After a storm at sea a lone man (Patrick Duffy) washes ashore on the beach, covered in seaweed, and is found by a man and his son. Then, we're introduced to Dr. Elizabeth Merrill (Belinda Montgomery), who is at a dinner party when her date gets an emergency call from the hospital he works at about a man who can't breath. She accompanies him there, to find that he doesn't have malformed lungs but gill like structures. She tells the hospital staff she can save him--and takes him via an ambulance back to the ocean, where he starts breathing the water. The next time we see this water-breathing man it's a few months later-and Elizabeth calls him "Mark Harris", though he doesn't speak. He seems to understand everyone, though, including the General that's in charge of the secret operation he's now apart of. They test Mark and want to use him to do missions for the military. He's only able to stay out of the water for a dozen hours, after which point he gets weaker and his skin becomes discolored. He also has webbed hands, his eyes change color when underwater, and has skin like a dolphin. He's also much stronger when in the water. At first he's reluctant to participate, but once Elizabeth tells him it's a request, not an order, he agrees. In fact, this is the first time he speaks. Mark is sent to investigate the disappearance of a new submarine in the Marianas Trench. It's there is comes across a mysterious submarine and follows it to an underwater base run by Mr. Schubert (Victor Buono of WHATEVER HAPPENED TO BABY JANE). Although he's invited brilliant scientists from around the world to live there and work with him he controls them with a special bracelet that basically makes them slaves, without a will of their own. When he tries, twice, to put it on Mark, he finds it doesn't work on him. Schubert explains how he believes humanity needs to start over-and plans to remotely take over operations of all the nuclear subs and fire those missiles, which will start WWIII and destroy mankind on the surface. Of course, Mark, who now has an attachment to humans-particularly Elizabeth-wants to stop him.
I remember as a kid being a bit bored by this movie-but forty-years later it remarkably holds up and held my interest. Mark Harris is an untypical hero and Buono provides a threatening, James Bond type villain. Recommended.