When scientists exploring the Amazon River stumble on a “missing link” connecting humans and fish, they plan to capture it for later study. But the Creature has plans of his own, and has set his sights on the lead scientist's beautiful fiancée, Kay.

In a tributary of the Amazon, a monster – half-man, half-fish – is captured and placed in a reservoir in a Florida national park to be observed by scientists.

Scientists surgically transform the Creature into an air-breather, but being able to live on land is not enough to make him comfortable with humans. Enraged, he turns his wrath on anyone who comes near as he desperately tries to return to the deep-water world where he truly belongs.