Twilight Sparkle has made the biggest mistake of her life, letting Trixie do research. Before she knows it, Trixie has used an experimental spell and accidentally switched places with her, a legal complication has made Trixie a princess, and things go downhill from there.

(Woo! A sequel to a sequel! You don't really have to read the first part of the series, but you kind of do need to read Equestria's Got Talent or this will be pretty confusing :P)