Deep in the Amazon jungle, a group of American scientists embarks on an expedition to explore a mysterious river surrounded by legends and myths about creatures inhabiting its waters. The researchers, led by Dr. Carl Maia and Dr. Mark Williams, stumble upon an extraordinary fossil of a hand that seems to belong to a prehistoric being. Determined to uncover its secrets, they embark on a journey that takes them to a remote lake, where they soon realize they are not alone.
As they immerse themselves in the murky water, the scientists encounter an amphibious being called the Gill-man, a creature that is half human and half fish, who has been living in isolation for millennia. What begins as a scientific interest turns into a deadly game, as the intrusive presence of humans threatens the Gill-man's habitat. Attraction and fear intertwine as Mark becomes fascinated by the creature, while his colleague, the beautiful Kay Lawrence, becomes the object of the being's obsession.
With its unforgettable design and a growing atmosphere of tension, this classic story blends adventure with horror, exploring the limits of human understanding and the struggle for survival. As the scientists confront their own fears and the imminent threat of the Gill-man, it becomes evident that they are not only fighting to uncover an ancient mystery but also to save their own lives.