On her sixteenth birthday, Sarina Slyfoot will be sacrificed to the Jack 'O'Skins, the terrifying monster who rules within the Veil, a barrier of mist that has imprisoned the elves of the Mire for over a thousand years. Rules are rules after all. The elves hate humans and Sarina is no exception. Suddenly, Sarina is made an offer by Alaina Grendulf, defacto ruler of the Mire, who will allow Sarina to remain in the Mire for one teeny-tiny favor: help Alaina destroy the Veil and Jack forever. Seems like an offer Sarina can't refuse, except Alaina is hiding a vital part of her plan, one that carries a heavy price. It'll take all of Sarina's cunning to solve the mystery of the Mist before she becomes its next victim... Welcome to the Mire, a land of magic and mayhem, where secrets never die, they only sleep beneath the blood-soaked soil of this enchanted swampland. And where the real monsters aren't the ones in the Mist.

Nightmares and Dreams are actual creatures who invade human minds as embodied fears and joys. But with the new BlissMax pill (guaranteed to give you good dreams all night!) the Nightmares are starving. Violet doesn't use BlissMax (or any modern niceties, thanks Dad), and one night, a Nightmare—the fear of hell—enters her enticingly un-medicated mind and gets trapped. Though Violet has a penchant for creepy things, having a Nightmare stuck inside her mind causes problems: narcolepsy and yet further distance from reality, to name a few, not to mention the Nightmare begins to starve in the wake of fresh human fear. In a misguided attempt to get him home, she gets dragged into the city of Nightmares, Halloweentown on steroids. Once there, it becomes apparent the Dreams and Nightmares hate each other and getting back (alive) only tips the list of growing complications.