She was born as a witch but moved to the human world after she married her husband, William Piper.
Marnie and her mother, Gwen arguing over why she and her younger siblings Dylan & Sophie can never go out for Halloween, and why Marnie cannot go to a costume party. Her mom offers no clear explanation.
Gwen's mother Aggie, who is an expert witch, shows up for her annual Halloween visit. The children are happier to see Aggie than Gwen is, as Aggie openly encourages the children to get more involved in Halloween.
Gwen and Aggie then get into an argument. Gwen insists Marnie will be raised as a normal person and not a witch. Aggie says she disagrees, but actually she is there for another reason: people have suddenly started mysteriously disappearing in her hometown. Gwen thinks they just moved, but Aggie says it is not that simple. Aggie desperately asks for Gwen's help, but Gwen declines. Marnie overhears the argument.
Gwen arrives in Halloweentown with plans to bring her children home, much to Marnie's objections. Gwen cannot find another bus back to the mortal world and decides to see if the mayor can do anything to help. She is shocked to see that the mayor is Kalabar, an ex-boyfriend of hers. Kalabar briefly leaves to handle another problem, Gwen and her children see Aggie walking somewhere with Luke. Sensing Aggie might be in trouble, they follow Aggie and Luke to an abandoned movie theater.
Gwen and the children enter the theater as Luke rushes out in fear. The demon freezes Gwen and Aggie.
Kalabar, noticing her, freezes her. As she is about to pass out, Marnie drops the talisman inside the jack-o'-lantern, which causes it to illuminate. This unfreezes herself and everyone trapped inside the theater, as well as severely weakens Kalabar. Gwen, Aggie, and the children confront Kalabar and use their combined powers to defeat him. The film ends with the family getting on the bus and blasting off to the mortal world.
Halloweentown II: Kalabar's Revenge
Sophie and Dylan realize that Alex, believed to be Kal's father, is actually a golem intended to distract their mother Gwen at a high school Halloween party. Kal puts his spell into effect, turning the party guests into the monsters they are dressed as, resulting in chaos. Dylan and Sophie hide from the monster humans including their mother.
Kal angrily confronts Marnie, who mocks him and demands the spell book. Kal produces slimy living serpent vines of dark magic and uses them to take both spell books in an attempt to prove his own superiority, which fails when Marnie takes them from him. Kal is sent away by the vines and the family breaks his spells in the mortal world and in Halloweentown.
Gwen uses a Witch's Glass to hunt down the missing student imprisoned by Dalloway.
Gwen has shown the evening's events to the Witches' Council. They return the Cromwell magic, Gwen returns with the imprisoned student, and the Council imprisons Dalloway in another Witch's Glass.
Return to Halloweentown
Marnie is offered a full scholarship for her good deeds and for her work in the previous years. Much to her mother Gwen's dismay, Marnie goes, and her brother Dylan is reluctantly forced along by Gwen to keep an eye on her.
Marnie, Dylan, Gwen, and Aneesa destroy the Gift. The Dominion attempt an escape, but are apprehended by Periwinkle who is revealed to be an undercover detective of the Halloweentown Anti-Dominion League.