- Directors: Don Barton, Arnold Stevens
- Producers: Don Barton
- Writers: Story, Ron Kivett, Lee O Larew, Screenplay, Don Barton, Uncredited, Arnold Stevens
- Genres: Fantasy, Sci-Fi
- Actors: Marshall Grauer
The film begins with Nazi mad scientist Dr. Kurt Leopold â€“ it is revealed later in the film that he graduated cum laude from MIT in 1934 â€“ in his lab, where he has lived alone for about twenty years. He is comtemplating his former colleagues’ laughter at his formula (described as “ZaAt” â€” read Z-sub-A, A-sub-T â€” but which he simply calls “Zaat”). His formula can turn a man into a walking catfish. He injects himself with the serum emerges from a tank as a giant fish-like creature.
His first act of revenge on society that he feels has wronged him is to release several smaller walking catfish around the town’s lakes and river (filmed in the St. Johns River near Green Cove Springs), an annoyance to the townspeople, and releases Zaat into the local water supply, rendering many of the townspeople ill. Leopold decides to kill the colleagues that laughed at his work. He begins with a character named Maxson. In a lake where Maxson is fishing, Leopold swims under Maxson’s boat, overturns it, and proceeds to kill Maxson and Maxson’s son. Maxson’s wife escapes, although she is in shock from the attack.
Leopold attempts to kidnap another mate: his choice is Martha Walsh, the lovely female member of a scientific team sent to investigate the weird happenings in the town (caused by Dr. Leopold). Leopold grabs her after her male counterparts leave her alone. Leopold takes her to his lab, but two of her companions (having unraveled the plot) are waiting there. Leopold kills them (including the town’s sheriff Lou) violently. He injects her with Zaat, readies her to be dunked into the tank, and makes his getaway, with canisters of Zaat. Martha’s transformation does not go as planned and she gets saved by one of her dying male companions from getting dunked in the tank as Leopold flees toward the ocean. Despite being saved from the transformation, she immediately follows Leopold trance-like into the sea. The movie ends ambiguously, with Leopold seen shot but not killed.