When a nearby volcano is about to erupt, the Jupiter 2 is in jeopardy. In order to avert this catastrophe, Don and Dr. Smith set out in the chariot with explosives to cap the volcano before it can cause any harm. When Don and Smith arrive at the volcano’s base they are taken prisoner by a tribe of cavemen. They see the cavemen worshiping a giant computer. The tribe's spokesman, Rongah, says the computer is Protineus, their leader. Protineus thinks the presence of humans contaminates their future destiny and orders Smith and Don to be put in quarantine in a cave.