This is the sequel to the Freaky Nightmares. If you haven't read it already, I would suggest reading it to understand what is going on.

When Canterlot's hot pony of pop, Sweetie Belle, gets a very strange letter from Derpy, she knows that something has to be done, and that she won't just sit there. She has to do something to save her only friend left, and somehow think of a way to do it. But when she arrives in Ponyville, things go differently than her expectations.