After the residents of the small mountain town of Pine Ridge anxiously gather to discuss the mysterious death of local Harold Banks, Sheriff Parker reveals that Banks died from a severe beating, prompting the townspeople to speculate over the recent spate of animal deaths and question whether the tales of an ancient Indian curse may be true. Teenage brother and sister Ann and Charlie Brown scoff at the legends, but Indian Joe declares that if the locals continue to disregard his ancestral burial grounds in Devil’s Crag, there will be more violence. After Parker dismisses Joe’s warning, a townsman advises the sheriff to question scientific researcher Wayne Brooks, who was seen quarreling with Banks earlier that week. When Parker questions Wayne, however, Wayne insists that he and Banks had a simple disagreement. Soon after, Professor Cleveland and his daughter Janet arrive in town and Wayne recognizes Cleveland as the famous archaeologist whose lectures he attended while in college. Wayne offers the professor his services and at dinner that night Cleveland explains that he and Janet have been searching for the remains of a Spanish conquistador, Vargas, later known as the Diablo Giant, who abandoned the military to hunt for gold in the mountains. Later, Wayne takes Cleveland and Janet to his cabin to show them the artifacts he has unearthed, the most important of which is a live reptile that Wayne believes is centuries old. Cleveland is excited by the reptile’s discovery and after piecing together a European crucifix from Wayne’s relics, insists that they return to the site where they were found. The next day after Wayne, Janet and Cleveland set up camp at Devil’s Crag, Parker arrives and reprimands Wayne for leaving town without his permission. The following morning as Wayne prepares breakfast, he hears a gunshot and discovers Joe nearby. After Wayne explains that he and the Clevelands are searching for ancient artifacts and will respect the Indian burial grounds, Joe thanks him for his honesty, but cautions him that the area is dangerous. Later, Cleveland and Wayne begin a methodical search of the area which continues for several days without success. On their final afternoon, however, Janet detects a metal object underneath an enormous log. Wayne and Cleveland dig under the log and discover an armored helmet, breast plate and several weapons, which Cleveland establishes are of Spanish origin. The men are more excited when they discover a skeleton, and Cleveland returns to camp to catalog the artifacts and begin his scientific paper. That afternoon as a rain storm threatens the site, Wayne finds an ancient axe handle, but is unable to dislodge it from the ground. Wayne returns to the camp, and soon after, the storm breaks and a bolt of lightning strikes near the log. The enormous figure of Vargas, the Diablo Giant, then rises from the ground clutching the axe. The next morning Cleveland and Wayne are stunned to find the axe gone and the ground disturbed. A medallion on the ground confirms Vargas’ identity, prompting the men to wonder if the giant, like Wayne’s lizard, has returned to life. Later when young Charlie comes by the camp, Cleveland, Wayne and Janet ask him not to reveal their discovery of the Spanish armor, arguing that it will bring townspeople to disturb the site. That evening, Vargas stalks the campsite and when the men discover the armor and medallion missing, they remain on guard. Further down the hill, Charlie frets about leaving Ann alone as he prepares for work, but she assures him she is safe. The following morning, as Wayne tells Cleveland they should report their suspicions of the awakened giant to Parker, the sheriff arrives with the news that Ann has been found brutally murdered. Parker arrests Wayne, claiming that Ann was clutching the Spanish medallion, and reveals that Charlie identified it as the one found by Wayne. Insisting that he is innocent, Wayne suggests that whoever stole the armor and medallion must have killed Ann. Parker agrees to question Joe, but when they find him murdered in his cabin, Parker takes Wayne into Pine Ridge. Cleveland follows them into town and after his departure, Janet is abducted by Vargas. In town, when Parker leaves Wayne unattended in his car momentarily, Cleveland appears and drives Wayne back to Devil’s Crag, where the professor reveals that he took a plaster cast of a huge footprint which he believes will confirm that Vargas has returned to life and perpetrated the murders. Parker and the townsmen follow Cleveland and Wayne, but when they learn of Janet’s disappearance and hear Cleveland’s story about Vargas, they help search for her. Soon the men corner Vargas, and he attacks and kills several before he is wounded and escapes, leaving Janet unhurt. While the injured men are taken back to town, Parker apologizes to Wayne for not believing in his innocence. Charlie asks to help search for Vargas in retaliation for Ann’s death, but when Wayne and Parker refuse, sneaks away on his own. Later the sheriff, Wayne and Cleveland hear shots and find Charlie badly wounded . While Parker goes for help, Cleveland remains with Charlie and Wayne pursues Vargas alone. Wayne catches up to Vargas at a windmill and after a brief fight, chases the giant to a bridge across a dam. As Cleveland, Janet and Parker arrive, the wounded Vargas topples off the bridge into the water below.