Herbie Popnecker; Pincus Popnecker; Mrs. Popnecker; Professor Flipdome
Herbie's teacher Dr. Plumduffle is in danger of being forced to retire because he can't handle Butch Nelson. Herbie goes with his neighbor, Professor Flipdome, to the center of the earth in the professor's Squirm-Worm. After some misdirections, including Hades, they make it to the center where they find that there is an exact fire-breathing duplicate for everyone on the surface. Herbie returns with Dr. Plumduffle's duplicate, who teaches Butch Nelson a lesson.