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.
Retirement has not been kind to Celestia. Pushed by boredom, she disguises herself as an average pony, and she heads west. Unfortunately, she's picked up a traveling companion that was not a part of the plan.