Dr. Van Gelder is leading a scientific expedition thousands of miles below the surface of the sea. For six months the small group has been living in isolation in the "Deep Star Six" station. Tension is rising and there are frequent arguments. Van Gelder is building a nuclear missile launch ramp for the US Navy. He is pressed for time and decides to speed up the work. Ignoring the advice of zoologist Scarpelli he dynamites an underground cave triggering terror. For thousands of years a monster from the depths has lived in the cave. Now that it is free, it unleashes its voracious anger on the divers. Half crustacean, half sea snake, the beast haunts the "Deep Star Six" station.