Returning from the planet Venus, an errant NASA spacecraft crashes into the ocean, spilling its radioactive cargo. Enveloped by a radioactive mass, a rabid weasel is transformed into a gigantic killer mutant. Prowling the countryside, the huge weasel kills and devours victims. The creature is captured by a disturbed scientist who plans to use its regenerative blood to amass an army of similar monsters, enabling him to conquer the Earth.