Sexy but unstable wife and mother Jude walked out on her family three years ago. Now, just as suddenly, she is back. But her husband, Robert, has fallen in love with Callie, an assistant principal at his sons' school. He asks Jude for a divorce. She responds by trying to turn her three boys against Callie, then by slashing herself and blaming her rival and finally by drawing her 12-year-old, Kes, into a murderous plot.

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.