A group of mice try to find shelter during a storm, ending up in the home of mad scientist Dr. Henry Jekyll. His cat is the only resident of the house. Unable to catch the mice, the cat enters Dr. Jekyll's lab and drinks his master's formula for creating monsters. The mice are helpless against the monster until Mighty Mouse flies down and clobbers the Mr. Hyde-like cat. After his severe beating, Mighty Mouse knocks the cat back into Dr. Jekyll's lab, where the cat knocks over lab equipment, spills flammable chemicals and destroys the lab.