After breaking her leg in the middle of the Everfree Forest, Applejack is forced to take shelter with Rainbow Dash in an abandoned cabin in the woods. Historically, this sort of thing has only ever ended well.
In the years since Luna returned from the moon, she’s always been quiet about the truth of her exile. Always keeping it to herself, hidden away. Keeping secrets. She never even told Celestia that something was up there with her.
Derpy's father says there's a monster in the basement. He says she must never go down there or it will gobble her up. She decides to investigate anyway.