Susan Winter undertakes an expedition to a remote area of Mexico, hoping to find her fiance, Bruce Barton, whos plane-crashed there some time ago. The area is suspected to have a good supply of uranium, so Susan has promoted this to wealthy Martin Melville to get money for her expenses. In addition to Melville, she is accompanied by guide Russ Bradford and pilot Lee Brand in his small-engine craft. But the plane crashes, stranding the four in an isolated valley, which they soon discover is highly radioactive and inhabited by mutated life forms - giant insects, enormous lizards, and a 25-foot-tall human male with a deformed face, just one eye, and only brute animal instincts to feed and protect its turf...