Twilight Sparkle was known for several things. Of those, 'being a nervous wreck under stressful circumstances' was high on that list.

An array of objects flew around the castle, grasped in her magic as she tried to get many things in order. Watching the chaos ensue, as if it was nothing new, was most of her friends. Within the center of the room they occupied, the portal to Canterlot High was powered up.

"Now, Twilight, do you mind explaining what it is that is going on so that we may help?" Rarity asked.

"Yeah!" Pinkie agreed, jumping in front of the panicked alicorn. Twilight grunted and teleported past her. "Are you going to visit the other 'usses' of the mirror world and need us to protect all of Equestria while you're gone?"

Twilight didn't even acknowledge the question as she continued around the room. Before long, Rainbow Dash and Applejack entered the castle to join the other four. "Sorry we were late. I was taking a really good nap," Rainbow Dash scratched the back of her neck as she landed.

"Ah basically had to shoot her house down to wake her up," Applejack muttered. It wasn't long before she noticed what was happening before her. "So what's wrong this time?"

Rarity stared idly at Twilight as the alicorn read some measurements. "We haven't been able to get it out of her yet."

Applejack sighed at the panic Twilight was in. She walked over to her and rested a hoof on Twilight's shoulder. "Twi, remember what Cadence taught you?"

Twilight turned to Applejack and stared at her. After a moment, she smiled and sighed. Going through the motions that her sister in-law taught her, Twilight cleared her mind and calmed down.

"All better?" Applejack asked. Twilight nodded and smiled shyly at her friend. "Now, care to tell us what's wrong?"

Twilight turned to the mirror and sharply inhaled. "I suppose nothing is really wrong. I was just trying to make sure everything went well."

Rainbow rolled her hoof in the air. "Went well for…?"

As Twilight opened her mouth, the portal glowed brightly, releasing the image of a light orange unicorn with a flowing red and yellow mane and tail. "Sunset Shimmer!" Twilight squealed as she rushed to the newcomer. Sunset shifted uneasily on her hooves before quickly finding her balance.

"Twilight, it's so good to see you again," she said, giving the alicorn a hug. She turned to the other five that gave her cautious smiles. She smiled apologetically and waved to them. "And...uh...all of you girls too."

Rarity was quick to accept that Sunset's reformation was strong and true. She approached the mare and examined her closely. After a moment, she asked Sunset, "Do you like the color blue? I see a real opportunity for something here."

Sunset chuckled. "You weren't kidding when you said that our friends aren't very different between worlds."

"Yep!" Pinkie popped up, spraying confetti across the room. "We are just as close to us as you could possibly get!" Sunset giggled as the pink pony wrapped a hoof around her. The others took a moment to meet and get to know Sunset as she gathered her bearings for magic and trotting around. Before long, she was back in the swing of being a unicorn.

"It's nice to see you getting along so well with my - our - friends," Twilight said to Sunset, "But we can all talk on the train."

"On the train? I wasn't told we'd be goin' anywhere," Applejack butted in.

Sunset looked at her hooves as Twilight explained. "Sunset Shimmer, the Princesses, and I were all talking recently through the journal and decided it was a good time for Sunset to come and visit. Luna was the deciding factor as she badly wanted to meet her."

"How long you gonna be here anyway, Sunny?" Rainbow asked as she idly floated past.

Sunset looked up, rather dejected. "About two months. Princess Celestia apparently has some really big project for me to do."

Twilight wrapped a hoof around Sunset's neck as the two began to exit the castle, the others following close behind. "I know you are thinking that she'll be mad at you, but I can assure you that she isn't. She's actually very proud of how you've grown since you ran away."

Sunset smiled and sighed. "Thanks, Twilight. I needed that." She looked around at their group for a moment. "Now that I think about being here for a moment, where's Spike?"

"He's in Canterlot, helping the Princess get things ready for your big project," Twilight answered. "She never actually told me what it is, but she seemed pretty insistent on having Spike help her get it set up while I came and got you."

"Does that have anything to do why we're coming along?" Fluttershy asked, walking up beside Twilight. "The whole trip is kind of last minute, and I don't want Angel Bunny to have to take care of everything while I'm gone."

Twilight put a hoof to her chin in thought. Looking towards the mountain that Canterlot rests upon, she could faintly make out the sight of a train winding its way through the various tunnels and channels to Ponyville. "I suppose we do have a little time left before our train arrives," she said, "If anypony needs to do something before we leave, now would be the time to do it."

Applejack, Pinkie, Rarity, and Fluttershy nodded and gave a quick goodbye before heading off towards their respective homes to let friends and family know what was going on. Rainbow, Sunset, and Twilight agreed to meet them at the train station and continued on.

"So what's it like in that mirror world, Sunset?" Rainbow asked from above, "I mean, Twilight's explained about the humans and some of the stuff you can do there, but what's it like to actually live there?"

Sunset laughed, "As a high-school student renting her own apartment, I can easily say that things could be easier." Rainbow looked at her confused. "I have to attend school to learn mostly useless topics while also regularly attending a low-wage job to pay for my overpriced, crappy home."

"Really!? That sounds like a load of manure. Now I feel really glad I've got an aerial contract with the Weather Team. My house isn't cheap, but at least I don't have to pay the full price for it."

"Rainbow, you basically live in a cloud mansion," Twilight said bemused, "The only reason you don't have to pay for that thing is because ninety percent of your salary goes towards that property."

"Look who's talking, Miss 'I got a crystal castle from a crystal box that a crystal tree gave me for absolutely free,'" Rainbow Dash joked.

Twilight proceed to pout, "I never wanted that castle."

Sunset patted Twilight's back as they got to the train station. "Hey it's okay. Nobody should have had to go through what you did."

Twilight flashed a small grin towards Sunset, "Thanks, but the same could be said about you." Sunset smiled back. The two shared another quick embrace as they waited for the others to arrive.

"So, you sure you don't know what this big project is? Not even why it involves us?" Rainbow asked as they stepped off the train in Canterlot.

"All Celestia told me is that this was something very important to her," Sunset responded, "She made a mistake several hundred years ago and she's finally getting around to fixing it."

"Now, what kind of mistake could take so long to patch up?" Applejack mused, "Is there some family problem she ain't told us about that happened back then?"

"I'm guessing there's some big monster bothering the towns ponies that she needs us to teach a thing or two," Rainbow Dash suggested cockily while mock-punching the air.

The group looked to each other and shrugged. Only time would tell what it was they were needed for.

Their walk through the capital city went uninterrupted. The various nobles and elites would give a generous curtsy or nod as Princess Twilight passed them. Sunset was lost in her memories as she noticed the various shops and leisure spots that she used to commonly visit.

As they approached the castle, the guards gave a modest bow as the group went in. It wasn't long before they stood outside the throne room. "You ready, Sunset?" Twilight asked. Sunset sharply inhaled and nodded. Twilight smiled and opened the doors.

"Hey, you made it!" was the response to the group's entrance. As they walked in, they noticed Spike sitting at the base of the throne, with no royalty in sight. He met them halfway, Twilight bringing him up onto her back.

"Where are the princesses?" Rarity asked.

"They're in the conference hall. Celestia said this matter was a little more serious, so they wanted some privacy," Spike told them.

"Does that mean you're not going to tell us in advance?" Rainbow asked with noticeable impatience.

Spike cast a glance to Twilight, "Ehm… I think I'll let the Princess explain." He scratched his neck and looked back to Rainbow, "It might sound a bit more… credible coming from her."

The group gave some confused and worried looks to each other, but continued into a side room next to the throne. Upon entering, they came across a heavily sleep deprived Princess Celestia examining a large quantity of papers rather closely.

"Princess Celestia?" Twilight peeped as she led the group in. The white alicorn's head rose quickly and a smile dawned on her tired visage.

"Twilight, it's so good to see you." She rose and embraced her former student. She glanced past to the other ponies, "And all of you as well."

Off to the side of the group, Sunset avoided eye contact with the demigod, something that Celestia easily noticed. She subconsciously approached the unicorn and nuzzled her, much to Sunset's surprise. "I'm especially glad to see you again, Sunset Shimmer."

Sunset ironed herself to hold back tears and returned the nuzzle. "I'm glad to be back."

Celestia returned to her full height, nearly stumbling backward in the process. She pressed a hoof to her forehead and yawned.

Rarity stepped forward, "Princess, have you been sleeping alright?"

"I can assure you, she has not," Came a boisterous voice from behind the group. They turned to see Princess Luna entering with a coffee pot grasped in her magic. She hovered the drink over to her sister, a muttered 'Thank you' being received in return. "Ever since Sunset Shimmer confirmed that she would be returning, my sister has been in a frenzy between her usual duties and entering the archives beneath the city."

"Has she slept at all?" Rainbow whispered to Luna, worriedly looking at Celestia as she returned to her papers.

"I'm afraid not," Luna shook her head, "While she has gone to her chambers to rest, I have not witnessed her dreams in almost three days."

Celestia returned to the group, her papers left behind, stacked neatly. "I will be fine, Luna. I have gone much longer without sleep."

"Regardless of such habits, dear sister, I can not guarantee your well being if you proceed much longer with this."

Celestia dismissed her warning, requesting a guard take the paperwork to her secretary. "I will sleep tonight, once I am sure that everything is going well."

"Well with what, sister?" Luna asked, shoving her face into Celestia's. "You may have told our draconic friend, but you've yet to explain to your own blood what you have been so adamant about for these past few days."

Celestia reared back and blinked in surprise. When she was sure that Luna's glare would not break, she sighed. "I suppose I have just been rather uneasy about this project."

"My project?" Sunset interjected.

Celestia gazed between her and Twilight's group, "Not… quite." Twilight and Sunset both raised their eyebrows to the response. Twilight began to speak, though Celestia beat her to it. "I'm sure that you all have some burning questions. If you'll allow me the time for us to trot to the archives to compose myself, I will explain everything I can."

Some murmurs came from the group, but nothing was said aloud as Twilight started the group towards the archives. Luna cast another glance at Celestia, who smiled assuredly in return, before quickly falling in next to Sunset Shimmer.

"Um, hello, Princess Luna," the unicorn said barely above a mutter, "I don't think we've met yet."

Luna turned to the orange mare and smiled. "Ah, of course. I almost forgot that you were the whole reason this fuss arose in the first place." Sunset winced at the gesture, something that Luna noticed. "Er, I mean no offense by that, little one. I, too, have some blame to hold in this recent occurrence, as it was my vote that bid you come back for a visit."

"About that," Sunset perked up a bit, seemingly out of eagerness, "Was there any reason you wished to do so? I had retreated into the mirror portal before the Nightmare Moon legends were even revived, so we hadn't a chance to really interact."

Luna's smile brightened, "My sister speaks many wonders of your knowledge and motivation. I can understand your prior shortcomings of impatience or misinterpretation as well, so I feel that we may get along more than the relation of I and Princess Twilight Sparkle."

"What do you mean you understand my shortcomings?"

Luna chuckled, "Well as a fellow do-gooder gone rogue on the basis of a temper tantrum and later reformed, I feel that we have one thing in common already."

Sunset followed the chuckle awkwardly. "Heh, yeah. Sorry to bring up any bad memories."

"It's quite alright, I have matured and realized that my bout was out of jealousy and corruption." She rested a hoof on the smaller mare's shoulder. "I can see that you have matured in a similar manner."

The quiet banters between several groups began to die down as they descended the cobblestone stairs into the Canterlot Archives, and eventually, Starswirl's personal wing by Celestia's request.

"You seem to be keeping your cool a lot easier this time around, sugarcube." Applejack teased as she nudged Twilight.

"I spent some time down here during my last visit. The place is amazing, but the initial excitement has died off." She looked back to Celestia, who was walking with her head down at the rear of the group. Twilight hadn't noticed earlier, but most of her regalia was currently removed. "I'm also fairly concerned with what Celestia has in store for us."

"She did somethin' wrong," Applejack stated. "You can see it in her eyes; they're swimmin' with regret."

"I hope it's something we can fix," Twilight said with one last glance back.

Before long, the group had come to Starswirl's athenaeum. Around them lie several magical artifacts that modern unicorns had all but forgotten. The air seemed to tense as Celestia walked through the group to an unassuming wall on the far side of the room.

Luna was the first to speak up, her concern finding its root around the long lost Starswirl the Bearded. "Sister, does this have to do with Starswirl?"

Celestia was late to respond, but a deep sigh signaled that she would. "I-I was a fool one thousand years ago."

"Tia…"

Celestia turned to look at the group, sadness having been drawn on her face, "I allowed Sombra his time to rule, I ignored Luna's plea for attention, but I also let my greatest friend grow distant due to one stupid mistake."

"Princess," Twilight spoke, "I'm sure nopony could have done everything perfectly in your shoes considering the events of the time."

"And I'm all too forgiving of our situation back then," Luna added, "Most of it was my own fault."

Celestia looked to the floor and sighed, as if accepting the comfort mentally. "Regardless, do you all remember the mirror that led to the good Sombra's world?" Mentioning it seemed to shatter her heart. The group, with the exception of Sunset Shimmer, all nodded and agreed. "And of course, there is the mirror that leads to Sunset Shimmer's world."

"Celestia," Luna stepped forward, a curious look on her face, "What happened to that mirror project of Starswirl's?

The white alicorn turned back to the wall. After a moment of thought, she flared up her horn. "I suppose it's time that I show you."

Within a moment's notice, the wall vanished and beyond its prior place of existence lay a vast room of mirrors, bookshelves, and foreign artifacts. There was likely over a hundred mirrors within the center of the room and the bookshelves spanned to the ceiling. The artifacts range from vaguely altered objects used by the ponies to mass piles of bolted machinery that would seem advanced to even the humans of Sunset's world.

Twilight, as was to be expected, was agape at the prospect of the room, though she was far from alone. The rest of her friends seemed to be just as stunned by the reveal, as was Luna.

"What was that old coot thinking?" Luna asked, more to herself than anyone.

"H-how many of these things are there?" Rarity stuttered out, "I'd rather not have a repeat of our run to our friend Sombra's world."

"I assure you that no such thing could happen," Celestia shook her head.

"Most of the mirrors are deactivated," Spike said, "Though some could easily be reactivated, a lot are failed experiments."

Sunset examined the surface of one of the mirrors, "The magic behind reactivating them is far from easy, Spike."

"She's not wrong," Celestia confirmed, "It took me almost a year to reopen Sombra's mirror the first time."

Moments more passed as the group got their fill of the ancient room. Though, the silence was broken by a rather unnerving comment from Rainbow Dash.

"So, are we going into one?"

The ponies apart from Celestia all froze. With reactions ranging from fear to curiosity, they all looked to the solar goddess for an answer. She responded by approaching the center of the room and pulling a cloth from atop a mirror away from the main bunch. The mirror she uncovered looked no different from the other hundred besides the fact that its surface had been recently dusted. From the shelves, she gathered several tomes and stacked them near the mirror.

"This," Celestia started, "is the mirror to what Starswirl gathered to call Terra."

Sunset's eyes widened, "That's simply an ancient word for Earth, the world I live on."

"Precisely." The Princess nodded. "Starswirl had visited your Earth several years before this, but found that the world was far too receptive to magic. If magic were to be introduced to your Earth in too heavy a dose and concentration, the dimensional barrier could collapse entirely. As such, he found the world unsuitable for our studies, especially since this was during their dark ages; when magic was theorized and could be used for evil and evil only."

"Then what about the Sirens?" Twilight asked. "He banished them there, but they could still use their magic."

"Their magic was weak," Sunset explained, "Human forms are very poor receptors for Equestrian magic, and it was only their necklaces that allowed it to transmit at all. Said necklaces were hyper-charged by your Equestrian magic via the passive magic of the Elements of Harmony."

"Correct. Likely, because of the Elements' use and the battle against the Sirens, Earth is only one bad magical flux away from our dimensions colliding in a similar manner to ours and Sombra's," Celestia added.

"That's beside the point!" Rainbow Dash butted in, "What does this 'Terra' have to do with Sunset's world and what do we have to do there?"

"Patience, Rainbow Dash!" Rarity yanked her down by her tail, "I know you're anxious, but we'll be off in due time, I'm sure."

Celestia placed a hoof on the stack of books, "These tomes will help you find information you need while on Terra." She avoided eye contact for a moment.

"Who exactly is going?" Sunset asked, "You said this wasn't my project."

"Princess Twilight and her friends."

The six friends stopped for a moment, before it sunk in. Rainbow Dash just smirked, completely aware from the beginning, while Fluttershy seemed to shudder.

"Why are we going, though?" Twilight asked.

"I-" Celestia started, but choked up. She took a moment to steel herself, "I need you to apologize to Starswirl for me."

Twilight and Sunset froze at the statement. The others seemed just as shocked and even Pinkie stopped moving to consider this new information.

"A-are you saying," Twilight breathed out.

"They're going to meet Starswirl the Bearded?" Sunset finished.

"Don't get too excited," Celestia said, her head dropping in mourning, "He is very much dead."

"Then isn't the trip kind of pointless?" Rainbow asked, oblivious to the sorrow that just spread through the air.

"Maybe, maybe not," Celestia answered. She wavered for a moment before yawning again. "Pardon, but I am in dire need of rest. I advise you all to study up on Terra and sleep well tonight. I'll explain more about Starswirl's disposition with the mirror tomorrow, as well as where to look for Starswirl."

Without so much as another look back, she trot out of the room and back up the stairs from where the group came. Luna hesitated to follow her, but decided against it.

"I will assist you however I can with learning about this world," she said.

"Are you sure that this will really be a good idea?" Sunset asked Twilight and Luna, "These mirrors are unpredictable from what I've learned back when I was Celestia's student."

Twilight approached the pile of books. Opening the first cover, she recognized the legendary scribbles that was Starswirl's writing. "I never learned what happened to Starswirl during his final days. This is Starswirl we're talking about. He may have left something for us regardless of what is current livelihood is."

Sunset gave her friend a cautious glance before sighing.

"My sister never explained Starswirl's demise to myself either," Luna said, picking up a book, "Their falling out remains a mystery lost to the tests of time."

"Well then!" Pinkie exclaimed as she popped out of the book Luna was holding, "Let's get studying! Who knows what this place has in store for us!"

The three studious mares looked to each other for a moment and nodded. If they were to fix one of Celestia's biggest mistakes, they would need all the preparation they could get.