Earlier...

Luna's chambers were a study in contrast to Celestia's, in more ways than one. There was the obvious difference in palette, of course—navy and blacks and aqua accents, as opposed to whites and golds and the burnished copper of sunrise. More than that, though, Luna's apartments were sumptuous, comfortable, royal. Where Celestia had a bed and a liquor cabinet, and simply wrote in mid-air without a desk, Luna's walls were lined in tapestries and dark velvet hangings, with thick curtains to block out the light from the windows. Her walls were lined with shelves, and the shelves were lined with countless books. The bed was four-posted, and hung with dark blue tulle. The desk was large enough for three ponies to have quite a bit of fun on.

Celestia had picked the books out of the archives for her, before her return. Half were ones she thought Luna would like, based on millenia-old memories, and the other half were ones she thought Luna would need to adjust to the new era. The bed and desk had been comissioned by the solar princess in the time leading up to the fateful thousandth Summer Sun Celebration, painstakingly crafted in a style a thousand years old.

They reminded Luna strongly of a bed she had wept bitter tears of jealousy into, and a desk that had seen the first drafts of a spell that had darkened her coat, sharpened her teeth, and granted her the power to challenge her sister. She quite liked some of the things she had seen more daring unicorn smiths do with steel and glass since her return, but she hadn't found it in herself to tell Celestia so.

Whether she liked the aesthetic or not, however, Luna slept in the bed. She thought she'd rather earned some rest after everything that had happened. Indeed; Celestia back in court, Twilight back in Ponyville—a good evening's work for a princess, and one that deserved a good day's rest.

The doors burst open anyway, with no respect for Luna's efforts. The slam of the heavy wood against the wall was muffled slightly by the tapestries, but it was still quite loud. Luna, curled up in the center of the bed, found herself suddenly standing in the center of the bed, hooves splayed and set in readiness, and wings spread to flee or fight. Her eyes focused on Celestia in the doorway, and later, she would find the time to be ashamed that she did not immediately relax upon recognizing her sister.

Celestia, however, either did not notice Luna's wild eyes, or did not care. She began to speak without preamble, “Pack your saddlebags. There are changelings in Roan Mesa, and you and I are going to dig them out.”

Without waiting for a response, the white alicorn turned on her hoof and strode off, leaving a wide-eyed Luna behind, wondering at the venom in her sister's voice.

***

“...and so, we ask all of you to accompany us to Roan Mesa, so that we might discover whatever truth there may be in our sister's supicions. We will meet all of you in front of Twilight's library in a quarter turn—Celestia is bent on proceeding immediately, and that is all the time we may grant you.

Yours, ever in gratidude,

HRH, Princess Luna Revenia”

Six ponies regarded each other with wide eyes as Spike finished reading the letter, Twilight's panicked flight momentarily forgotten. It was Applejack who spoke first.

“Welp. If ah run, ah can just make it. Ah'll see y'all there.”

That fractured the stillness. All six ponies made for their homes and the Harmony Crisis Bags they had taken to keeping in perpetual readiness, but not before Rarity could arch an eyebrow and smirk at Twilight while the purple unicorn levitated Spike onto her back. Twilight, for her part, did her best to ignore it. World-threatening crisis now, panic later—that was a rule that had served her well time and again.

When she arrived back at Golden Oaks, she ripped a well-worn checklist from the day desk by the door.

Spike distracted by the topaz cache? Check.

Saddlebags? Check.

Quills? Check.

Ink? Check.

Hermetically sealed backup ink? Check.

Parchment, and likewise backup parchment? Check.

Field notebook? Check.

Duplicate backup notebook stowed in the safe as insurance against the inevitable accidental destruction of her current notebook? Check.

Bandages in case Fluttershy runs out? Check.

Concentrated alfalfa energy bars in case Applejack runs out of apples? Last rotated 37 days ago, so Check.

Needle and thread in case Rarity loses hers? Ow, and Check.

Daring Do short story collection to keep Rainbow from wandering off? Check.

Spare packet of confetti for Pinkie Pie? Check, and check.

Somehow, Twilight managed to close her bags and step out her front door. She was alone.

***

Twilight was the last to arrive, despite being the meeting point being outside her front door. In fact, she closed her door just as the quarter-turn ended, and any attempt anypony might have made at asking her what the earlier running and yelling and screaming had been about was cut short by the arrival of a starry black cloud, and the princess that assembled herself from it.

Luna opened her eyes as she coelesced, and smiled. The Elements had risen to her call! Logically, rationally, she had known they would, but... it was gratifying, nonetheless. Her smile faltered to a slightly uncertain one as she tried to find the words to address her assembled saviors.

“My,” there was just the slightest of pauses, “friends, we—I—am happy to see you are all ready to proceed. Gather close, please, and w—I will conduct you to Celestia. There is no time to lose. We will answer your questions in time, Rainbow Dash—for now, we must make haste.”

Dash was scowling as she closed her mouth again, and narrowed her eyes suspiciously at Luna. When the rest of her friends crowded wordlessly around the dark princess, though, she joined them. Luna's form puffed back into insubstantiality, surrounded the six ponies, and began to shrink. Years of acrobatics had given Rainbow a cast-iron immunity to motion sickness, but as Luna's magic shrunk the group to a singularity, the pegasus could feel her stomach being left behind.

***

Luna's presence around them was cool silk, and so the heat hit them like a body blow when they arrived. The starry mist deposited them on a sun-baked flat of cracked earth. To their left, railroad tracks gleamed in the merciless light, and on the horizon, a mesa rose from the plain, its yellow base fading to a red top. To their right, the insubstantial presence of Princess Luna fled to the shade of a rocky outcrop to coalesce as a pony, her eyes shut, her mane and tail lank, and her body already showing a sheen of sweat.

In front of them stood Princess Celestia. Her coat was as pristine as any day in court, and her mane waved gently in an unfelt breeze, just as it had for every day of Twilight's life. She did not squint, despite the harsh glare of the sun, but her stare was hard and flinty, and her mouth set into a stern line, with just the hints of a twisted scowl at the corners of her lips. Twilight reached up and pressed a hoof to the top of her head, as if to hold in the wild snarls of her mane that threatened to overwhelm her at the sight.

The air shimmered before the Princess, and Twilight distracted herself by examining the spell. She recognized it as a simple cantrip for altering the refractive index of a material. It was something she had used herself to make tiny adjustments to the glass of instruments in her lab, but Celestia was strong-horning the air itself into acting as the lenses of a pair of binoculars—not only holding part of the air in her iron field and forcing it to act as one unit, but applying a gradient adjustment effect to create the magnification she desired. She was peering down the railroad line, where Twilight thought she could make out a dark smudge on the horizon through the shimmer of heat rising from the ground.

Celestia did not look back over her shoulder at the new arrivals. She didn't even acknowledge her sister's arrival—she simply began to speak.

“I have had the town in sight for the last quarter-turn, and there has been no movement. There is a train at the station, but it is cold—no smoke, no steam,” the magnification spell in front of her muzzle winked out, but she continued to stare at the smudge (Roan Mesa, I presume, thought Twilight) unblinking. “We should overfly the town, and see what else there is to see.”

Rainbow Dash immediately swept into the air, coming to a hover above and in front of Celestia.

“All right!” she cried, grinning and saluting in mid-air, “Which side do you wanna take, Princess?”

Celestia was frozen in place, staring at Rainbow. It didn't really look that way, of course—in her surprise, muscle memory had taken over, conditioned by countless court sessions made of up even more countless* attempts to put her off-balance. Celestia's face never missed a beat, no matter what inchoate flailings made up her thoughts.

After about three moments too many, she finally found her voice, sliding automatically into the diplomatic case. “Ah, Rainbow Dash,” she finally spoke. A gentle smile spread across Celestia's muzzle. It was a number fifty-seven, gentle humor.

“I'm afraid you might be a bit... visible for reconnaisance work. You... stand out,” Celestia paused, and one corner of her mouth twitched up in a sly half-wink, “Impressively, of course, but it draws attention.” Celestia frowned in thought for a moment before continuing, “If you would, however, take a position at about two hundred hooves, and watch for movement from the town...?”

Rainbow's ego visibly deflated, then reinflated as Celestia spoke. She saluted again before shooting straight up into the sky to take up her assigned task for as long as she could stay interested in it. In the meantime, Celestia frowned, and paced over to her sister.

“Luna.” She paused politely to allow the darker alicorn to reply, but Luna merely nodded, and waited silently, watching her sister warily. Celestia had been expecting as much, so she continued without further preamble.

“We need to talk.”

*Ruminations on the difference eventually led one of Celestia's previous students, Cardinal Counter, to publish several rather influential papers on countable and uncountable infinities.

Author's Notes:

Merry Christmas, my dear, delightful readers.

This throws the model of my update timing completely out of whack, though.