Princess Twilight Sparkle of Equestria went into the human world to get her crown back, but it was a confused human Twilight Sparkle who returned to Equestria. Now the former human is stuck as a blank flank unicorn, and the element of magic is in the human world for the next thirty moons.

Can you say, Equestria is doomed?

Based on Equestria Girls but you DON'T need to have seen the movie to understand what's going on. Also I started to write this story just after the first movie, hence why this in an alternate universe and has differences with canon in later Equestria Girls movies. And is not like the premise of the story, that the way the mirror works is different, is not an AU to start with.