The workroom was filled with thrumming, near-blinding energy. At the heart of the blazing aura, a pony hovered in midair... a pony encircled by kaleidoscopic whirling rings of magic potential, in every color and form that magic could take... a spectrum of power, of sheer potential...

A pony whose eyes were glowing beacons of radiance, whose forehooves lifted triumphantly...

... a pony wearing a magician's pointy hat, and a star-spangled cape. The cape wafted majestically on an invisible breeze...

"BEHOLD!" the blue-coated, white-maned pony shouted, her voice booming and echoing. "At long last! The Grrreat and Powerful Trrrixie possesses awesome power! Absolute power! Power undreamt of by pony kind!"

Her blazing eyes swept downward. She stabbed a forehoof at the lavender pony standing before her, gazing up in wide-eyed astonishment.

"Are you listening to me, Twilight Sparkle? 'Cause Trixie can make you listen! Trixie can do whatever she likes! From this moment forth, there is no will in the universe save Trixie's! Undoubtedly, Trixie is the greatest equine who has ever lived!"

She flung her hooves wide. All about her, a shower of glittering fireworks exploded.

"Trixie..." Twilight called up, in a worried voice.

Trixie gazed down upon her.

And then put her forehooves on her hips.

"Oh c'mon, Twilight," she said conversationally. "You've gotten so serious lately. That was worth a laugh at least." Raising a hoof, she adopted her stage persona once more. "Nay! It deserves far greater! Showponyship such as this deserves a standing ovation! Encore performances! A guaranteed tour!" She pointed at Twilight again. "Do not arouse the wrath of the Great and Powerful Trixie! Be thankful Trixie grants you audience today... instead of three weeks from now!"

She lofted both hooves dramatically. "The Great Trixie has... spoken!"

On the other side of the practice room, Tempest Shadow stood with her head lowered, her broken horn blazing. With Starlight Glimmer casting a conic shield on her horn to keep her magic stable, Tempest had been sending a beam of raw power into the hoof-carved focus gem fastened to the collar of Trixie's robe.

And Tempest had finally had enough.

With a toss of her head, she abruptly cut the power. The blazing energies surrounding Trixie whirled away into nothingness. Caught by surprise, the blue pony shrieked, struggled in midair, and then plummeted towards the stone floor, forehooves covering her eyes...

And was snagged by Starlight's cyan magic, inches from pancaking on the slate tiles.

Tempest smirked.

"Pay no attention to that pony behind the curtain..."

Trixie stood up, ostentatiously dusting herself off, then lofted her snout in disdain. "Hmff!" Then she nodded gratefully to Starlight. "Thanks, bestie! At least somepony here appreciates my efforts as guinea pig for this project."

"Not a problem, Trixie," Starlight Glimmer said. "Just try not to get too carried away, huh?" Starlight looked over at Twilight, who was busy copying figures from several measuring instruments onto a clipboard. "What do you think, Twilight? We're able to keep the effect stable, with only a single gem this time, and pretty much indefinitely. That's progress... isn't it?"

"Maybe..." The lavender pony shook her head, looking over her figures. "But progress toward what? It feels like we're just refining things, without really understanding what we've discovered."

"Ahem! What Trixie discovered!"

"By lucky accident..." Tempest muttered sourly.

"Sorry, Trixie." Twilight nodded. "But even with all the work we've put into this, I'm still not certain it amounts to much..."

"It certainly looked impressive to me," said a regal voice from the doorway. The ponies all turned, and found the tall, brilliantly-white Sun Princess striding into the room. Her prismatic mane swirled about her; her golden armored shoes clicked lightly on the slate floor.

"Princess Celestia!" Twilight anxiously riffled through pages on her clipboard. "I'm so sorry! Are we late for our presentation? I wanted us to do one more run-through... I might have lost track of time..."

The Princess put up a forehoof, stemming the flood of apologies.

"I was free a little sooner than expected, so I came by early. And you needn't worry about perfection, Twilight. I can see the four of you, working together, have already made remarkable progress."

Twilight sighed.

"I suppose, but... it's still not clear what we can do with this effect. I mean, yeah, we can split magic into its constituent spectra. We can put that into a storage loop, charge it all the way up into a huge potential force. But then how do we refocus and control it? How do we direct it to perform actual spells? How do we make it useful?"

"That's tomorrow..." Celestia gently chided her. "Remember to be proud of what you've accomplished today."

"You're right." Twilight nodded. "It's just... I feel like there's something missing here, something important. And..." She winced uncomfortably. "I'm also wondering if we're the right ponies for this. I mean, we are only students..."

"You are more than merely students, Twilight," Celestia said, "as I've told the head of the Research Magic division -- on more than one occasion! I like to think of you as our Advanced Projects group. You're exploring aspects of magic that ponies are only beginning to understand. And doing so with all the tools and techniques of modern science and research. And, most importantly, you're approaching this with open minds, fresh perspectives. Not to mention a certain willingness to freely experiment, to try things out, even where there's a significant risk of failure..."

She glanced at Trixie as she said this. Trixie shamefacedly tried to hide behind her hat.

"Thank you, Your Highness." Twilight smiled uneasily. "Still, I'm not sure what we should try next. And I don't want us to get stuck, just because we've run out of ideas. Do you know anypony who might be able to suggest something? Maybe someone from the Research Magic division?"

Celestia looked sadly thoughtful.

"There is one pony," she finally said, "who I'd would willingly recommend to help you. But locating her, I fear, may be a challenge."

She glanced around at all of them, and seemingly came to a decision. "Twilight," she went on, "I need to ask something of you... of all of you. I need to ask a personal favor."

"Of course, Your Highness," Twilight said, startled. "What is it?"

"You know that I've guided the studies of several ponies here at the School. What you may not know is that I had a personal protégé once before." Nodding at Twilight's look of surprise, she continued. "Her name was Sunset Shimmer. She had great magical potential, a willingness to learn, and depthless devotion to what she saw as her role as my student. Unfortunately, I didn't properly understand that devotion, not until far too late. We had a... disagreement, and she abandoned her studies, leaving here to seek a path on her own."

"Oh! Where does she live now?" Twilight's horn sang. Her magic brought over a chart of the known lands of Equestria.

Celestia shook her head kindly. "That won't help, I'm afraid. She left here by a... rather unusual means."

She turned, heading for the doorway.

"Come with me, all of you. There's something I need you to look into..."

They followed the Princess as she returned to the royal Audience Hall. There she strode in, with Twilight and her friends close around her.

"Give us the room!" she called out authoritatively.

Her staff hurriedly collected their books and scrolls, and filed out as quickly as possible. The guards waited until the room was clear, then excused themselves, shutting the doors behind them.

And Celestia led the way over to a tall mirror, with an ornate, gem-studded, horseshoe-shaped frame, standing behind a pillar on the left side of the hall.

"The Speculum Vacivitas," she stated. "One of Star Swirl the Bearded's final masterpieces. And despite appearances, it is more than just a mirror. It has the power to create a portal linking our dimension with another: an entire world, suiting the needs of the caster who summons it. Sunset Shimmer, in her anger at me, activated the portal and went through it. What became of her thereafter, I am uncertain."

Twilight was staring at the mirror, her mouth hanging open, in awe at the sight of a relic of her hero's knowledge and expertise.

So Starlight cautiously raised a forehoof. "Why, Your Highness? Can't you just go in after her?"

"Unfortunately not," Celestia replied. "The world that Sunset created actively excludes me -- and my guards and staff. I suppose it's a side-effect of our disagreement. And there is a time limit as well: on the third night following the portal's opening, it closes again. And then it cannot be reopened for an extended period of time. The Research Magic division examined the mirror's workings, and calculated the duration to be thirty moons."

Celestia sighed unhappily.

"At first, Sunset's behavior so enraged me, I was rashly tempted to leave her to her fate, to let her learn from her own mistakes. And there were other... pressing distractions at the time." She smiled at Twilight. "But I quickly came to my senses. She meant too much to me, I could not simply abandon her. Yet there was no way for me to reach her. Every means I used to contact her produced no result. In the end the portal closed, and she was trapped on the other side... in whatever world she created."

Tempest had a thoughtful look on her face. "Thirty moons... two and a half years? How long ago was this?"

Celestia nodded approvingly. "The portal should have reopened some time ago. But nothing has happened, then or since. I've had the mirror watched. I've had its mechanisms rechecked. As far as anyone here knows, it is in proper order. I could only assume, therefore, that Sunset either wasn't able to return... or worse, didn't want to."

She looked at Twilight, a sadly apologetic look in her eyes.

"I didn't mention her to you, Twilight, because I didn't want you to feel there was any competition here. You and Sunset are equally important to me. Both as my students... and as ponies I care deeply about."

"I understand, Your Highness," Twilight said seriously.

Celestia stared into the mirror.

"I deeply regret that my last words with Sunset were so heated, so harsh. I would give much to find her, to reassure her that there is not -- and never has been -- any ill will between us."

"But that was so long ago, Your Highness!" Starlight said.

Tempest nodded. "And if there's been no word of her since..."

Twilight sighed. "I suppose it must be far too late now."

Celestia gave her a look... and a smile.

"Perhaps not, Twilight..."

Her horn shimmered, and Celestia's magic summoned over a large hardbacked book, with a flaming sun icon on its cover. She flipped it open and paged through it.

"This is a journal. One of a pair I gave to Sunset as a gift. Anything written in one appears in the other. I've used it multiple times to try to reach her, both before the portal closed and after, but have gotten no response... until just this morning, when this appeared."

With the tip of a forehoof, she indicated a paragraph on the left-hand page of the Journal, written in a crabbed, uncomfortable script... almost as if the writer was still unfamiliar with using a pen.

"... I'm waiting right on the other side of the portal. Please don't leave me here. I know I can make things right again. I'll do whatever it takes..."

Twilight's eyes went wide. "It looks like Sunset wrote this... right after the portal closed! But if this message only appeared today... then that means..."

"... the two time streams!" Starlight said, seeing it. "There and here! When the portal closed, they became disconnected! So more than thirty moons can have passed here..."

Twilight nodded in amazement. "While they haven't, over there."

Tempest stared at the mirror. She cautiously put out a hoof, touching its surface. "So that means... Sunset might still be on the other side, right after the portal closed."

"Ahem!"

They all looked at Trixie, who was staring at the mirror, her head tilted, a puzzled look on her face.

"Trixie is a little confused," she stated. "If time can become... unstuck, or whatever it was." She waved a forehoof dismissively. "... which Trixie is not certain she believes anyway... then what point is there in having any kind of time-limit on this thing at all? And why thirty moons, specifically?"

Realizing the others were looking at her, she hunched nervously, removing her hat and holding it contritely. "Um... just asking. Trixie will be quiet now."

Celestia eyed her briefly, then smiled. "That, my little pony, is a very insightful question. To my knowledge, it has never been raised in any of the reports from the Research Magic division. This is why I would like to give this to the four of you... in your capacity as my Advanced Projects group. I would like you to examine the mirror yourselves. Approach it with no preconceptions. See if you can find a way to reopen it, to reconnect it with Sunset's world. And, assuming you're able to do that..."

She gazed at the mirror, her voice soft and pained.

"Find Sunset Shimmer. Let her know she's still welcome here... if she wants to return..."

Twilight swallowed nervously. Then she drew herself up proudly.

"We'll do everything we can, Your Highness! We'll find Sunset, and bring her home."

"Thank you, Twilight. I know you'll all do your best. And you, Trixie --" Celestia added, looking at her.

"Me?" Trixie squeaked.

"I would like you, in particular, to examine the mirror's workings from your... uniquely practical perspective. See if you notice anything the Research Magic division might have overlooked, hmm?"

Trixie's mouth fell open. Then shut. She fiddled with her hat for a moment. Then she nodded, her voice solemn and determined.

"Trixie will do everything in her power, Your Highness."

Three days later, Trixie was long past regretting those words.

She sat at the worktable in Twilight's tower room at the School's library annex, with books and scrolls stacked all around her. Her head was nodding from exhaustion. Her vision was blurred, her eyes felt like sandpaper. She'd stared at more journal entries and diagrams and research reports than she'd ever thought existed.

All concerning one blasted magic mirror!

She glared crossly at the mirror, sitting on the other side of the room where Celestia's guards had moved it so Twilight and her friends could study it in detail. And they had... they all had. They'd examined the mirror's workings, run tests on its aura, even cautiously attempted re-invoking the portal spell.

All to no avail. The mirror remained stubbornly inactive.

So, in typical Twilight fashion, they'd turned to research: gathering and poring over every scrap of information they had: on Star Swirl's final years, on the mirror, on anything that might be relevant.

And none of them more determinedly, more stubbornly, than the Great and Powerful Trixie herself.

Trixie felt like the mirror was taunting her, laughing at her miserably inadequate skill at magic. Much of what she was reading was well beyond her level anyway, as a stage magician. Oh, Trixie was quite at home with mechanisms: enchanted gems, boxes with false backs and hidden compartments, trick blades, and so forth... all the apparatus of the trade. She'd built most of her equipment herself, with her own hooves.

But real magic... artful constructions of spellwork and precision-crafted components that took years to learn and decades to achieve mastery in... it might as well be ancient Ponish to her. Apart from the few practical spells she'd learned to work with her horn.

Trixie was uncomfortably close to admitting she was unqualified for this task... that the Great and Powerful Trixie might be... well, not so great after all...

Her head sank wearily onto her forehooves. Her eyes slid closed.

Trixie is just resting her eyes for a moment, she told herself. The Great and Powerful Trixie doesn't fall asleep on the job. She doesn't shirk, doesn't dawdle...

... and she definitely doesn't daydream...

Her thoughts wandered. She drifted into a light doze...

And a mildly scornful regal voice spoke from behind her.

"We see that thou are hard at work."

Trixie looked up. She saw a dark blue alicorn approaching, wearing black peytral and crown, and silver shoes. The alicorn's large cyan eyes gazed at Trixie in a manner that was at once coldly aloof, and yet somehow strangely comforting... as if she was always just on the verge of breaking into a proud smile.

"Princess Luna?" Trixie smiled in relief. Of her two teachers, Trixie definitely preferred the one who appeared to her when her thoughts wandered during the day. The other one... the one who appeared at night, in her dreams, her nightmares... Trixie shuddered, shying away from the memory.

The Princess put a comforting hoof on her shoulder. "What troubles thee, Trixie Luna Moon?"

Trixie stared at the piles of books and scrolls morosely.

"Celestia asked me to figure out this mirror, to make it work right again. She wants me to discover something that dozens of expert research ponies weren't able to find."

"And are you not perfectly suited for this task?"

"No." Trixie admitted sadly. "Trixie is no mage. Trixie has no real skill with magic. Just stage magic: tricks with props, sleight-of-hoof, illusion and misdirection. Just trickery... half-baked, barely-good-enough trickery!"

"And is that not why you have been tasked with this?"

Trixie looked up at her, surprised.

"But I thought --"

"What do you bring to this task, Trixie?" Luna interrupted. "What is there here that would not reveal itself to anypony but you?"

Trixie thought about that. "So... I need to stop thinking like a mage... and start thinking like a stage magician?"

Luna nodded. And then she leaned close, whispering urgently in Trixie's ear.

"Find the key, Trixie Luna Moon! Find the key, open the door..."

Luna's eyes suddenly grew wide, sad and frightened.

"... and then come find me," she begged. "Please... come find me, Trixie. It is so cold here... and dark... so cold and lonely..."

Her hoof suddenly lifted from Trixie's shoulder.

Trixie cried out, not wanting her beloved teacher to leave:

"Luna!"

Trixie's head snapped up. She stared around the room uncertainly, wondering who had just shouted... and why her eyes were wet with tears. Scrubbing her face angrily with her hooves, she stared down at the book in front of her. And slammed it shut in annoyance. She wasn't going to find her answers there. Trixie needed to start thinking about this her way, as a stage magician.

"Find the key," Trixie muttered to herself. "Find the key, open the door..." She wasn't certain where that thought had come from, but it seemed important. She struggled to hold onto it, to feel it out. "Find the --"

She suddenly swung to stare at the mirror... at a half-baked hunk of junk botched together by some old mage centuries ago, who'd never even heard of writing anything down clearly, or making it easily understandable...

... and, perhaps, had a reason for that obscurity...

"Wait a second..." Trixie breathed.

Moments later, she was across the room, pry-bar in hoof, grunting loudly as she struggled to lever the cover off the mirror's base.

"Thank you all for coming," Trixie said archly. "Trixie is very tired, and does not wish to have to explain this more than once."

"Are you okay, Trixie?" Starlight asked, worried. "You look awful!"

"The Grrreat and Powerful Trrrixie knows no rest until she has achieved perfection," she intoned. "And Trixie has finally figured it out!"

"You know how to reactivate the mirror?" Twilight asked eagerly.

Trixie held up a hoof. "All in good time, my attentive audience! Now, Trixie set herself the task of understanding why Star Swirl's mirror only opens for three days, then closes for thirty moons. Trixie particularly wondered at the thirty moons part: it struck her as being unusually exact -- and also familiar. So Trixie looked through all the scrolls and diagrams on the mirror and got nowhere... because they were prepared by tenured researchers who saw Star Swirl as a brilliant sorcerer, a great pony, a grand figure out of history. One and all, they look upon his work as untouchable, unexplainable, masterful genius...

"But Trixie is a stage magician. So Trixie said to herself: who is this Star Swirl character, anyway? Some old spellcaster. Trixie certainly never heard of him. He built a magic mirror? Hah! Trixie could manage far greater with her eyes closed, chained upside down over a vat of chocolate pudding!"

Twilight was staring at Trixie in shock, mouth hanging open. Her right eyelid twitched.

Starlight put a comforting hoof on her shoulders. "Breathe, Twilight... remember to breathe. Go on, Trixie!"

"So, the Great and Powerful Trixie approached the mirror as she would any other trick of the trade. Trixie doesn't believe what she reads about ponies, she doesn't believe what she's told about them. She only believes what she can see, what she can touch with her own hooves! And Trixie doesn't need to know every little detail about the mirror. She only needs to know one thing, the most important thing: how the trick works!"

She led the way over to the mirror, where the levered-off panel had been tossed aside to reveal a fiendishly intricate complex of wiring, lenses, and crystal gems built into its base. Even cataloguing its design had taken the research ponies volumes of detailed lists and figures.

But Trixie hadn't needed to consult any of it.

Trixie touched her forehoof to one gem in particular. Not a particularly large or fancy one. Quite nondescript, really. But one she had quickly zeroed in on as being out of place.

"Know what this is, Twilight?" she asked smugly. "Oh, don't even bother guessing! It's a timing gem. Trixie uses them herself in her act. They can make a trick self-working but then there's no room for error, for playing the crowd. You have to time the performance perfectly, or be caught flat-hoofed, with no hope of recovery."

She tapped the gem, smirking. "And this gem in particular... it's just about the longest-period timing gem that's self-sustaining. Anything longer would need recharging and adjustment. And, more importantly, the gem draws no power. Which is important, because this circuit doesn't provide any. It's just a timing loop. Star Swirl probably just grabbed this gem off the shelf because it was available, and he needed as long a cycle as possible without it being too noticeable.

"And the delay cycle of this gem?" She grinned. "In seconds, it runs to just under thirty moons. Which is why Trixie recognized the duration."

Trixie looked around, at the astonished expressions on their faces. "What? You're surprised the Research Magic ponies didn't see this? How could they? They started from the assumption that Star Swirl was brilliant, that he had a reason for everything, and they were unworthy to question it. So they made their so-called analysis, they estimated the cycle of the timing circuit, they wrote it down in their report... and didn't even bother to ask why it was there in the first place!"

Glowering, Trixie placed her forehooves on the gemstone.

"And if we simply remove this gem..."

She twisted the gemstone, plucked it from its socket. There was a sudden stillness as the nearly inaudible hum of the mirror died away.

"Trixie!" Twilight gasped.

"Relax, Twilight," Trixie muttered, tossing the gem aside. "Trixie has done this before, just to be sure she was right. And now, we replace it with the shortest-period timing gem we have, merely to complete the circuit..." Choosing another gemstone from a box nearby, she slotted it in place and twisted it, locking it in.

The mirror powered up. And with an inrush of magic potential, the plane of the mirror became a swirling magenta vortex, then settled down to a curtain of shimmering light.

"Voila!" Trixie said, lofting her hooves dramatically. "One open portal!"

Turning to her friends, she stabbed a hoof at it. "We could have gone through this thing at any time! Right now, if we wanted to!"

The other ponies stared at her, in stunned amazement.

"Star Swirl..." Starlight finally asked, "... he deliberately added a lock-out delay? Why?"

Trixie shrugged.

"How should I know? Maybe as a way to discourage silly ponies from going through the portal, because then they'd be afraid of getting stuck on the other side. After all, it was only after Sunset went through that Celestia called for the analysis that found the lock-out was thirty moons, instead of forever."

Twilight was agape, utterly floored by the implications. "That's... amazing work, Trixie... really... well done..."

Trixie smiled archly. "Why, thank you, Twilight!"

And then she yawned, and her eyes crossed.

"And now," she murmured sleepily, "the Grrreat and Powerful Trrrixie... wants some peanut butter crackers..."

She collapsed in a heap, and snored gently.

Starlight checked on her, then looked at the others in relief. "She's just worn out. I'll put her on the couch so she can rest for a bit. And then I'll put the cover back on the mirror."

"I'll lend a hoof," Tempest said. Together, the two of them lifted the weary showpony and lugged her away...

... leaving Twilight looking up in concern at Princess Celestia, seated quietly beside her. The Princess was wide-eyed, utterly speechless.

"Um... Your Highness?" Twilight asked gently.

Celestia sighed. "That old fool. He was thinking of me," she said. "Star Swirl knew how fascinated I was by the possibility of other worlds beyond the mirror. But he also knew I'd never abandon my throne if there was even the possibility I might be trapped forever on the other side. And as a result..." She shook her head. "Just when you think you've already made enough mistakes!"

"Maybe it's not too late? We can still go through and find her."

Celestia got up and walked to the mirror. She put her hoof to its surface. It still rejected her, the shimmering surface pushing back against her attempts to cross it. Presumably it would still reject her guards as well, and anyone else Sunset was familiar with...

... but not Twilight.

Celestia turned back to her.

"I shouldn't have to ask this of you," she said. "You or your friends. I shouldn't have to put you at risk like this. But... right now, it seems I must. Find her, Twilight! Please find her. She can help you, I know she can."

"I will, Your Highness." Twilight nodded. "I promise!"

A short while later, Tempest Shadow and Twilight were standing before the mirror, wearing hastily-packed carryalls.

"Tempest and I will go through," Twilight said, trying to sound more confident than she felt. "See if we can find Sunset on the other side. If we do, we'll come right back. If not, we'll... figure something out. Get a message back, keep looking for her. Starlight, you and Trixie stay here and keep an eye on this end of the portal. We don't want it closing on us while we're over there."

"You got it, Twilight." Starlight nodded. "I'll let Trixie know when she wakes up."

"Spike," Twilight said to him, "you and Grubber stay close to Starlight. Help her and Trixie. Do whatever they need you to do."

"You are coming back, right, Twi?" Spike was wringing his claws anxiously. "You're not gonna get lost over there?"

"Don't worry, bud," Grubber said, patting his shoulder. "The boss ain't gonna let anything happen to her!"

Hearing this, Celestia apparently decided to make it official.

"Tempest," she said sternly, "I am placing you in charge of Twilight's well-being. See to it she returns safely, whatever happens."

Tempest nodded. "Understood." She glanced at Twilight. "So don't give me any trouble, huh? Unless you want to get carried back."

Twilight smiled up at her. Then she turned to face the mirror.

She gritted her teeth.

Sunset went through this -- and she wrote back from the other side, so it has to be safe... right?

Tempest glanced down at her. "Nervous?"

"A little," Twilight allowed.

"Good. Nice to know I'm not the only one."

As they'd planned, Tempest stepped up to the mirror first. She put a hoof cautiously through the shimmering plane of the portal, feeling it out.

And then, without hesitation, she dove straight through it, disappearing from sight.

Twilight blinked, startled.

She waited a couple minutes, as Tempest had suggested. And then, when the other pony hadn't returned, she stepped to the mirror herself.

Gingerly, she pushed her way through it. And was gone.

Unable to take it any longer, Spike dashed across the floor towards the mirror. "Twilight!"

"Spike! Wait!" Grubber chased after him. Lunging, he just managed to catch hold of the dragon's tail... but Spike had already plunged through the plane of the mirror. The portal's suction hauled Grubber in as well, with a loud yelp that was abruptly cut off.

Silence fell.

Starlight nervously exchanged a look with Celestia.

"Well..." Starlight said, "at least this way they'll have help, right?"

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