When Heather and Wally's parents inform the two that they are expecting guests from out-of-town, they must hide Denver that the local movie theater, which is believed to be haunted.

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.