After his father’s recent death, Zach Cooper and his mother Gale move from New York City to the peaceful town of Madison, Delaware. While settling in the neighborhood, Zach meets his neighbor Hannah, but her overprotective father warns him to avoid contact with them. The next morning, at Madison High School, where Gale works as vice-principal, Zach befriends Champ, a clumsy student. That night, Hannah invites Zach to an abandoned amusement park where they start to befriend. However, when they return home, Hannah’s father warns again that Zach stay away or something bad will happen.
The following night, Gale leaves to supervise a dance at the school, leaving Zach with his aunt Lorraine Conyers. Fearing that Hannah is in danger, Zach tricks her father into going to the local police station while he and Champ enter in his home. While investigating, they find a bookshelf with numerous manuscripts, each one cataloging entries from the Goosebumps franchise and mysteriously locked. In response to Champ’s curiosity, Zach unlocks The Abominable Snowman of Pasadena, releasing the story’s titular monster. With Hannah’s help, they track the creature to a local ice rink where Hannah’s father appears and reimprisons it in the manuscript.
Hannah’s father unwittingly reveals that he is R. L. Stine, the creator of the Goosebumps franchise, who initially wrote the stories to cope with severe bullying, but the stories’ monsters eventually became real as a result of his imagination, forcing him to imprison them in their manuscripts. Back home, the group encounters Slappy the Dummy from the Night of the Living Dummy saga, who has now been freed from his manuscript. Seeking revenge on Stine for his imprisonment, Slappy incinerates his manuscript and flees with all the others. Stine and the kids are attacked by the lawn gnomes from Revenge of the Lawn Gnomes, being forced to escape. Slappy begins releasing several monsters, causing panic and destruction around Madison. Meanwhile, Lorraine is attacked by Fifi the Vampire Poodle from Please Don’t Feed the Vampire!.