ZAAT

In his laboratory, mad scientist Dr. Kurt Leopold contemplates his former colleagues' derision of his formula ZaAt, a compound that can transform humans into sea creature hybrids. He injects himself with the serum and immerses himself in a tank, emerging as a catfish-like monster. In his new form, Leopold releases walking catfish across the town's lakes and rivers, and releases "Zaat" into the water supply, rendering many of the townspeople ill. Meanwhile, Sheriff Lou Krantz and marine biologist Rex Baker investigate strange occurrences with the local catfish and waterways.

Leopold turns his attention to killing those who mocked his work. He murders former colleague Maxson and his family on a boat, followed by associate Ewing in his home. Afterward, Leopold kidnaps a young female camper at a lake. Taking her back to his lab, the doctor straps her into a mesh basket beside a large water tank, with the intention of making her his mate. However, due to her struggling, the equipment malfunctions, and her corpse, partially transformed, is pulled from the tank.

Baffled by the deaths, Rex contacts an organization known as INPIT, which dispatches scientists Martha Walsh and Walker Stevens to investigate. Following the kidnapping of Martha, Leopold heads again towards his lab, surreptitiously followed by Walker, who has picked up Leopold's radioactive trail. The doctor arrives at the lab with Martha, where Rex and Lou happen to be searching. Lou attempts to apprehend Leopold outside his laboratory but is killed in the process; inside, Rex also tries to stop the doctor as he prepares Martha for her transformation. After seriously wounding Rex, he injects Martha with "Zaat", readies her to be dunked into the tank, and makes his getaway with canisters of the compound, while Walker attempts to stop him before dying from an earlier snake bite. Martha's transformation fails, and she is saved from the tank by a dying Rex, although she appears to be in a trance and immediately follows Leopold into the sea.

Leopold, in catfish form, kidnaps Martha and attempts to transform her into his mate, but her transformation fails. After a struggle resulting in deaths, Martha—now in a trance—follows Leopold into the sea.

The ending leaves the mutated Leopold and Martha disappearing into the aquatic unknown, suggesting the monster's legacy continues beneath the waves. The failed transformation and trance-like state of Martha reflect the destructive consequences of Leopold’s obsession and the unchecked dangers of mad science.