Two years after Halloweentown II: Kalabar's Revenge, Marnie Piper (Kimberly J. Brown) prepares to begin a new school year. She asks the Halloweentown Council to work toward openness between Halloweentown and the human world. She proposes bringing a group of Halloweentown students to her own high school in the mortal world. The Council is initially apprehensive, mostly due to the legend of the Knights of the Iron Dagger: a fanatical knighthood who wanted to destroy all things magical. The Halloweentown Witches' Council, however, agrees to accept the plan after Marnie mistakenly bets "all the Cromwell Magic (paranormal)" that her plan will work. If she does not prove she is right by midnight on Halloween, then her entire family will lose their magical abilities. Marnie, regretting what she said to the Council, wants to cancel the program she has made, but her grandmother Aggie (Debbie Reynolds) opposes, saying that they have nothing to fear.
Every DCOM ever aired... minus the commercials.
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.