Sunset Shimmer brings Princess Twilight Sparkle, Starlight Glimmer, and Spike the Dragon from Ponyville, to come with her to stay with their human friends from Canterlot High (except for Twilight's human counterpart, Sci-Twi, who is away on holiday) to celebrate Halloween. The Dazzlings (now reformed) turn the Girls humans into REAL monsters for Halloween - for REAL; but the subjects still have their normal personalities, and the subject will also transform into their real human form and can turn back into their selected monster form whenever they want. When the Girls were transformed, they are amazed by their new forms, and are excited to test out their new spooky powers. Because in their new forms, Twilight Sparkle (witch), Spike (in his dragon form), Rainbow Dash (vampire), Rarity (mummy), Fluttershy (ghost), Pinkie Pie (werewolf), Applejack (Frankenstein monster), Sunset Shimmer (swamp monster), Starlight Glimmer (alien) , and the Great and Powerful Trixie (lizard creature with chameleon abilities) go to the world of Monsterdum and team up with the Grimwood Girls, and new characters like a motorcycle daredevil human named Henry, a young world-traveling adventurer named Zoe, and Zoe's 3 companions who are wild cats: Jason the jaguar, Tahlia the tiger, and Leonard the lion, to defeat 2 new enemies: a mad scientist named George Sullivan who is one of the archnemesis of the Grimwood Girls, and an evil warlock named Scarface who wants to take over all of Monsterdum.
