Princess Twilight's friends are missing, sucked into a TV world where all of their deepest secrets and insecurities are physically manifest. Dark, cartoonish versions of each mare await Twilight Sparkle and if she doesn't save each one they'll be consumed by the darkness of their own hearts. The bizarre rules of the TV world have no sense of up or down, but the shadows waiting to stop her certainly want Twilight to see her friends' hearts laid bare before they're destroyed.

(Persona 4 Crossover: No knowledge of the "Persona" series required to enjoy this. I'm just borrowing their over-arching concepts.)