A crustaceous cave-dwelling carnivorous sea monster jangles the nerves of the undersea engineers assigned to install a deep-sea missile base on the ocean floor. The engineers and scientists are also there to explore the possibility of building an underwater colony, but unfortunately, they discover that the earth's crust at their chosen site is unusually thin. Curious, they break through the crust and discover an enormous cavern below. If they had watched Alien and it's numerous rip-offs, they probably would have known better than to go looking within.