Minnesota, 1990. Detective Bruce Kenner investigates the case of young Angela, who accuses her father, John Gray, of an unspeakable crime. When John unexpectedly and without recollection admits guilt, renowned psychologist Dr. Raines is brought in to help him relive his memories and what they discover unmasks a horrifying nationwide mystery.

The story follows a family of inbreeds who have been afflicted by a genetic disorder known as ‘Merrye syndrome’, named after the family in which the disorder developed. This malady causes its victims to enter a state of age regression that starts at the age of ten and continues throughout the remainder of the person’s life, rendering them with the intelligence of a child. The final generation of the family has been entrusted to the care of the family chauffeur (Lon Chaney Jr), and all is well for these odd people until a greedy branch of the family decides that they want to relieve the family of its home. Mental illness has always, and will always be, a fascinating subject for horror movies as it probes into the unknown and Spider Baby makes best use of that fact.