"Hello my dear, I am Discord, and welcome to the Velvet Room"
It's the summer of 2012 in Equestria, the mare known as Twilight Sparkle moves into a new house in the town of Ponyville, where she wakes up in the middle of the night and finds herself going outside and entering a massive tower at the center of town.
She wakes up the next day, assuming it was all a dream, but she feels a burning sensation on her hoof, which turns out to be a changing burn mark, almost like a long clock, ticking down from "364 Days" she finds out that all ponies in town have these marks, so she must gain allies and fight through the tower to find out more about the mysteries of Ponyville and, possibly, save them all.

(Inspired by the Persona series of games, which I recently got into, you don't have to play the games to enjoy the story, but it'd help in understanding the concepts and elements present here.)
(Rated T for minor swearing and some innapropriate content)