When the Potter family moves to their new apartment, Anne asks her son, Harry, Jr., to keep an eye on his younger sister, Wendy, while the grownups unpack. As Wendy walks into the laundry room to retrieve her ball, a troll steals her identity, and inhabits her body. Harry, Jr. sees Wendy’s ball and demands that his sister stop hiding. When Wendy appears laughing, Harry, Jr. remarks that her disappearing act is not funny. When Wendy joins the rest of the family to eat, Anne notices Wendy’s troll ring, which Wendy claims to have found. Her father, Harry, Sr., serves fast food hamburgers. When her family assures her the food will be tasty, Wendy dives into her meal like an animal. As her family looks on in amazement, she snatches a bag filled with more hamburgers and runs out of the apartment, setting off the fire alarm. When Harry, Sr. chases after her, he trips over neighbor Peter Dickinson, who has slipped on the stairs. Dickerson warns that he is single and does not want noisy children to interfere with his active social life. Harry, Sr. also meets neighbors Barry “Uncle Duke” Tabor, a retired marine, and lovers, William Daniels and Jeanette Cooper. Soon, Mrs. Eunice St. Clair, another neighbor, comes down the stairs and asks if anybody has warned the firemen about the false alarm. Without waiting for a response, Eunice speaks to the fire department. Inside the apartment, Wendy creates havoc, turning boxes upside down until her parents calm her. The next morning, when Anne suggests that Wendy have something for breakfast, her daughter does not respond. Anne tells her husband if Wendy continues to act strangely, she will take her to see the doctor. Wendy opens the refrigerator, and her hand recoils when she touches a cold milk bottle. When Harry, Jr. offers to help her, she grabs him by the collar and throws him against the wall. Wendy leaves and invites herself into Peter Dickinson’s apartment where, much to his annoyance, she plays hide-and-go-seek. When Dickinson tells her he has no candy or toys, Wendy changes back into a troll, and zaps him with the troll ring. The jolt transforms Peter Dickinson into a family of trolls nestled in a strange forest. Wendy pats some trolls on the head as she leaves. Meanwhile, Harry, Jr. stops by Eunice’s apartment, looking for his sister. Eunice St. Clair has a live mushroom-like creature named “Galwin” that keeps her company while she paints. Eunice hides Galwin under a lampshade. Harry, Jr. admits he has just moved in and has no friends. His sister, Wendy, has been acting weirdly, and it scares him. Eunice invites the boy to visit her anytime. Outside, Wendy barely misses being hit by an oncoming car. Another tenant, Malcolm Malory, tells Wendy that she should be more careful, and Wendy calls Malcolm “brother elf.” In the apartment, Wendy only acts strangely around Harry, Jr., and he is only too happy to go to the store on an errand to get away from her. Wendy visits “Uncle Duke” Tabor, and throws him around the room once she transforms into a troll. Uncle Duke shoots the troll but his bullet has no effect. The troll bends the rifle in half and aims its ring at the victim. Harry, Jr. returns to the building and hears Uncle Duke’s screams. He bangs on the apartment door, but his parents do not hear him because their stereo is turned up too loudly. Wendy appears and growls at her brother. Her troll ring gives him a jolt, knocking him unconscious. Harry, Sr. carries his son to his bed once Wendy informs him that Harry, Jr. is lying in the hallway. Meanwhile, Uncle Duke’s apartment has been transformed into a lush troll world. Wendy has invited Malcolm, an English professor, to dinner. Her parents were expecting another child, and are surprised to find that Malcolm is a dwarf. Soon, Wendy persuades Malcolm to recite a poem “The Fairy Queen.” Malcolm’s recitation stirs a frenzy in the two new apartment building troll forests. Eunice feels their vibrations and blows into an animal horn to quell their powers. In his room, Harry, Jr. watches a sci-fi movie in which creatures and humans become “pod people” from the planet Mars. He suspects something similar has happened with his sister. The next day, he visits Eunice, and asks if she is a witch. Eunice admits that she started out as a princess but fell in love with a wizard named “Torak.” In order to learn more about his world, she studied magic with a teacher named Galwin. Harry, Jr. asks Eunice to teach him magic but she declines, saying there is no time. Wendy visits neighbor Jeanette Cooper and brings her flowers. They talk while Jeanette gets ready for an audition. Eunice tells Harry, Jr. that the apartment mischief started on Val Pirgis Night also known as Witches’ Sabbath, when all supernatural beings come out to party. Wendy returns to troll form and transforms Jeanette Cooper’s apartment into a troll colony. Except for some well-placed foliage, Jeanette is nude and dances seductively for the troll. Soon, William Daniels stops by to find out why Jeanette failed to show up at a volleyball game. After several knocks, the door swings open by itself. William follows the sound of Jeanette’s laughter as her apartment transforms into a lush forest. Daniels runs after the scantily clad Jeanette, playing hide-and-go-seek. Meanwhile, Wendy notices Malcolm in the hallway locked out of his apartment. She claims the door is open, and they walk inside. As Malcolm draws Wendy a picture of a bunny, he reveals that as a child he wanted to be an elf and live in a land of unicorns, dragons and flying horses. Doctors tell him he is seriously ill but he still waits for the day he can live among other special creatures. Wendy is touched by his story and transforms into a troll. She turns Malcolm into an infant troll surrounded by forest. Eunice knocks on Jeanette’s door but gets no answer. She discovers most of the neighbors are missing. Later, as a troll sneaks into Eunice’s apartment, she is ready and vaporizes the creature with her magic trident. The next day, Harry, Jr. tells Eunice that he saw Wendy at night as a troll. Eunice explains that a long time ago fairies and humans controlled the Earth together. When the fairies wanted more power, there was a war, and they lost. Torak turned into a troll and during the seventy-two hours of the Witches’ Sabbath, he is transforming the various apartments into fairy worlds that will form a single fairy universe that will eventually have the power to burst forth into a fourth dimension. Eunice explains that Wendy is being held captive somewhere so Torak can anoint her princess of the fairies. Eunice gives Harry, Jr. a magic trident to destroy Torak, and warns that his adversary will shapeshift into something big, mean and nasty. Meanwhile, Eunice lets down her hair, and transforms into her younger self. She warns Torak not to harm Harry, Jr. or a terrible fate will await him. However, Torak captures Eunice’s trident and chases her out. His cronies force Eunice back to her apartment, and Torak zaps her with his ring. Harry, Jr. hears the commotion, and discovers Eunice’s apartment has been taken over by the fairies. Now in the shape of a gnarly tree, Eunice orders Harry, Jr. to save his sister. In Eunice’s fairy world apartment, Harry, Jr. hears voices calling his name. He finds Wendy sleeping in a glass coffin surrounded by a force field. The trident disarms the field and Harry, Jr. carries Wendy away. However, several menacing creatures chase the Potter children. Outside, bystanders watch in horror as the building sways and leafy tentacles emerge from the roof. Torak warns the largest creature not to harm Wendy but it does not listen, and Torak impales the creature with Harry, Jr.’s magic trident. The strike gives Harry, Jr. and Wendy enough time to escape and return to their parents. Reunited, the Potter family peers outside their front door to find all the foliage from the fairy world gone. As they pack up their belongings and load them in the car, a young Eunice compliments Harry, Jr. on a job well done. After the Potters and bystanders leave, two policemen search the apartment building. When one officer opens the door to the laundry room, he sees a fairy world. Torak zaps him with his troll ring and the laundry door slams shut.