A detachment of U.S. Marines is put on board an Air Force TR-3B. Their vessel is forced down in a dense jungle inhabited by crocodiles, killer cats, poisonous snakes, giant anacondas, savage cannibals and headhunters, and a legendary beast called Mapinguari. Their mission leads them to a lost city that is invisible to radar, flying aircraft, and orbiting satellites. The ancient city with its Mayan pyramids and primitive stone buildings is occupied by hundreds of natives and a group of German civilians. Corporal Steiner and beautiful Ingamar Dornbusch must descend the stone stairway to Ixalba, the ancient Maya's 'Place Of Fright.' Fleeing from a gang of armed treasure seekers, Private Mead and a group of civilians discover an underground city where hundreds of human skeletons are strewn about the streets. Will the unknown forces that claimed the lives of those ancient people also claim their lives?