No, wait. That's the parody trailer. The real trailer is here.
In the movie, a villain named Sunset Shimmer steals Twilight Sparkle's crown. Sunset then escapes through a mirror/portal to the human world. Twilight has to chase after Sunset Shimmer and retrieve the crown, but there's a catch. She has to get the crown within three days, or she'll be stuck in the human world for a month.
The human world ends up being Canterlot High School. Twilight doesn't know how to use her new body or anything else in this strange new world, but she manages to track the crown all the way to the lost and found box in Principal Celestia's office.
Since no one has claimed the crown, Principal Celestia is planning to give it to the
Twilight has a tough task ahead of her, considering the fact that Sunset Shimmer is the most popular girl in school. As a villain, Sunset Shimmer is sort of a toothless tiger; she spends most of her time making evil faces and vague threats. The real villainy is committed by Sunset's two goons. They use their camera phones to record embarassing footage of Twilight, and the video gets spread all over school.
In the meantime, Twilight meets the human versions of her friends, who are all fighting with each other. Twilight has to re-introduce herself to them and solve their friendship problem. It turns out that the fight is over some bogus text messages that were sent from Pinkie's cell phone. The five girls apologize for the mixup.
Now that our six heroes are friends again, they work together to put on a musical number in the school cafeteria. This boosts Twilight's popularity, and it looks like she might win the contest after all! At the very least, she's caught the eye of the random hot guy in school that she bumped into earlier.
Sunset Shimmer's goons destroy the school gym, and they frame Twilight for the crime. For about half a minute, it looks like Vice Principal Luna is going to expel Twilight for trashing the gym, but the random hot guy shows up with proof that Twilight is innocent. She hugs him, and he asks her to be his date to the dance.
Twilight and her friends do another musical number, while cleaning up the destroyed gym. The entire school pitches in, and the dance gets un-cancelled. All the students are happy about this, and a lot of them cast votes for Twilight as the most popular girl in school.
Everyone leaves school to get ready for the dance. Twilight has a minor ethical dilemma at this point, and she decides to tell her friends that she is really a pony princess from another world. At first, they think she's crazy, but she manages to prove her story with her talking pet (Spike). All six of them once again agree to be friends forever.
They all go back to school for the dance. It is revealed that Twilight won the vote for Princess of the Fall Formal. She gets her crown back, but then a lot of drama happens. Sunset Shimmer's goons kidnap Twilight's pet, and Sunset threatens to destroy the portal back to the pony world. Things look bad, but Twilight is inspired by the power of friendship, and she makes a passionate speech about how she'd rather be stuck here forever than let Sunset win.
A mad scramble for the crown ensues, and Sunset gets her hands on the crown. She uses it to turn herself and her goons into giant dragon-like monsters. She also turns everyone in the school into her mindless zombie slaves. Don't ask me how or why this happens. The movie doesn't explain. Now that Sunset has conquered the human world, she vows to conquer the pony world.
But wait! When Sunset tries to use her evil magic on Twilight and the others, it has no effect! That's because they're protected by the power of friendship! Twilight makes another passionate speech about friendship. Twilight and her friends somehow make a rainbow-colored cyclone of friendship. They also turn into strange half-human, half-ponies here, for no particular reason. I'm not kidding. All of them grow tails and pony ears, while three of them grow wings. At no point does anyone express any surprise over the fact that six people have become horse women.
The friendship cyclone turns everyone back to normal. Sunset Shimmer starts crying like a maniac, and she apologizes for everything. Twilight explains that Sunset Shimmer isn't really a bad person. She was only being mean, because she doesn't have any friends. Everyone agrees to be Sunset's friend from now on.
Now that the villain has been defeated, everyone celebrates at the dance. Afterwards, Twilight says goodbye to all of her new human friends. She returns to the pony world, where she meets the pony version of random hot guy. Her friends tease her about him, and the movie ends.