The citizens of mountain town Pine Ridge, California, are concerned about a series of livestock mutilations in nearby Devil's Crag.

Local geologist Wayne Brooks is told of Banks' death by friends Anne and Charlie Brown, two Pine Ridge siblings.

Brooks tells Cleveland of a local Native American legend that evil spirits will rise up in Devil's Crag.

Dr. Cleveland reveals that he is searching for the remains of a Spanish expedition that he believes reached Devil's Crag 500 years earlier.

Brooks finds a rock formation similar to the one in which the lizard was entombed and discovers the handle of a massive, still intact axe he believes belonged to the "Diablo Giant".

Brooks is forced to leave because of a large electrical storm, just before the "Diablo Giant" Vargas rises from the site's detritus.

Later that night, the body of the 500-year-old Vargas, revived by a lightning strike, stalks the group and eventually kills Anne Brown.

Sheriff Parker accuses and arrests Brooks for Anne's murder because a medallion (one of his excavated artifacts) was found clenched in her hand.

Brooks is eventually able to kill Vargas, forcing him to fall to a watery death from a bridge spanning a raging waterfall.

The make-up effects were created by Jack Pierce, known for his Universal Pictures' classic monster makeup for Boris Karloff's Frankenstein (1931), The Mummy (1932), and Lon Chaney Jr.'s The Wolf Man (1941).

[3] In describing the film, critic Glenn Erickson wrote "Basically a light entertainment ‘posse western’ about a big, murderous bruiser who’s been sleeping in the woods since the 16th century, Giant from the Unknown can’t be described as brilliant or inspired," but added that "[o]n the plus side are the pleasantly silly performances," that "Richard Cunha’s direction is sound and his camerawork exceptionally good," and "Albert Glasser’s attention-getting music score is a big plus — it adds the mystery, suspense and danger that’s sometimes lacking in the visuals.