As one story ends, another begins. After a disagreement with her mother, Luster Dawn decides to continue with her plans, regardless of permission, and that is to go through the mirror portal and study this strange new world.
Twilight Sparkle seemed to be the only creature to have both wings and a horn; because of this she was forced to remain hidden under the orders of her guardian, the Empress Cozy Glow.
Dealing with school rivalry? Easy. Saving your school from a magic related problem? Piece of cake. Trying to control your feelings around your crush? Twilight would rather handle another problem like the previous two than deal with that mess.