While Dr. Smith has the Robot do his chores for him, he suddenly becomes hypnotized by a strange humming sound and wanders away. He walks straight into a transparent cage and the door shuts behind him, trapping him inside. Will runs to help him, but a creature arrives and the boy must flee for his own safety. The monster pounds on Smith's cage; Smith comes out of his trance and screams in terror when he sees the creature. A noble-looking man appears and points his glowing staff at the monster. The monster quiets and walks into another cage nearby. Then both the man and the monster vanish.
Meanwhile Will has run back to the Jupiter 2 to get John and Don to rescue Dr. Smith. Don thinks seeing Smith in a cage is very funny. John lets Smith out, and Smith explains how he saw the man and the monster disappear. John worries something dangerous may be going on, and orders everyone back to the Jupiter 2. The Robot examines the cage and reports that it is a sophisticated device used to catch live animals. The noble-looking man later appears at the Jupiter 2 and greets the Robinsons. He says he has been watching them for some time now, and introduces himself as the Keeper. He explains how travels around the universe visiting every planet and collecting two of every species of animal. The Keeper then vanishes again saying he will return to talk to them again.
The Keeper returns and talks to Don and Judy. He tells them he would like to have them as part of his collection. They refuse, but the Keeper warns them that they have made a poor choice. Meanwhile, Dr. Smith makes plans to steal the Keeper’s spaceship so he can fly himself back to Earth. The next day the Keeper appears to speak to Penny and Will. He lets Will play with his glowing staff and catches an animal for them as a demonstration. He then gives the children a tour of his spaceship, showing them many interesting animals. He tries to lure them into a couple of empty cages, but the children are suspicious of his motives and run away. The Keeper makes use of his staff to draw them back to him, but Dr. Smith, also drawn by the staff's power, accidentally knocks it out of the Keeper’s hand, freeing them all from the trance and giving them time to dash back to the Jupiter 2.
Will and Penny tell their parents what happened, and John and Maureen both realize the children are in danger and should be guarded at all times in case the Keeper tries to hypnotize them again. The Keeper attempts to lure the children out of the ship at night, but John makes sure they cannot leave their rooms. The next morning, the Keeper uses his staff to call to the children again, and they manage to sneak out to go to him. The Keeper nearly gets them onto his ship, but John and Don arrive in time to rescue them by breaking the Keeper’s staff. The Keeper decides the Robinsons are too troublesome to deal with any further, and says he will leave the next morning. He goes back to his ship and the Robinsons return to the Jupiter 2 thinking the ordeal is over. That night, however, Dr. Smith and the Robot sneak onto the Keeper’s ship in the attempt to steal it. Smith does not understand the controls, though, and accidentally opens all of the cages releasing the animals. When he realizes what he has done, he flees the ship interror.
- Various sources provide a dash or a comma in this episode's title, "The Keeper - Part 1" or "The Keeper, Part 1". The episode itself does not have any punctuation, just offering "The Keeper" and "Part 1" on different lines. This wiki uses a colon, "The Keeper: Part 1", because it is listed this way at the Internet Movie Database.
- The Keeper's menagerie includes aliens from previous episodes such as the Cyclops (but presented as human-sized) and some Rubberoids.
- Along with "Return from Outer Space" and "The Keeper: Part 2", this is one of Bill Mumy's three favorite episodes.
- This is LIS' only two-part episode. It is also regarded as one of the best, giving all the cast their moment to shine and avoiding overuse of Dr Smith and the Robot.
- Starring as 'The Keeper' is the capable Michael Rennie, who is most famous for playing Klaatu in the sci-fi classic The Day the Earth Stood Still. In a nice touch (and saving money on recording new themes) the somber 'Klaatu' theme from the same movie plays over each scene where the Keeper communicates with his home. A further odd connection is that Jonathan Harris worked previously with Rennie in The Third Man TV series.
- During the escape scene, only four creatures are observed leaving the Keeper's spaceship. They exit over and over, courtesy of careful editing. They are the Cyclops, now normal sized, the seaweed creature from "The Raft", the large hairy creature from who appeared in the first scene of this episode, and the "Rubberoid" creature from "Wish Upon a Star". The flying creature that is zapped by the Robot at the beginning of the second part of this episode, and the original lizard from the beginning of the first part of this episode, are not seen escaping from the Keeper's spaceship at this time.
- The "Rubberoid" creature was wearing the same clothing that he wore in "Wish Upon a Star". Why would he be wearing clothing if he was an animal in The Keeper's zoo? Zoo animals don't wear clothes.
- The Keeper's spaceship is an unusual ring design, perhaps 30 yards in radius, that is obviously too small to hold the number of specimens the Keeper claims. Perhaps it's merely a shuttlecraft or another example of Irwin Allen adding questionable interior space to a vehicle as in the Jupiter 2's power core.
- The Keeper asks Don and Judy to join his menagerie in Part I of this episode, but in Part II he rejects them when they offer themselves in place of Will and Penny.
- The Keeper considers the Robinsons to be “rare and valuable specimens” that he very much wants to add to his collection. Why is he so fascinated by them when he himself looks exactly like a human being?
- The Robot claims that “If I do not complete my task at exactly 14:15 I will blow a computer in miscalculation!” Would that actually happen, or was he just being silly? It would really be lousy if the poor Robot’s computers exploded every time he didn’t do things precisely on schedule.
- Will, Penny and Doctor Smith are all obviously very vulnerable to the effects of the Keeper’s staff. When the Keeper used his staff at the beginning of the episode to trap Smith in the cage, why was Will unaffected? He was standing directly beside Smith, so he certainly would have heard the summoning as well.
- When in the Keeper's trance, why does Smith walk zombie-like with his arms straight out in front of him? Will and Penny were summoned just as often, and they moved rather normally.