A scientist researching the effects of a new synthetic nutrient ends up releasing a giant spider in the American Southwest in this classic sci-fi featuring Clint Eastwood.

A rogue scientist near a small desert town arouses the suspicion of the town's doctor when his lab assistant is found dead from a case of acromegaly, which took only four days to develop. As the doctor investigates, aided by the scientist's new female assistant, they discover that something is devouring local cattle and humans in increasingly large quantities.