In the small town of Wardenclyffe on Halloween Night, two boys named Sonny and Sam find a manuscript in an abandoned house that was previously owned by R. L. Stine called "Haunted Halloween." When they open it, they release Slappy who plans to create the Halloween Apocalypse with the help of his Halloween monster allies. Now, Sonny and Sam, alongside Sonny's sister, Sarah and Stine himself, must work to thwart Slappy's plot before all is lost.