Grandmother Aggie sweeps granddaughters Marnie and Sophie off to Halloweentown to investigate a series of strange occurrences. When they arrive, the vibrant community seems to have lost its magic, and its once-colorful citizens have become gray—literally—and boring. But why? It isn't long before Aggie discovers that her spell book has been stolen, and the trio sets out to find it. They discover than an angry warlock plans to permanently turn every human into whatever costume they are wearing at midnight on Halloween and has taken the book so Aggie can't use it to reverse his spell. It's a race against time, as Marnie must use her skills to defeat the young warlock threatening to change Halloweentown for eternity.