Twilight gingerly stepped off the train to Canterlot. She was worried. Some nagging, unconcious gut feeling told her something odd was going on.

But why should she be worried? She had been invited to Canterlot by her mentor to simply take part in a mission. A mission which was a matter of such extreme urgency that Twilight had to be on the train that very afternoon. And which nopony could know about, not even Spike. She'd even asked Twilight to burn the letter she'd sent it on her . . .

Okay, yeah, this was kinda creepy.

But if there was one thing Twilight has too much of, it's faith in in her teacher. Trying to assuage her worries, she collected her bags and headed for the castle.

For some strange reason, Twilight was directed to Celestia's private rooms. Even her brother, who was often chattier than Pinkie if you played the cards right, was locked up tight. He merely said that "the Princess is not coming out of her chambers".

Twilight knocked once, very lightly.

No response.

She knocked again, louder.

Still no response.

She knocked loud enough that one could actually hear the sound of hoof on wood.

The door squeaked open, and Luna stuck her head out. "Finally!" she hissed, and then abruptly yanked Twilight inside.

Twilight briefly glanced around the room. It was impolite, she knew. but she couldn't resist the temptation to see how her mentor really lived. There were books, mostly. Just books. There was a bed and a fireplace, but they seemed to be afterthoughts in a monument to the bibliophile. Twilight, naturally, was thrilled.

Celestia was consulting -surprise, surprise- a book. She looked abnormally worried.

Luna poked her. Celestia lifted her head from her book, peered around, and finally caught sight of Twilight. "Ah! Twilight! I'm glad you've arrived!"

Luna leaned down and whispered, "Celestia's been . . . researching. I haven't seen her in this state for a long while."

Twilight's eyes glimmered. "She's- researching?" She suddenly felt a deeper connection to Celestia than ever before.

Celestia trotted around the room, gathering strange magical instruments. At last she turned back to Twilight, a harried expression on her face. "Twilight, as you probably gathered from the letter, I need some help with an mission."

Twilight leaned forward hopefully. "Yes?"

"Well, it's not really a mission," Celestia added. "It's more of an . . . experiment."

Twilight stood on tippy-hooves. "Ye-es?"

"I need you to temporarily take my place as Sun Princess of Equestria."

" . . . what."

That one word basically summed up what can best be described as an explosion in Twilight's mind. She was awash in confusion, disbelief, doubt that such a thing was possible, worry about her teacher's sanity, fear that it might actually work and she would be stuck with far too much responsibility, and at the very back of her mind, a tiny, squashed-down bit of ambition.

"I told you she wouldn't go along with it!" said Celestia, glaring at Luna.

"As I recall, you were the one who contacted her, sister," said Luna. She turned to Twilight. "Hear us out, please."

The door was right behind her; she just had to walk away, head back to Ponyville, and wait for Celestia to come to her senses. Very easy. Just a few steps . . .

"All right," said Twilight, sitting down.

"My dear sister has been feeling . . . a bit overworked recently," began Luna. "It's not easy, you know, being the Princess of Equestria."

Celestia grunted in agreement.

"So I suggested that she, well, take a vacation," said Luna.

Twilight raised one eyebrow. "That's what all this fuss is about? She wants to take a vacation?"

"Patience," said Luna. "It would not be easy for Celestia to, what's the phrase, fit in. She could disguise herself, but she would still need to look after the sun every day."

"There is a spell," she went on, "that could solve all these problems. It transfers an alicorn's power to another pony. It has been performed successfully in the past, and can be reversed very easily once Celestia comes home. She will, for all intents and purposes, be a normal pony. However, she needs somepony to transfer her power to. That's where you come in."

"So . . . I'll be like you? An alicorn? And be in charge of Equestria?" Twilight asked hesitantly.

"Don't get too excited," said Luna, smiling. "We would like to transfer her power to you, yes, but we will not to give you too much responsibility. I will simply report that Celestia is . . . ill, and must not be disturbed. Nopony will have to know that Celestia is gone. I will look after the kingdom. You'll just have to raise the sun every day."

"I've recorded my methods, although I'm not sure if you'll find them descriptive enough," said Celestia. "Luna will be there to help you out, of course, but if you can't handle it . . ."

"Relax, sister," said Luna, winking at Twilight. "I haven't seen you this worried about something since Starswirl the Bearded asked you to marry him."

Twilight could not help blurt out, "WHAT?!"

Celestia giggled. "I turned him down, of course, but he was rather insistent."

Luna pressed on. "Anyway, we've agreed that you're the best candidate for the position, and we'd like to carry out the transfer as soon as possible, if you're willing. Celestia won't be gone longer than a month, I'm sure."

"A month?" squeaked Celestia. "I can't be away that long, Luna-"

Luna shushed her sister and looked at Twilight. "We won't do it if you don't want to, of course, but we thought you might like it."

Sometimes we surprise ourselves. Twilight thought that she might be a bit more worried about taking control, carry out a strange magical procedure, and all the other myriad risky things involved. But there was no doubt in her mind.

"I'll do it."

Thirty minutes later, they were ready to begin. Celestia had taken Twilight down a secret passage that looked like it hadn't seen the light of day in several centuries. The room they were in looked suspiciously like a dungeon, and there were several inexplicable brownish stains on the stone floors that Twilight tried not to think about.

Celestia and Twilight were lying side-by-side on two tables, surrounded by a bevy of strange instruments, books written in dead languages, and obscure charts. Luna was bustling around, making sure everything was in proper order.

"You two ought to be connected by threads of mercury and silver," she muttered, "and we've got the serum of poison joke, to make the transformations go faster, right, and I think we'll just need a few more pegasus feathers . . ."

Celestia had taken off all of her royal jewelry and her crown. She seemed embarrassed about it, but Twilight didn't think it made her look much different. She was shifting back and forth on the table and glancing at Twilight with an embarrassed look on her face. Occasionally she would flutter her massive white wings, or gently poke her horn.

It was strange to think that Celestia was just as anxious as Twilight, if not more so.

"Alright, I think we're ready to go," said Luna at last. "I'll just summon down the lightning-"

"Lightning?!" cried Celestia. "I never heard anything about lightning!"

"Oh, it'll just be a quick jolt, nothing too painful," said Luna. As Celestia continued to protest, she couldn't help but smile and added, "Joking! Joking!"

"So . . . there's not gonna be any lightning?" asked Twilight.

"I didn't say that . . ." said Luna, but then she gave Twilight a friendly wink. She levitated a dusty tome into the air and flipped to the right page. She paused only to look at the two ponies on the table and ask, "Are you sure you want to do this?"

"Positive," said both at the same time.

Luna nodded and began to chant in some strange language. The wires and instruments began to hum lightly. Celestia smiled encouragingly at Twilight. The purple unicorn breathed slowly, trying to keep calm. it began to get easier, gradually. her eyelids grew heavy.

Luna's chanting grew to a crescendo. Twilight felt herself beginning to vibrate slightly. But she didn't care. She was just too tired . . .

Then she passed out. Next Chapter: Good Morning Sunshine Estimated time remaining: 21 Minutes