Will, John, Don and the Robot take the chariot to search for potable water. When a volcano erupts nearby, they narrowly avoid injury. They are soon forced to take shelter in a cave from the increasing temperature and repair their air conditioning unit. While John and Don attempt repairs, Will and the Robot decide to explore the cave. The Robot tells Will a large source of drinking water is nearby so they decide to see if they can find it. They find the water at the bottom of a long shaft, but a sudden earthquake knocks them both into the shaft.They call for help, but when no one hears them they decide to seek another exit.
Will and the Robot find themselves in an underground tropical jungle on the shores of a vast underground lake of drinking water. As they go through the jungle, the Robot detects a human presence nearby. Will sees a pretty little girl asleep on a bed. Acting under advice from the Robot, Will kisses her and she awakens. She tells them that she is a princess, and asks who Will is and where he came from. When Will tells her he comes from Earth, the Princess seems sad. She rises and asks Will and the Robot to follow her.
Meanwhile, Don and John have finished repairs and become concerned that Will and the Robot have not returned. They track them to the bottom of the shaft, but Don's leg is severely injured in another earthquake. With difficulty, they continue to search, but they are soon taken prisoner by soldiers.
The princess takes Will and the Robot to her fortress where they are introduced to her Major Domo. Will is then taken to John and Don, where the Major Domo explains that their race intends to conquer the universe with a vast army of powerful soldiers that have been bred for generations. The invasion begins when princess awakens, and Earth will be the first planet to be invaded.
The Major Domo insists that Will must marry the princess, but Will resists the idea. When asked why, he explains that he just wants to stay a regular boy and live with his family and have fun. The princess does not know what fun is, so Will tries to explain it to her. As she begins to understand, she releases Will and helps him free Don and John so all can escape. The Major Domo, meanwhile, continues planning his offensive and sets his soldiers on John, Don, Will and the Robot. During the fight, the Major Domo is killed, and the little princess goes back into her magical sleep as her kingdom crumbles around her. When the Robinson party returns to the chariot, they set out just as the cave collapses behind them. The men travel back to the Jupiter 2 to a warm welcome.
- Some of the set pieces are reused Alpha Control and Jupiter 2 equipment.
- Cave temperatures are typically in the low 60s F but after Don and John have been in the cave for some time working to repair the chariot's air conditioner, they are shown sweating.
- The underground jungle is so well-lit it appears to be above ground.
- When Will Robinson descends down the spiral stairs into the control room with the Princess, much of the instrumentation, controls and equipment are from the "Voyage to the Bottom of the Sea" set. In particular, the large serrated edged separation doors that isolate the forward viewing compartment on the "Seaview" from the rest of the submarine.
- Kym Karath, who played the part of the Princess, was in the movie "The Sound of Music" with Angela Cartwright.
- The planet Priplanus seems to have had a lot of strange warrior cultures that have come to odd ends and which seem to lack other members of society such as farmers, laborers, and merchants. In this episode the people are technically advanced and yet still believe that a little girl has the right to rule over them. The plot seems very much to be in the Flash Gordon mode, right down to Royal Dano's Ming the Merciless-like character.
- Footage from "There Were Giants in the Earth" episode reused when the chariot enters the cave.
- The overhead light above the bed of the sleeping Princess suspiciously appears to be the lower exterior power unit of the Jupiter 2 full size mock up, complete with the same sequential flashing light pattern.
- As an environmental control robot, shouldn’t the Robot have been able to give his friends a little more warning that the volcano was going to erupt?
- Did the Robot really need to be specifically told that the Robinsons were looking for drinking water? He’s lived with them for a whole year now. Surely he knows what they need to survive.
- Why is Will so much more upset about the idea of having to get married then about the idea of Earth (and the rest of the universe) being conquered?
- This is not the first time a boulder has fallen on Don and hurt his foot. Don really needs to start watching out for those falling rocks!
- What about the water?! The men left in search of water, had an adventure, came home empty-handed, and nothing further was said about it! What was the point of that whole trip? It's surprising that they return home rather than go on further in search of water.