A geological expedition to the Amazon uncovers fossils of a skeleton with webbed fingers, which shows a connection between terrestrial and aquatic animals in the mythical Black Lagoon region. Professor Carl Maia and his friend David Reed, members of the Marine Biology Institute, get money for a new expedition in search of more evidence. They travel back to the location, in the company of Kay Lawrence, Reed's girlfriend. When they reach the archaeological site, they find out that the men who were there had been killed by a mysterious half man, half amphibian creature.