Chrysalis kidnaps Rainbow Dash and holds her for ransom. This same Rainbow Dash also happens to be engaged to the immensely powerful Alicorn Princess Twilight Sparkle. Chrysalis is not a very smart changeling.
Princess Twilight Sparkle wakes up to find herself in an alternate Equestria, where everypony is nice, nothing ever bad happens and everything is just 'perfect'. Naturally the place bores her to death.
Just when everything seems like it's just fine in Equestria for a change, something begins to threaten the entire universe - and only two ponies are capable of preventing it.
Tirek threatens to take over all of Equestria with his magic-stealing powers. In order to keep him from obtaining their alicorn magic, the four princesses must hide their power away... in an old potato.
Twilight Sparkle just loves the new dress Rarity gave her for her birthday. So much so, in fact, that she's got to get her groove-thing on. Naturally, this leads to horrifying consequences.
Twilight is angry. Normally, a more calm approach would be useful, but when a pony... When a pony commits THAT, well, the chips are off the table. Justice must be extracted.
What happens if almost one hundred Fimfic users get together to write a single story simultaneously? This happens! Also Lord Apep ate the Sun, and Twilight should probably do something about that.