Harriet is a long-time friend of fellow witch, Aggie Cromwell. One Halloween, Harriet bumps into Aggie when she returns from the human realm. The two exchange pleasantries, during which Harriet admits that she did not have time to visit the mortals this year because she has been doing volunteer work at the Headless Shelter - having baked muffins for them, without having realized that they had no way to eat them, as they have no heads. Harriet then bids Aggie farewell and the two part ways, not realizing that Kalabar had been watching them from his office above.
Soon after, Harriet is cursed by Kalabar, becoming twisted and feral. Aggie takes note of this when she bumps into Harriet again while showing Marnie, Dylan, and Sophie around Halloweentown. Immediately she rushes off to tell Kalabar, who encourages Aggie to give him a couple of days to sort things out. Unbeknownst to Aggie, Kalabar is the one responsible for Harriet's transformation, and mere moments later he claims Harriet as one of the many creatures he has taken as his prisoners.
When the time comes, Luke guides Aggie to the Halloweentown theater, where she finds Harriet frozen in a state of paralysis. In trying to save her mother, Gwen is paralyzed by Kalabar as well, with Aggie meeting the same fate moments later, leaving it up to Marnie, Dylan, and Sophie to save Halloweentown from its impending fate.
Ultimately, the kids manage to gather the necessary ingredients for the potion that ignites Merlin's Talisman, and with Luke's help, they succeed in placing it in the pumpkin in the town square - but not before Kalabar reveals himself and paralyzes both Luke and Marnie. Harriet is then freed from her curse and leaves the theater alongside Aggie, Gwen, and the rest of the captured Halloweentown residents, witnessing Kalabar's defeat at the hands of the Cromwell-Piper family in its entirety.