While trying to steal the Keeper’s ship, Dr. Smith accidentally releases all of the Keeper's specimens. Smith is frightened by the animals and runs back to the Jupiter 2 and he orders the Robot to erase the incident from his memory banks. Later on, John and Don encounter some of the more dangerous animals and are forced to destroy some. They cannot understand why the Keeper should have released his animals and Don suspects Smith was probably responsible. The Keeper, furious, comes to the Jupiter 2, determined to discover who was responsible. When Smith is revealed to have been responsible, the Keeper demands reparation from the Robinsons since many of his specimens have been killed; the Robinsons must surrender Will and Penny to him. John and Maureen naturally refuse, but the Keeper warns them that until the children are delivered to him he will not recapture his animals. With so many dangerous creatures loose on the planet, the Robinsons’ lives are in constant danger. Having delivered his ultimatum, the Keeper orders Dr. Smith to come to him that night and departs.
When Smith arrives, the Keeper demands he help secure the delivery of Will and Penny, or else Smith will be fed to a horrible monster. Smith is terrified and agrees. Back at the Jupiter 2, Judy and Don decide to offer themselves to the Keeper in Will and Penny’s place. John and Maureen also have the same idea, and all four end up at the Keeper’s ship. The Keeper considers it a noble gesture, but refuses them, saying he only wants the children and no one else. As the Robinsons leave his ship, the Keeper's staff falls to the ground, which shuts off the force field keeping the horrible creature in its pen. Moments later the terrible monster attacks and injures the Keeper. Meanwhile, Doctor Smith talks the children into giving themselves up to save their family. Smith begins to walk them over to the Keeper’s ship, but Don intervenes. John and Maureen are very concerned when they return and find the children missing. Maureen goes back to the Keeper’s ship looking for Penny and Will and finds him hurt. She helps him, and the Keeper is touched and surprised by her show of compassion. She then tells the Keeper of the missing children, and he begins to call back the escaped animals just as Don, Will, and Penny are being menaced by a giant spider. The next day, the Robinsons awaken to discover that the Keeper has collected all his animals back onto his ship. He comes to tell them that he has decided to leave, but intends to leave one creature behind to punish them. When the Robinsons go to investigate, they find that the Keeper has left Dr. Smith—imprisoned in a cage.
- Although the first part of this story uses an arabic number, the second uses a Roman number in the title.
- Along with "Return from Outer Space" and "The Keeper, Part 1", this is one of Bill Mumy's three favorite episodes.
- At the beginning of the episode when the robot kills a creature that's attacking Dr. Smith, the electrical bolts coming out of the robot's claws are straight like laser beams. In other episodes the robot's electrical attacks look more like lightning bolts.
- When Dr. Smith persuades the children to go with him to see the Keeper and try to persuade him to collect all his animals, the journey is portrayed as a long one on which the party has to stop and rest. The journey begins in daylight but when they stop to rest the second time it is night. During the rest of the episode, the journey from the Jupiter 2 to the Keeper's spaceship is portrayed as being a short walk.
- The scenes with the giant spider and Professor Robinson attempting to blast it with rifle and grenades were judged too terrifying in the original release and partially cut! The giant spider prop had been seen in an episode of Voyage to the Bottom of the Sea (TV show).
- The cage which the Keeper leaves Dr Smith in at the end of this episode is kept by the Robinsons and-in a great example of continuity- used later to try and trap the invisible Bog Monster in 'Ghost In Space'. (This is one of the few example where the show doesn't pull strange equipment out of thin air- despite it being sorely needed in some episodes by the Robinsons! Case in point- the Space Pod).
- Instead of trying to trick Will and Penny, why didn’t Doctor Smith simply inform the Robinsons that the Keeper had threatened his life? Surely they would have protected him.
- When the Keeper is attacked by the giant claw of his remaining specimen, you can see a filament or string attached to the top of the claw
- At the beginning of the episode when the animals are shown leaving the Keeper's spaceship they are all small enough to fit through the spaceship door, which is just large enough for a couple of people to walk through. Yet when we see the animals outside the spaceship they are all gigantic--far too large to be able to fit through the spaceship door. Also, when the giant lizard appears at the beginning of the episode John implies that it's the same lizard they saw in Part 1, which was small; but no explanation is ever given as to how the lizard increased in size.
- At the end of the episode when the Robinsons discover Dr Smith trapped inside the Keeper's cages, the camera zooms in to Smith's face - you can see the reflections of the cameraman and crew in the glass cage.