In the small town of Wardenclyffe, on Halloween night, two boys, Sonny and Sam, venture into an abandoned house during a junk collection job. While exploring, they uncover a hidden, locked manuscript. Unbeknownst to them, the book is a magical artifact that once belonged to R.L. Stine. Ignoring the warnings, they unlock the book, accidentally releasing the vengeful ventriloquist dummy named Slappy. Slappy's plan is to bring all of Halloween decorations to life and induct the boys into his family of monsters, creating chaos in the process.

Meanwhile, Sonny's sister, Sarah, is struggling with her college application essay, the typical stress of a high school senior. The siblings, along with Sam, soon realize the gravity of the situation caused by Slappy's release. As bizarre, supernatural disturbances increase across town, Sarah's desire for a normal Halloween and future aspirations are sidelined. The trio must figure out how to thwart Slappy's plot before the dummy's dark magic spirals further out of control, threatening to consume the town in eternal darkness.