For over a week, the princess of the night had been making small amendments to her message in the sky. First it had been to add "SISTER" at the end, then it had been a "PLEASE" in front, and then two days later she added "WE MISS THEE".

It became a little game for Celestia to guess as to what could be added.

The night sky consequentially became much more devoid, except for the neat constellation in the West.

Luna had also moved the polar stars back to their correct position, as to not hinder travel.

It was all so overwhelming for Celestia. After a week, the message that could not be unseen ate at her heart. It irked her that her sister still wanted her to come home. Or maybe she was just lazy and hadn't replaced the sky; the message hadn't changed that night.

Even in the daylight, the sun princess could not forget about what she saw in the evening. It was burned into her retinas, practically.

When Celestia fell asleep that night, she was greeted with an unfamiliar dream.

She looked over towards the horizon, but found none; instead, she could see forever, in all directions. The flatness of the land added to her disorientation.

"Where is the bookshelf?" she thought aloud.

The ground she was standing on was uneven, but it's equilibrium was most definitely a plane perpendicular to her own stance.

It was field, not unlike any she had seen before. There was nothing in the distance, though. It simply continued on forever.

It was a strange dream no doubt. It wasn't scary or stressful, but it was... eerie. She looked down at the ground, but instead of the usual, uncomprehending and probably blissful insects, all she found was... grass. This wasn't normal. There were bugs there, just a moment ago, and the princess knew it. Where had the bugs went? Were they crawling on her?

She turned around and looked on her back, and lo and behold... there were none. She was relieved. She hated bugs crawling on her.

She felt something on her thigh, and arched her neck down to see it. It was a blade of grass. It ticked her, and made her uneasy.

Then, crawling from the blade, unto her coat, she found a bug: a large, orange and black bug. There were so many bugs now that came from the grass.

The princess wanted to scream, but something kept her from using her voice. It was an awful feeling, truly, to be helpless with nopony around.

And just as it began, it stopped. She turned and saw a comforting face, just as she always had when the bugs crawled on her: her sister, Luna. Luna smiled at her sister, and her sister smiled back. It was perfect.

"Sister, thou art dreaming. There is no need to worry," said the midnight blue pony.

"What? What art thou speaking of, sister? The bugs were on us."

"Where art thou, then?" asked Luna. She knew her sister was not coherent. It was still funny to mess with her, though.

"We are in the field by the castle. The grass needed us to sing it a song, just as the birds do every morning," the white one said in a sing-song voice.

Her sister chuckled at the nonsense. It was just so fun messing with her sister in a delusional state.

"We were about to sing our song, until thee showed up. What art thou doing in the field, Luna?"

"We wished to speak to thee after all this time apart..." she said longingly. Luna then used her magic to “awarify” her sister, as she liked to call it.

"What...? Luna, what art thou doing here?..."

"It has taken us so long, sister. But have found thee. We wish to speak with thee."

"And? Get on with it..." Celestia said reluctantly.

"Dost thou seest this place?"

"Yes?"

"'Tis not of thine own mind, sister. 'Tis of ours."

"What be thine point?"

"Sister, dost thou knowest what is special about this place? Look into the distance."

"...There is no true horizon. Again, what is thine point?"

"Dost thou knowest why there is no horizon?"

"Because... the plane is flat...?"

"Thou art correct."

"We fail to see thine point, sister."

"Yes... what would happen if thou went in that direction, and thou never stopped?" Luna pointed her hoof some miscellaneous may.

"Then, we suppose, thou would reach the end," she quickly correct herself, "We mean, we mean, thou would go on forever."

"Yes, and would thou ever be able to explore that direction in eternity?"

"Well, of course! It would take an entirety, though."

"Yes. So then is it possible to explore that direction as well as that direction?"

"Well... I mean...."

"The answer is no, sister. If thou went in one direction, attempting to explore it, and thou could not turn back, thou willst only know one direction."

"We still do not see thine point, sister...." But she did. She knew exactly where her sister was going with this, and she did not like it.

"Sister, is it not obvious? Thine mission is silly! Thou cannot know everything, so stop trying! Even if thou couldst explore the universe, thou cannot explore the infinitesimally small, no matter what the circumstances.

"Please, sister, come back to us! No, to me; come back to me. We cannot do this without thee. Please, sister!"

Luna's pleads took a chisel to Celestia's heart and threatened to shatter it completely after Luna had already cracked it with her star banner. She couldn't take it, and began to get defensive. "'Tis not about thee or some silly quest for knowledge... it's about us, and... me, being happy. We are not happy... I mean, I am not happy."

"No, sister, thou spoke correctly the first time. We are not happy, neither one of us."

"'Tis because thou art pestering us! We do not need thee! This is not thine problem, it is ours! Go away and rule the kingdom, thou... thou... idiot!"

Luna scoffed, but still tried to keep her demeanor. "That was... uncalled for, sister. We only want thee back, with us. Please. We love thee deeply, and it pains us that thou cannot be here, where thou belongs."

"How dost thou knowest where we belong?"

"Because the kingdom is failing without thee, sister! Please, the ponies do not trust us. How could thou hast thought they would? They still hate our night, and now they suspect us of treason because of thine absence! Is that what thou wanted? Is it? Tell me."

"'Tis not...."

"Then come back, please...."

Celestia did not reply.

"Please, sister. We cannot live without thee. Please. We plead with thee."

Again, Celestia did not reply.

"If... if we admit thou art right, willst thou return? Please?"

Her sister was usually unusually stubborn, so Celestia knew Luna was desperate. Without this dialogue, though, even in the dreamscape, Celestia saw it on her face. She was worried and sad. Worried for... her. Luna was worried for her. And Celestia wanted to help Luna, too. "We... we…” Celestia stammed.

"Please?"

"Yes."

Luna swallowed. "Thou art not lying?" she asked, hesitantly.

"Yes.... We will return—for thee—sister."

"Are we dreaming?" Luna joked, slightly ruining the moment.

Celestia chuckled. "We will return, we promise, sister. We are sorry that we have hurt thee, it was stupid. We see that now."

"Thank thee, sister, thank thee. Can we... fix the sky, now? Art thou really coming back?"

"Yes, sister, we promise. We will return."

"When can we expect thee back?"

"We think it to be only a few days. Do not worry, we will not lie to thee."

Celestia walked over and hugged the younger sister, to which she responded in the same manner.

After several long moments, they stepped out of the hug.

"We will wait for thee," was the last thing Celestia heard before she woke up with a start.

She only remembered bits a pieces of her dream, but the message was clear. She needed to go home.

Author's Note

If Luna hadn't said that you couldn't turn back, the obvious solution to the "exploration" problem mentioned above is to spiral outward, but Luna was just trying to get her point across. Of course, there are infinities which cannot be listed, such as listing all the decimals (known as uncountable infinities), but that's a little hard to visualize. (aka I'm too lazy to think of a physical analogy)
I guess you could say that, technically, spiraling is cheating or improper, as your "center" is not actually going to occupy every space on the plane after infinite time, but who cares, honestly? Besides, that kinda falls in with listing the decimals, so... I don't even know anymore, do what you want with it!
Ugh... I hate ad hoc stuff.

And, wow, this took a far different turn than I was expecting... so... uh... sorry?