On Halloween night, three siblings — Marnie, Dylan, and Sophie — learn that their grandmother, Agatha Cromwell, is actually a witch living in Halloweentown, a community where spooky beings have separated themselves from Earth's history to escape humans' fear and persecution.
This comedy-adventure follows Marnie and her younger siblings as they follow their estranged grandmother—a good witch—to her home in Halloweentown, where witches, goblins, vampires and the like lead very normal lives. On her 13th birthday, Marnie learns that she comes from a witch family and that she, too, is a witch. When she discovers that an evil force is plotting to take over Halloweentown, she must help her mother and grandmother save the day.
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.
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.
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.