Harry Potter Sr. and his wife Anne are moving into a rented apartment in San Francisco, bringing their son Harry Jr. and daughter Wendy along. While the family settles in, they encounter a mysterious figure on the sidewalk. Wendy goes to the laundry room to get her uniform, only to meet a troll named Torok, who uses his magic ring to possess her and transform her into his own form.
Wendy struggles to break free from the troll's control, but it is too late when she realizes what has happened. The troll takes over her body, transforming her into one of its kind and using her form to create an alternate reality where all inhabitants are transformed into trolls.
Harry Jr. notices something is wrong with his sister and seeks help from a good witch named Eunice St. Clair, who lives in the building next door. Eunice quickly recognizes the danger and helps them escape from the troll's realm.
As they make their way back to the apartment, they encounter other residents of the building, including a group of goblins who have also been transformed into trolls by the evil wizard Malfoy. The Potter family must work together to defeat the wizard and restore order to the city.
With the support of Eunice and the other residents, the Potter family is able to defeat the troll and restore peace to San Francisco. They return home to find that their new neighbors, the Goyles, have also been affected by the troll's transformations and are now living in their apartment.
The Potter family decides to move into the Goyles' apartment, hoping to start anew in a safe and welcoming environment. As they settle in, they discover that the troll's realm still exists, but they are determined to fight against it and protect their new neighbors.