Citizens of Pine Ridge, California worry about livestock mutilations and a recent murder in nearby Devil's Crag. Many suspect the deaths are rooted in something supernatural. A Native American named Indian Joe [Billy Dix] confirms the town's fear of a tribal curse. But to prevent panic, [Bob Steele] as Sheriff Parker banishes Joe from the premises. Parker is also wary of a local geologist, Wayne Brooks [Ed Kemmer]. There were rumors of animosity between him and the murder victim. Meantime, Professor Cleveland [Morris Ankrum] and his daughter Janet [Sally Fraser] arrive, inviting the possibility of scientific study in Devil's Crag. Dr. Cleveland plans to carry out archaeological research, but the sheriff warns him about the recent trouble in that area.
Later at dinner with Cleveland and Janet, Brooks, an ex-student of Cleveland's, offers to show him his collection of local artifacts. After dinner, Brooks takes them to his laboratory. As Cleveland examines Brooks' finds, Janet opens a box with a small lizard inside. Brooks explains that he found it inside a rock, where it had been in a state of suspended animation for a long time. As they converse, Indian Joe peers through a window, watching closely.
That evening, when Brooks and Janet return from a double feature, Cleveland calls them inside. He has pieced together broken fragments into a cross and now theorizes that conquistadors visited the area hundreds of years before. The next morning Wayne guides Cleveland and his daughter to Devil's Crag, while Sheriff Parker follows them in his police car. Parker pulls up and chastises Brooks, reminding him that Devil's Crag is a danger zone. But Cleveland produces a research permit from the Commissioner of Public Lands and reassures Parker that they are armed and can defend themselves. Though unconvinced, Sheriff Parker leaves the small group to set up camp.
The next day, while Brooks examines the area, Indian Joe fires a rifle in Brooks' direction but claims he was just hunting rabbits. He then asks Brooks if he is there to rob Native American graves. Brooks assures him that he and Cleveland are only looking for Spanish artifacts. Joe agrees to hunt elsewhere, but warns Brooks the place is evil.
Brooks returns to camp. He informs Cleveland that the area is physically changing and suspects a recent electrical storm may have created the disturbance. So they break out metal detectors. However, an initial search reveals nothing. Then strictly by chance, Janet detects metal underground. The spot is excavated, revealing a cache of Spanish artifacts, armor, weapons, and bones. Brooks then discovers a rock formation similar to the one in which his lizard was entombed. He also unearths the handle of a massive axe, still intact. He believes it once belonged to a local legend known as The Diablo Giant. Unfortunately, Brooks is forced to exit the premises due to an electrical storm. After he leaves, the Diablo Giant (aka "Vargas") rises from the site's detritus.
The next day, the group finds a large indentation in the ground. They also note the absence of the body armor and other artifacts unearthed earlier. They discuss the possibility that, like the lizard, Vargas is alive, having just been revived from a state of suspended animation.
That night, a centuries-old Vargas, awakened by a lightning strike, stalks and eventually kills a local teenager, Anne Brown [Jolene Brand]. Sheriff Parker arrests Brooks and accuses him of Anne's murder. An old medallion, one of Brooks' excavated artifacts, is found clenched in the victim's hand. But after another brutal death, searchers conclude the Diablo Giant is roaming the wilderness. Pine Ridge locals help the sheriff hunt down Vargas, a one-man crime wave. Brooks eventually finds and kills Vargas, forcing him to fall from a bridge spanning a raging waterfall.
