Twilight Sparkle read a book about two unicorn sisters that controlled the raising of the Sun and Moon in Equestria. Over time, the sister responsible for raising the Moon started to resent the other sister that was in charge of raising the Sun. Although the citizens of Equestria loved playing and enjoying the day her older sister brought forth, they ignored and slept through her beautiful night.
One night, the younger sister refused to lower the moon. The older sister tried to reason with her, but it was of no use. The younger sister's bitter heart transformed her into the wicked mare of darkness, Nightmare Moon. Reluctantly, the older sister used the most powerful magic known to pony kind, the Elements of Harmony, to defeat her younger sister and banish her to the Moon. Ever since, the elder sister had been responsible for both raising and lowering the Sun and Moon and harmony had been maintained in Equestria ever since.
Twilight seemed to have remembered hearing about the Elements of Harmony before and rushed home to her library to investigate further. Along the way, she ran into a few of her peers offering her to join them for Moon Dancer's get together. However, Twilight came up with the excuse that she had to study. After Twilight left, the ponies accused Twilight of being more interested in books than friends.
Twilight barged into her library, knocking over her assistant Spike in the process. Twilight asked him to help her find the old copy of "Predictions and Prophecies", telling him to ignore the present for Moon Dancer that he squished when Twilight knocked him over earlier. Once the purple unicorn found the book she was looking for, she read up on the Elements of Harmony and their connection to Nightmare Moon. Apparently, on the longest day of the thousandths year, the stars would aid in Nightmare Moon's escape and bring about eternal darkness.
Twilight immediately asked Spike to write a letter to Princess Celestia warning her about the return of Nightmare Moon. Worse yet, the longest day of the thousandth year coincided with the upcoming Summer Sun Celebration that was only two days away. The Princess responded to Twilight's letter immediately, as the studious unicorn predicted. However, Princess Celestia seemed unphased by the news and suggested that Twilight should stop reading those dusty old books. The Princess charged Twilight with the task of overseeing the preparations for the Summer Sun Celebration being held in Ponyville that year. Most important of all, the regal pony urged Twilight to make some friends.
Twilight reluctantly followed (most of) the Princess' orders. The purple unicorn planned on checking up on the preparations as quickly as possible and then heading back to the library she would be staying in to find additional proof of Nightmare Moon's return. However, Twilight was convinced that the fate of Equestria did not rest on her making friends.
Twilight's first stop involved dropping by Sweet Apple Acres to ensure the food for the celebration was taken care of. There, Twilight met an orange earth pony named Applejack. After they introduced themselves, Applejack introduced the unicorn to the entire Apple family and invited Twilight to sample some of their food. Each family member had an apple-themed food stuff to present, so Twilight knew that the food situation was taken care of and said that she must be on her way. However, the Apple family managed to convince her to stay for brunch and the purple pony, after initially declining the offer, reluctantly stayed.
Next on the list of things to do was check up on the weather. A pegasus pony named Rainbow Dash was responsible for keeping the sky clear. However, there were still several clouds present when Twilight arrived. Right after Twilight commented on the poor job the pegasus was doing, Rainbow Dash herself flew right into the purple unicorn. The two landed in a muddy puddle, so Rainbow Dash brought over a raincloud to wash Twilight off and dried her by conjuring up a small whirlwind. This left Twilight with ridiculously frizzy hair causing both Rainbow Dash and Spike to laugh hysterically.
Twilight introduced herself and mentioned that she was there to check up on the weather. Rainbow Dash said she could take care of that in a jiffy, right after she finished practicing for the Wonderbolts. The acrobatic team was going to appear at the celebration the next day and the blue pegasus wanted to impress them with her moves. Twilight remarked that the Wonderbolts, the best flyers in all of Equestria, would never let a pony that could not even keep the sky clear for a single day join their ranks. Rainbow Dash said that she could easily clear the entire sky in ten seconds flat, and when Twilight asked her to prove it, she did. Twilight was in awe of Rainbow Dash's skills. Before she left, the pegasus said she could not wait to hang out more with Twilight and the unicorn continued checking on the preparations.
Decorations were next on the list and seemed to be coming along wonderfully. Twilight tried to introduce herself to the beautiful unicorn in charge named Rarity, but Rarity was horrified to find Twilight's hair in such a mess from the purple pony's earlier encounter with Rainbow Dash. Twilight was whisked away by the beautiful unicorn that Spike had suddenly become enamoured with.
Rarity took Twilight to her boutique where she made her try on a myriad of different outfits and several different hair . The fashion pony was thrilled to find out that Twilight was from Canterlot and proposed becoming the best of friends. However, when Rarity went off to get some rubies for Twilight's outfit, the purple unicorn made her escape fearing that Rarity would come back and do something horrible like dying Twilight's coat a new colour.
The last item on the list to check up on was the music. Twilight noticed a yellow pegasus carefully directing a choir full of singing birds that got scared and flew off when Twilight introduced herself to the yellow pony. The pegasus seemed a little shy so Twilight asked her what her name was. The yellow pony let out a barely audible answer that her name was Fluttershy, but when Twilight asked her to repeat her name louder, Fluttershy did not increase the volume of her voice and eventually locked up completely. It was not until Twilight mentioned leaving and Fluttershy spotted Spike that she finally spoke up. Fluttershy had never seen a baby dragon before and started giving Spike a lot of attention. Twilight started walking home and along the way Fluttershy asked about Spike's entire life story which he gladly shared with the inquisitive pegasus.
When Twilight finally made it home, she came up with the excuse that Spike needed to get his sleep and sent Fluttershy off to do other things. However, Twilight's plan to quietly spend the evening reading up on Nightmare Moon was foiled when Pinkie Pie, the local party pony, decided to throw a gigantic party for Twilight in the library she was currently staying in. Pinkie Pie knew all of the ponies in Ponyville, so when she saw Twilight, she knew that she must be new and probably did not have any friends yet and thought a party would be a perfect remedy for Twilight's loneliness. Pinkie even invited the four ponies Twilight met earlier in the day that were in charge of the various components of the Summer Sun Celebration: Applejack, Rainbow Dash, Rarity and Fluttershy. Twilight seemed less than thrilled about the extra guests in her library. After accidentally drinking a glass of hot sauce, Twilight ran up to her room in tears and in pain.
Twilight tried to get some sleep, but the party going on downstairs made it impossible to do so. Spike explained that the ponies stay up all night on the eve of the Summer Sun Celebration to watch Princess Celestia raise the Sun, but Twilight still refused to join in.
Once it was almost morning, all of the ponies gathered together to watch the Sun rise. The Mayor of Ponyville welcomed everyone and formally announced the beginning of the Summer Sun Celebration. To commence the rising of the Sun, the Mayor introduced Princess Celestia. However, the Princess was nowhere to be found. In her place, a mysterious black pony materialized out of the darkness.
The dark pony seemed a little insulted that her subjects acted as if she was not royal enough for them and that no one seemed to know who she was. However, Twilight did know who she was and revealed to everyone that she was Nightmare Moon. Content with her introduction, the Mare in the Moon revealed that this would be the last day any pony would ever see and from that point on, the night would last forever.