained from adding that she wasn't doing all that well.

“Alchemy? Isn't that hard? Ooh! I'd give anything to learn that. Father Watcher doesn't like me focusing on things that are outside my schedule.”

“…father who?”

The princess seemed to mirror her own confusion, and asked, “Isn't there a Father Watcher on your side? He's the one in charge of my care and scheduling.”

“No,” Twilight said, “Nopony like that. Why do you call him 'father'?”

“He insists. He's some kind of church official, but Celestia allowed him to take control of my princess training. He's a really good magic user; I always wanted to be as good as him, but he keeps telling me it's not right for a young princess to worry about casting magic. I don't like going against him. He's… there's something off about him. I mean, he's really nice, one of the nicest ponies I know, and I know he only wants the best for me, but still…”

Twilight stored this away and said absentmindedly, “I wonder what else is different about our worlds?”

After a pause, her double said, “Let's find out.” Taking advantage of Twilight's stunned silence, she began reexamining the mirror and continued talking, “If this thing lets us see and hear each other, surely we can use it to get through, too!”

Twilight opened her mouth to protest, then stopped. She thought about it for a moment. Parties, parades, spa treatments? No hard studying? It was just the break she needed.

Idiot! Twilight scolded herself, You can't go shirking your duties. You still have to prove to Celestia that you deserve to be her student!

On the other hoof, this was a completely unknown situation. Would it really be “shirking her duties” if she stepped through to examine it a bit closer? She could stay for a few days and come home with a full report for Princess Celestia about what she had learned. And if she so happened to enjoy some of the luxuries, who could blame her? A good experiment was well-rounded, after all. The more she thought about it, the better an idea it seemed.

“I think I have an idea,” Princess Twilight said, “There are some words on the top of the frame over here.”

“Yes, there's some over here too,” Twilight told her, “It looks like it's written in Equestrian, but I don't recognize any of the words.”

Princess Twilight tried to sound out the message from her side, “Erised stra… ehru oyt ube… cafru oyt on… wohsi.”

When she finished it appeared that nothing had happened. But there was an unmistakable feeling of discomfort in the air that had been absent before. Twilight felt it, and judging from how the princess herself was fidgeting, she did on her side too.

A soft draft blew against Twilight's face, sending a shiver down her spine. It took her a moment to realize it came from through the mirror. Stepping forward, she slowly reached out towards the surface of the glass with her hoof. Instead of touching a solid surface, her hoof passed through the open portal. The pane of glass was gone.

Twilight stepped back in awe, allowing an equally surprised Princess Twilight to come forward and try herself. She put one hoof through the door and took her first step into Twilight's world. She let out a gasp at the reality of it and leapt through without a second's hesitation. The twins were now standing face to face on the same ground.

“You are real,” Princess Twilight breathed. Then all at once she exclaimed with glee, “I can't believe that really worked!”

“Me neither,” Twilight murmured, sitting down. She needed a moment to take it in.

“So?”

Twilight was taken aback at the question. “So what?” she asked.

“How are we going to do this? Won't it be odd if there are two Twilights wandering around?” As Princess Twilight struggled to find the best way to go about learning of this new world she was in, Twilight herself looked passed her and into the open portal. Beyond that was a world free from cares or worries, where she was treated like a princess.

“Give me your outfit,” she said.

“Excuse me?”

“Give me your outfit. We're going to switch places. At least… for a while. Nopony would know, and Princess Celestia isn't going to return until noon the day after tomorrow. That gives us all of today and tomorrow to explore each other's world.”

Princess Twilight seemed hesitant. But then she nodded and reached up to take off her brace. “That's a wonderful idea,” she insisted, “I had to think about it, but if I can't trust myself then who can I trust?”

In another moment Twilight was wearing the heavy silver brace and the matching shoes. The brace was heavier than she thought it would be, and that made it all the harder to walk in the slippery boots.

“They're pegasus-made,” Princess Twilight explained, “So they aren't very good at… what's the word?”

“They don't have any traction,” Twilight finished for her, “I know how the Princess' shoes are made, thank you.”

She looked the young princess over. Without the boots and the modified yoke around her neck, she really did look a lot like Twilight. Her hair was done up a bit differently, but Twilight didn't think anypony would notice. In an odd moment of surreal clarity, she realized that the princess herself was probably experiencing the same sensation of looking at herself from an outside perspective. It was the first and only time they'd ever get to really look at themselves without the use of a mirror.

A regular mirror, that is.

“Are you ready?” Princess Twilight asked, and Twilight nodded despite the butterflies in her stomach.

It took all of her courage to step through the mirror. Mind you, she wasn't about to let herself look weak in front of the princess… even if it was only herself. She put one hoof through, and in leaning her weight on it almost slipped when it threatened to start sliding. Curse these boots! Not wasting another moment, she pushed herself forward through the frame and into the foreign world.

Turning around gave her the image of herself in a mirror. If she didn't feel the silver pulling against her neck, she would have sworn there was nothing out of the ordinary about this mirror at all.

“We meet here this same time the day after tomorrow,” she reminded her replacement. Princess Twilight agreed, and they bade each other goodbye. The princess was the first to go, picking up the Illumination Stone in her mouth as she exited and gracefully nudging the door close.

A pit formed in Twilight's stomach as she realized she had no clear idea of how a princess was supposed to behave. But she pushed that aside. Being courteous should be enough to keep from drawing too much suspicion. She was here to enjoy her time – or rather she was there to discretely collect information about another world, she reminded herself – and worrying about the small things was not going to help.

Resolving to let events play out naturally, Twilight turned and slid out of the room, grabbing the glowing stone with her magic as she did so.

Author's Note

Questions, criticism, suggestions? Leave them below! All comments welcome.

This was an idea I had even before Equestria Girls and its own magic mirror were announced, but I never actually acted on it. Of course I'm far from the first person to use mirrors as a portal, but I'd like to think this was a rather unique spin on it. Let me know if anyone else has done something similar.