Turns out Rainbow Dash and Rarity are a married couple, and have been since before Twilight moved to Ponyville. Twilight never knew this. Hilarity ensues, as well as a few feels.
Looking for a way to get over her fear of public speaking, Sweetie Belle borrows an instructional book from Twilight's library to help out. One thing leads to another and some how ends up binding King Sombra to her as her shadow...