Teenager Sarah Quinn (Madison Iseman) lives with her mother Kathy (Wendi McLendon-Covey) and younger brother Sonny (Jeremy Ray Taylor) in Wardenclyffe, New York. Sarah is attempting to get into Columbia University by writing an essay about fear. Kathy agrees to watch Sonny’s friend Sam Carter (Caleel Harris) while his father goes out of town.
Sonny and Sam are attempting to start a garbage cleanup business, and are called after school to clean up an abandoned house. Inside, they find a locked manuscript and open it, causing Slappy the Dummy (Mick Wingert) to appear. They unknowingly bring him to life by speaking magic words found in his pocket. The book is later stolen by a bully named Tommy Madigan (Payton Wich).
At home, Sonny is working on his science project, a miniature version of Nikola Tesla’s Wardenclyffe tower. Slappy then reveals he’s alive to Sonny and Sam, and gets their trust by using his magic to do the boy’s chores and homework. Meanwhile, Sarah goes to a party to meet her boyfriend Tyler (Bryce Cass), but sees him kissing another girl.
The following day on Halloween, Slappy uses his powers to attack Tyler and sabotage Sonny’s science project, making the group realize he’s evil. The kids capture Slappy and try to dispose of him, but he escapes. Later that night, they find an article online about the events that happened at Madison, Delaware and try to contact R.L. Stine (Jack Black) after realizing the book they found was an unpublished Goosebumps manuscript called Haunted Halloween. Stine, now living in a cabin in the woods, hears their message and departs for Wardenclyffe.
Slappy goes to a local drug store, and uses his magic to bring Halloween costumes and decorations to life. He also transforms an employee named Walter (Chris Parnell) into a hunchbacked ogre. He then travels to the Tesla tower, using it to channel his magic.
Sarah, Sonny and Sam retrieve the book from Tommy’s house while all the town’s Halloween decorations come to life. They soon discover that the book can trap the monsters inside. However, the book is stolen and Kathy is kidnapped by the monsters.
The kids’ neighbor Mr. Chu (Ken Jeong), a Goosebumps fan who is thrilled to be trapped in a living Goosebumps story, helps them craft monster disguises to safely navigate the town. The kids head for the Wardenclyffe tower while Stine arrives in town.
At the tower, the kids encounter Slappy and Walter and discover that Slappy has turned Kathy into a living dummy. Sonny and Sam overload the reactor while Sarah fights Slappy. She defeats Slappy by kicking him into the electrified coil atop the tower and opens the book, combining it with the reactor’s energy to suck all the other monsters into the Haunted Halloween manuscript. Kathy and Walter return to normal. Stine arrives and congratulates the kids for defeating the monsters, as well as offering Sarah writing advice for her essay.
Sometime later, Kathy and Walter start dating, Sonny wins the science fair and Sarah is accepted into Columbia University. Back in Stine’s cabin, he finishes a new book. Slappy appears and reveals that he survived and wrote a book of his own where Stine is the main character. Slappy then opens his book and sucks Stine inside.
Starring: Wendi McLendon-Covey, Madison Iseman, Jeremy Ray Taylor, Caleel Harris, Chris Parnell, Ken Jeong,