An underwater research station headed by Dr LaSatier (Gary Merrill) finds evidence of a very fast-moving undersea craft, and request assistance from the United States Navy. which sends its top submarine officer, Commander Wayne (Scott Brady). He is able to confirm that there's no submarine, on our side or in the Soviet navy, that can match the size and speed of what has been tracked by LaSatier's people -- and the United States Navy wants to know what this object is. The visitor finally reveals itself as a saucer-like creation that sets down on the ocean floor near the sea-lab. Head diver Hugh Maddox (Mike Road) and photographer Sandra Welles (Wende Wagner) investigate the saucer and find a chamber containing an odd-looking capsule, which they bring back to the laboratory. No sooner do they get it aboard, however, then they realize that the capsule is getting larger, and starting to emit dangerous high-frequency sound-waves. Before anyone can prevent it, the capsule bursts open and a powerful and dangerous extra-terrestrial amphibian bursts out, attacking the scientists and crew and isolating them on the ocean floor. The creature's obvious intelligence, coupled with the fact that it carries a deadly microbe, convinces Wayne that the Earth is facing a full-scale invasion.