A man found in the water is brought to an emergency room. Dr. Elizabeth Merrill who was called in to look at him discovers that he is not human, he needs to be placed in the water to live like a fish. His physiology is different, he has webbed hands and is sensitive to light, when he is in the water he is strong and when he is out of the water for a long time, he could die. They believe that he comes from the undersea civilization of Atlantis. They give him the name Mark Harris. Now he and Elizabeth go out in a special submarine and discover strange things. He would occasionally run into Mr. Schubert, a devious man, who is coming up with a new scheme that Mark always thwarts.