After Marnie, Dylan, Sophie, Gwen, and Aggie defeated Kalabar, Aggie moved to the human realm to be with her family. Having spent the last year away from Halloweentown, Aggie begins to miss her old friends, and after fighting with Gwen about taking Marnie to Halloweentown for a year to be properly trained as a witch, Aggie uses her crystal ball to check in on Astrid. At the time, Astrid is going door-to-door, asking for donations on behalf of the Halloweentown school - ultimately arriving at Gort's house. Though optimistic, Astrid is ,as expected, denied a donation.
Soon, Kal, Kalabar's son, arrives in Halloweentown and casts the Grey Spell, which takes effect on Astrid and makes her dull and grey - just as the creatures of Halloweentown view mortals. When Marnie and Aggie to investigate what is going on, they run into Astrid, who complains about the vibrant nature of her shoes - though they quickly transform, as the Grey Spell finally takes complete hold of Astrid.
In the end, the Piper-Cromwell family defeats Kal and returns to Halloweentown to set things right after permanently opening the portal between the realms and saving the human realm from the Creature Spell. Astrid is then returned to her light-hearted optimism, and joins Gort as the rest of the town celebrates their liberation.