The Potter family move into a new apartment complex in San Francisco. While unpacking, their young daughter Wendy is attacked by a grotesque little creature, who had long ago been transformed from a powerful wizard into a troll. Using a magic crystal green ring, he captures Wendy and possesses her form. After meeting the other eccentric tenants, the family notices Wendy's unusual behavior (roaring, biting, tossing people across rooms, punching people in the groin), but they attribute her behavior to the stress of the move. The only one that notices something is terribly wrong is Wendy's brother, Harry Potter Jr.