A boy genius, Freddy Facknitts, invents a time machine that sends Denver and Morton Fizzback into the past and into the middle of a crazy caveman competition called Mammothball.

When the boys enter Wally's dog, Rocky, in a dog relay race, they must also disguise Denver as a dog so that Rocky will perform better.

Denver and Mario take jobs as children's entertainers, only to face to wrath the wrath of Douggie, the Dancing Clown, who attempts to sabotage the duo's entertaining efforts.