A nearby volcano is about to erupt, and the Jupiter-2 is close enough to be in jeopardy. In order to avert this catastrophe, Don and Dr. Smith have 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.
Meanwhile, the crew of the Jupiter 2 find they have lost communication with the chariot and John goes out in the space pod to search. Will and the Robot decide to conduct a secret search of their own and find the chariot covered in vines. Apparently, it has somehow become 'primitive.' John meets up with Will and the Robot, and Will tells his father what he has seen.
Locked away in the cave, Don and Smith dig a hole so they can plant an explosive. They set the timer it and run for cover. Don almost falls to his death, but in a strange act of bravery Dr. Smith pulls him to safety and saves his life. They then discover that the entrance to the cave has collapsed and they are sealed inside. Having no method of escape, there is little they can do but sit and wait in the hope that they will be rescued. Since they have a lot of time on their hands, Don and Smith get to talking and they decide that they are friends.
John and Will sneak up on the cavemen while the Robot disguises himself as one of them and blends in. Rongah spots John and takes all three of them prisoner. John decides to challenge the authority of Protineus, claiming that the Robot can do anything that the computer can do. The Robot then goes about impressing the cavemen with a variety of magic tricks until Protineus becomes annoyed with him and shrinks him.
While the cavemen are distracted by the Robot's show, John rescues Don and Dr. Smith and manages to cap the volcano just in time to prevent an eruption. Protineus mends his ways and vanishes along with his tribe of followers. The Robot is returned to normal, and Don and Smith become enemies once again.