After a violent storm at sea, the inert body of a man is found on the beach near the Naval Undersea Center. Equipped with webbed hands and gills instead of lungs, he can breathe underwater, swim faster than a dolphin and dive to depths of at least seven miles. He is nursed back to health by Doctor Elizabeth Merrill and given the name of Mark Harris. In return, Mark agrees to help the United States Navy recover a lost missing submarine carrying top military officials.
