Twilight Sparkle finds herself being led to question things she's never questioned and try things she'd never tried by Rarity. But slowly Twilight begins to feel like she's being corrupted, and begins to wonder if Rarity is a vampire.
With the discovery of Twilight and Rainbow Dash being sisters and the return of Princess Starry Night, Equestria has a lot to take in and adjust to TWO new princesses.