On her 13th birthday, Marnie learns she's a witch, discovers a secret portal, and is transported to Halloweentown — a magical place where ghosts and ghouls, witches and werewolves live apart from the human world. But she soon finds herself battling wicked warlocks, evil curses, and endless surprises.

The Cromwell clan lives in the real world, except for their grandmother who lives in Halloweentown, a place where monsters go to escape reality. But now the son of the Cromwells' old enemy Kalabar has a plan to use the grandmother's book to turn Halloweentown into a grey dreary version of the real world while transforming the denizens of the real world into monsters.

Marnie Piper prepares to begin a new school year, she asks the Halloweentown Hot Witches' Council to work toward openness between Halloweentown and the mortal world. She proposes to bring a group of Halloweentown students to her own high school in the mortal world.

As Halloweentown prepares to celebrate its 1,000th anniversary, Marnie Piper and her brother Dylan return to Witch University, where trouble is in session from the Sinister Sisters and from someone who's plotting to use Marnie's powers for evil.