When Penny is helping her family look for plant and mineral samples, she is startled by a strange cave boy. When she tries to run away from him, she becomes ensnared in a net. An odd man by the name of Farnum B. appears and informs her that she is going to be part of his zoo.
Farnum B. transports Penny inside the zoo and shows her around. After the tour, he locks her up in her enclosure—a rather nicely decorated teenaged girl's bedroom. Farnum B. also catches Judy and Don and locks them up in the cage next door. Shortly afterwards, Smith, Will and the Robot are captured as well.
Will and Penny manage to escape and they go to find the cave boy Oggo. Penny begs him to help them get out and he agrees to show Will the exit. The alarm bells ring and Farnum B. runs to stop them. He and Will fall through a dimensional door and end up stranded on an odd, prehistoric world—Oggo's home world.
Dr. Smith escapes from his cage and when he discovers that Farnum B. is gone, he decides to run the zoo himself. He figures that by using his crew mates as exhibits he can make a lot of money on alien planets. He enlists the help of Oggo and reprograms the Robot to go along with his plans. When Don, Judy and Penny find out that Smith is free, but that he plans on keeping them captive, they are very upset. Penny is even more distressed when she learns that Will is lost, but she cannot convince Smith or Oggo to listen to her.
Dr. Smith gets Oggo to fly the spaceship to an alien world where he opens the zoo and puts his friends on display. Penny is frightened, Judy is offended, and Don is just plain furious. Penny confronts Oggo and asks for his help again, but he runs away. Penny turns to the Robot, asking him to ignore his new programming and help them escape. The Robot agrees and releases his friends. Don takes control of the ship, forcing Smith to close down the zoo.
Penny goes to Oggo and once more begs him to help them find Will and Farnum B.. Oggo is touched by her honest emotions and shows her how to rescue them. When everyone is safely back on the ship, Farnum B. returns them to the planet where the John and Maureen are waiting. Farnum B. apologizes for his actions and promises to mend his ways. He adopts Oggo as his son, which makes them both very happy. Dr. Smith also learns a lesson himself after Farnum B. tosses him out of the spaceship in mid-air.
- When Farnum B shows Penny her cage, it is very much a 1960s stereotypical view of a teenage girl's room. There is no CD player, PC, nor cordless phone. But, it did have a record player.
- Farnum brags that he has the only traveling intergalactic zoo. I’m afraid this may be incorrect. Didn’t The Keeper from season one also have an even larger, classier zoo on a spaceship?
- When the Robot had to duel Mord in the fighting ring, why was he attempting karate? Why not just blast him?
- How long were Smith and the others gone? It seems to have only been a day or so at the most, but if that is true, when did Smith have the time to make up that “See the Humanoids!” advertising poster with the picture of Don on it?
- Why does Oggo want a shiny pink jacket?
- Why would the aliens pay money to go inside the zoo to see humans when they could just stand outside the zoo and look at Doctor Smith for free?
- in this episode, several interesting and very "alien looking" aliens appear, some of them reused from other episodes, some of them never seen again on the series.