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 unknowingly release Slappy, who plans to cause a Halloween Apocalypse with the help of his monster allies. Now Sonny and Sam, alongside Sonny's sister, Sarah, must work to thwart Slappy's plot before all is lost.
- Jeremy Ray Taylor as Sonny Quinn
- Madison Iseman as Sarah Quinn
- Caleel Harris as Sam Carter
- Wendi McLendon-Covey as Kathy Quinn
- Chris Parnell as Walter
- Ken Jeong as Mr. Chu
- Avery Lee Jones as the voice of Slappy the Dummy from Night of the Living Dummy.
- Jack Black as R. L. Stine
- Frank Welker as the voice of the Gummy bears.