My faithful student,

I am writing to inform you that I may be out of communication for a short time. I have been spending what little free time I have over the last few decades on a project that I believe has the potential to revolutionize Equestria, and it's reached a delicate stage that requires my direct attention. Any letters to me will be forwarded to Princess Luna for the duration; she will be taking care of the day-to-day running of the country while I am occupied.

Do not neglect your studies. I look forward to hearing your reports on the magic of friendship, and sharing with you what I have been working on, when I return.

Princess Celestia

• • •

Twilight sighed as she placed the letter back in its file. Ten years, she thought. Ten years since she'd received the letter, ten years since she'd heard anything from her missing mentor. Ten years since Princess Luna had summoned her, frantic in the cool, restrained way of royalty with an image to maintain.

• • •

"She told me she would be working in her subbasement office that night, and that she didn't wish to be disturbed." The younger princess led them through the lower levels of Canterlot Castle. The air grew decidedly musty and damp; it was clear to Twilight that ponies rarely came here.

The pair came to a heavy oaken door, and Luna's horn glowed and the door slowly creaked open. "After a day, I began to worry. She hadn't shown for any meals, though I could still sense her presence within the castle. I came down to check on her, and there was no response when I called. When I entered the room, I found it as you see it now... empty."

Hesitant, Twilight entered her mentor's private study and looked around, not sure what she would find. There was a desk with some neatly-stacked scrolls and parchment, a cushion before it, and a bookcase along the wall filled with well-worn tomes. The walls were bare, rough-hewn stone, and Twilight realized they must be at the very foundation of the castle.

"When I've attempted to perform a locator spell on her, I am drawn here, but there are obviously no signs of her. The results of the spell are unusual, as well – rather than specific location, I get a generalized sense of her presence, but no direction to guide me to her."

Twilight moved to the desk, Luna watching from the door, and inspected the papers. They were riddled with bizarre language, words and acronyms she didn't recognize, and complicated diagrams filled with unfamiliar symbols, but she thought she could make out the basic idea. "It looks like she was trying to construct some sort of... calculation machine?"

"That is the best I have been able to determine," Luna said, hints of sorrow and confusion tracing through her voice. "My sister had not involved me in this work. I do know she had started it long ago, while I was still imprisoned. I personally lack the relevant knowledge or know-how to be able to understand her notes, and I have been reluctant to share them with anypony. My sister clearly wanted this kept secret and for all I know it could be dangerous to the wrong pony.

"I believe you may be the right pony."

Twilight turned. "Princess?"

"Celestia spoke of you often. I know how impressed she was with your abilities and accomplishments, how proud she was of your progress. She trusted you implicitly and therefore I do too." Luna bowed her head for a moment, then looked at the young mare before her with a steady gaze and a lone tear dripping down her muzzle. "I brought you here to ask if you would take Celestia's notes, her research, and attempt to follow in her hoofsteps, as it were. I am a thousand years out of date. It would take me decades or centuries to catch up to the point where I could hope to understand enough. You are knowledgeable, bright, and more familiar with my sister than any other pony. It's my hope you may be able to shed some light on where she has gone, or what has happened to her."

It was Twilight's turn to bow her head, appearing lost in thought. When she looked back up, her eyes were shiny with tears, her brow furrowed with determination. "Of course I will, Princess Luna." She hesitated before admitting, "I don't know what I'd do without her."

Luna smiled a small, sad smile. "Nor do I, Twilight Sparkle. See if you can find her."

• • •

Ten years. Twilight knew she was being needlessly maudlin and found it hard to care. She shook her head, dropped the file back in its cabinet, and stood for the first time in hours. What time was it? In the basement of the Ponyville library, she had no way of knowing how long it had been. She idly flexed each leg, working out the kinks in her joints, before moving away from her workspace's pool of light, towards the base of the stairs in the darkness.

• • •

For a time, Luna had maintained the pretense that Celestia was away on an extended diplomatic tour of major allies around the world, but she could only maintain the fiction for so long. Twilight and the other Elements of Harmony had stood with her when she announced that Celestia was, in fact, missing, and the Regent of the Moon would be assuming responsibility for Equestria. Many of her subjects reacted with heartbroken dismay at the news their beloved ruler was never returning, though Twilight took it as a challenge, a reason to redouble her efforts.

Celestia's notes were intricate, impressive, and incomplete. Some intellectual leaps were required as Twilight worked through her research, and she worried that she may have missed things the ancient alicorn had taken for granted. Partly, her worries were fueled by the nagging suspicion that it was too easy; even with the gaps in the source material, everything had been laid out before her – she simply followed the trail. It probably helped that she'd always had a mild interest in more advanced technology, and she'd adapted her basement lab, unused since she had attempted to determine the roots of Pinkie's unusual precognitive abilities, to her new line of work.

• • •

At the top of the stairs leading from the basement to the library proper, Twilight turned and paused, looking out over her lab. The machinery all hummed warmly, and the large screen she had erected against one wall depicted an overhead view of Ponyville, updated by magic sixty times per second to create the illusion of real-time movement. It wasn't actually Ponyville, of course; she wouldn't need a roomful of arcane equipment to power a simple scrying spell. She had used Celestia's techniques to create a simulation of the small town and its inhabitants and left it running constantly. She justified it as a long-term experiment to see how accurate the simulation could be, but were she completely honest with herself, she'd have to admit she had no idea what else to do. She had reached the end of Celestia's notes and been left with no obvious next step nor any useful clues as to the alicorn's fate.

Tiny pony-shaped figures moved about their business, unaware they existed only in the heart of a machine – unaware of anything at all, in fact, as they were by no means sentient, just programs following their set directives.

Sometimes it made Twilight uneasy, seeing her friends and neighbors move about on the screen, acting much as they would in the real world. None of their real-world counterparts had any idea this existed, of course: Twilight had taken the princesses' wishes to heart and not revealed the nature of her work to anyone, even her friends. They all knew that she was doing some kind of research for Princess Luna, and that it had to do with Princess Celestia's disappearance, but she had explained that for everyone's safety, she had to keep it to herself. They had all understood and agreed to stay out of the basement, some more reluctantly than others.

Rainbow Dash in particular hated the idea of being left out of something so potentially awesome, though as time had passed and she'd seen it appeared to amount to nothing more than extra studying, she'd cooled down on the idea of helping out. What had she been expecting, some grand adventure to hunt down the Princess? Inspecting the overview of her simulated Ponyville, Twilight found Rainbow's simulated double lazing on a simulated cloud puff and shook her head, grinning. It's a fairly accurate simulation, all right.

She turned back to the door and went into the main room of the library, her hooves clopping softly on the hardwood floor. It was clearly late; the room would be pitch-black if Spike hadn't left a few candles flickering, cutting into the dimness. She moved to a window and nosed the curtain aside, looking out into the night. Clear and crisp, with few clouds in the starry sky. Luna's moon shone brilliantly over Ponyville and Twilight gazed up at it, musing on how much had changed since the first time she'd stared sleeplessly up into the sky here.

Sighing again, Twilight headed for bed, passing Spike. He'd grown quite a bit since Celestia's disappearance and now slept in a full-size bed next to hers. She tried to move as quietly as possible, not wanting to disturb him. She had no idea when he'd gone to bed, as the prohibition against entering the basement applied to even him. About to climb into her own bed, she froze, silent, as the bed behind her creaked.

"Twilight?" Spike's voice sounded bleary with sleep.

"Hey, Spike," Twilight murmured softly. "I'm sorry I woke you, go back to sleep."

"Okay, Twi..." He yawned and rolled over, swaddling himself in his blanket. "Sweet dreams..."

"You too, Spike." She yawned as well and pulled herself into bed, settling beneath the covers. Letting her eyes drift shut, she calmed her mind, allowing her thoughts to quiet in preparation for sleep.

A sudden low rumbling snapped her eyes open and she pushed herself up, startled. Across the room, Spike turned over in his sleep and let out a tremendous fiery belch. Even in her half-asleep state, Twilight reflexively ignited her horn and grabbed the note that formed out of ashes in midair, bringing it over for her to read in bed. Spike slumbered on, oblivious.

Seconds later, Spike sat up in bed abruptly, startled awake himself by a loud banging noise. He looked around wildly, saw Twilight's bed was empty, the covers thrown back; saw the front door of the library had been blasted off its hinges; saw a singed roll of parchment laying on the floor between the two beds. He jumped out of bed and picked it up, unfurling it to read, and his eyes went wide.

"No way."

• • •

My faithful student,

I need your help. Please come to my study.

Princess Celestia