There was a time when a unicorn named Shining Armor and his followers were known as the "Rainbow Six." He was the first unicorn to wear a crown, and he was the ruler of Equestria. But that didn't last long. His heart was inspired by stories of the evil Discord and King Sombra were and he decided to bring them back into existence. To do this, he gathered all the ponies of Equestria together in one place so that they would be easier for him to control. He had everypony wear a crown and make the sign of friendship with their hooves as they waited for his magic to take effect. It didn't work like it should have. Instead, everyone's powers turned against each other!
To make matters worse, Shining Armor and his followers was forced to flee, but not before he told Nightmare Moon about what happened. She was furious at her subjects' betrayal and couldn't wait to get back to Ponyville and teach them a lesson. But she wanted to ensure there wouldn't be another chance for them to turn on her again, so she ordered the entire population of Equestria to sleep. When they woke up, everything would be changed forever...
***
Nightmare Moon's magic took effect immediately upon its release, and Twilight Sparkle and her friends found themselves banished from their home world. They tried to find a way back, but nothing worked. The only pony who could help them was Princess Celestia, but she refused to do anything until the spell was broken. Her brother had forbidden it for good reason: if anypony was ever able to return to Equestria, they'd never leave again.
For several months, Twilight Sparkle led her friends on an epic quest through time, space, and alternate dimensions, trying desperately to find a way back. Along the way, the ponies discovered a world where the Cutie Mark Crusaders had been cut out of reality because of their lack of cuteness. In the same place, they rescued a dragon named Flurry Heart from the clutches of a murderous unicorn named Raisin Cake. As luck would have it, they soon met up with a purple-maned unicorn named Scootaloo, who looked very familiar to Twilight...
They also journeyed to a land that was ruled by a group of griffons known as the Griffon Kingdom. There, they helped a royal pegasus named Chocolatine become the princess she always wanted to be. And then they went looking for the legendary Master of Masters, a pony who had once saved Luna from the moon. Finally, after many trials and tribulations, they came face-to-face with Princess Celestia herself.
In the end, however, the ponies couldn't convince her to break her brother's law. So instead, Twilight Sparkle and her friends set out to find a way to break it themselves...
***
Nightmare Moon took full control of Equestria when she used her magic to force everypony to make the sign of friendship and then turn on each other. She couldn't let anypony escape, even Princess Celestia. And that included Twilight Sparkle, who was caught by the spell and put under Nightmare Moon's power.
The evil princess transformed Twilight Sparkle into her own version of Princess Cadance and made her go undercover as the princess's assistant. She forced Twilight Sparkle to learn everything she could about her new role as princess, so that someday she would be ready to rule Equestria alongside her sister. But there was one thing Twilight Sparkle wasn't willing to give up: her friendship with Spike. Despite the fact that he was now Nightmare Moon's minion, she still had feelings for him, and she kept going back to visit him in the dungeon.
Meanwhile, Princess Celestia was struggling to keep the ponies from turning on each other. All the while, she was being watched by a mysterious figure known only as the Black Prince.
When Twilight Sparkle finally managed to free herself from Nightmare Moon's spell and regain her true form, she realized how much she missed her friends. And when she reunited with them at the library, she knew exactly what she needed to do: she needed to break her sister's spell.
First, though, she needed to figure out what had made her so powerful. That meant she needed to study the magic book that Twilight hadn't read yet — the magic book that had mysteriously appeared in Princess Celestia's palace. After reading it, Twilight came to understand that the spells within it weren't meant for her at all. The magic book was created to be used by a unicorn whose name started with the letter T and ended with an R.
That explained why Twilight Sparkle hadn't been able to cast any of the spells in the book: she wasn't supposed to be doing it! Now that she knew what she was supposed to do, she figured it shouldn't be too hard to figure out how to undo the spell that had taken over Equestria. But before Twilight Sparkle could get to work, she had to deal with Nightmare Moon.
The evil princess was planning to use the Elements of Harmony to turn herself into the ultimate magical creature: the Trixie. She would then use her new body to destroy the world and take Equestria for herself. But before Nightmare Moon could begin casting her spell, Twilight Sparkle managed to trap her in an enchanted mirror.