The story is pretty weak and pointless.
1.Sunset Shimmer stole the crown. So what?
She goes to ower world and loses the crown for a contest. Wat?
Why didnt she keep the crown at the first place instead of trying to win it?
2. Twillight competes againt Sunset on a popularity contest
Wat? Twillight isn't even a student. This is bullcrap
3. Twillight's element is enought to give enought power to Sunset to conquer Equestria?
Sunset says she wants to rule over Equestria and that's the reason why she stole the element.
Well. Can you please explain WHY DAFUQ DID YOU returned to owner world, lost the crown just so you can get it again instead of staying on Equestria at first place? Btw. Is Twillight's element enough for such task? Cuzz' there was an episode were she needed to rescue the other ponies from Discord's spell so she could turn him into stone.
Oh. "Principal Celestia, I just found this crown on the floor. I think someone lost it"
"Oh. I know. I'll use it as a reward for the winner of the gala instead of waiting for someone to ask for the crown"
4. No ponies could go with Twillight to the other dimension because...
Some bull crap excuse that noone cares about and it's there just to fill an empty space.
5. "You can only travel on this portal for the next 3 days, then only after 30 moons"
I TOUGH YOU CONTROLLED THE MOON POSITION LUNA. ARE YOU A MORON?!
By the way, didn't anyone else on the school even found the obvious portal that is open 3 times per month? Wow
6. "I couldn't ask for the crown because I would be forced to say that I'm actually a Pony"
You know what? I give up.
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There are many other problems in the movie. Starting with the music that is an insult for the series, just like the entire storyline. Good thing we can make a theory in were Pinkie Pie is a psychic since she in her human form knows that Twillight is actually a Pony and in the end she knows the love storyline between Twillight and the other blue haired guy