A restless teenage cave man (Robert Vaughn) has an urge to travel to the forbidden land "beyond the river", where a fabled monster can kill with just a touch. The first journey ends with an unfortunate death (B actor Beach Dickerson) in some quicksand (The "Sucking Sands"), so there's lots of discussion about the status quo, which the tribe elders say must be maintained.
Eventually the teenager journeys again to the forbidden land, there's a bear attack (Dickerson in a bear suit), an attack by wild dogs (no doubt liberated from the local pound), and a final confrontation with the dreaded beast. The beast is nothing but an old man in a radiation suit who represents the last survivor of a long-ago nuclear war.
Beach Dickerson used to tell hilarious stories about this movie, including a description of his four roles (he dies in three of them, and attends his own funeral). Vaughn adds some credibility to the proceedings, and the luscious Barboura Morris appears in a small part. Okay, it's really cheap, but it's also fun, and Vaughn is pretty good under the circumstances.