Three young boys, Rocky, Colt and Tum Tum together with their neighbor girl, computer whiz Amanda are visiting Mega Mountain amusement park when it is invaded by an army of ninjas led by evil Medusa, who wants to take over the park and hold the owners for ransom. Kids and retired TV star Dave Dragon, who made his farewell appearance at the park at the time the ninjas appeared, have to break Medusa's vicious plans.

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.

Marnie puts her magic on the line to get several Halloweentown teenagers transferred to her high school in the mortal world. But soon, Marnie and her new friends are in trouble and out of luck when the evil Knight Of The Iron Dagger appears. He's determined to destroy all things magical, and it's up to Marnie to save her high school, the human world and her magic.

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.