A mysterious blip on the Jupiter 2's radar is seen by Judy and Maureen, but when Don checks it, there is no sign of anything unusual. Meanwhile, Dr. Smith observes an alien ship touch down. When he attempts to flee, he is struck by a beam from the ship and taken aboard.
Inside the ship two aliens question Dr. Smith. They are in need of a humanoid brain to repair their ships guidance system. Since size is a factor, the aliens propose removing a portion of Smith's brain, but he quickly suggests a better alternative: Will Robinson's brain. To ensure his cooperation, the aliens place a metal ring around his neck and release him. Smith soon learns that the collar causes him excruciating pain when he delays in his task.
Later, Will and the Robot are talking when Smith arrives. He calls the Robot aside and orders him to remove the ring, but the task is impossible. When he cannot later find Will, Smith tries to lure Penny away from the camp, but Don's timely appearance on the scene prevents her leaving with Smith.
Will has gone off rock hunting, and when he does return, his family search for him. Dr. Smith comes across the boy first, though, an cunningly manipulates him into accompanying him to the alien craft by telling him that his family is in danger. Will's parents arrive just as Will is taken prisoner.
The aliens are very pleased with Will Robinson's brain, and plan to take him with them on their journey as a living part of their computer. They are aware of the arrival of the Robinson party in the chariot, but have no fear of them, confident that they can destroy them at will. The Robinsons attempt to retrieve Will with the Robot, but he is soon disabled by the aliens. Allowed to speak with his parents, Will reassures them that they will be safe now. As the aliens prepare to leave with Will, they demand he calculate a complex mathematical problem required in computing their course, but Will is so consumed by his imminent separation from his family that he begins to cry and cannot focus on the task demanded. The aliens have no choice but to release Will; he is in fact totally useless to them.
Don: "So that's the monster?
Smith: "Now that doesn't look very monstrous does it?"
Don: "Neither do you,"
- When the sparks start to fly out of the alien spaceship's electrical components at the end of the episode, it becomes apparent that the alien spaceship does not have any fuses or circuit breakers in it. Just like the Jupiter 2.
- It is possible that this episode may have been used to "kill off" Dr. Smith had the decision not been made to keep him as a permanent cast member.
- The aliens seen here are based upon the bubble-headed alien seen in the last moments in the Pilot episode. They have been modified to have no mouths and wear long flowing robes, greatly improving their sinister appearance.
- This episode marks the first time Dr Smith attempts to save his own life by trading that of a Robinson (usually Will or Penny).
- When Smith suggests he get computers from the Jupiter 2 for the aliens, they dismiss the use of such "primitive toys" as "a million times a million of them" would still not meet their needs. The Jupiter 2's computers are Burroughs B205s, which could execute operations at kilohertz frequencies. Current microprocessors of course operate at gigahertz frequencies or about a million times faster than the B205.
- Don flies the Jetpack in this episode. He will do it again in "The Raft".
- The aliens' spaceship is perhaps the most original design of the series. Apparently lacking rocket motors or other conventional means of propulsion it looks truly non-human. Perhaps that's why it was not reused in the series as so many other props and models were.
- During the final scene, as the Robot is seen walking among the rocks near the alien spaceship, Bob May's legs can be seen sticking out of the bottom of the Robot's costume.
- This epsiode is the first in which Smith remarks "The pain, the pain!"
- Maureen Robinson drives the chariot in this episode.
- In one scene with Will and Dr. Smith, there are stalagmites among the large rocks in a desert like setting. Normally associated with dissolved limestone in a cave or lava tube formation in a volcano here on Earth. With Will being a geologist, you would think perhaps he would have been curious about that.
- When Doctor Smith first meets the aliens he is carrying a laser as a sidearm. If the Robinsons have so little trust in Smith, why have they allowed him to have a gun?
- Didn’t Smith realize that the Robinsons would kill him when they found out he had given their son to hostile aliens?
- If the aliens exist in a fifth dimension “where size has no meaning,” why did they insist on having a smaller person to run their computer?
- In this episode, the Robot claims that he can read minds!
- At the end of the episode, Will cheerfully tells his parents that the aliens were after his brain. His parents respond by laughing.
- The alien control necklace was also used in the episode, "All That Glitters."
- When Dr Smith is sent back by the aliens to procure Will, he is "boinked" out of their spaceship onto the ground, face down. If you look closely in the next shot, you will see a towel under his upper body and head, protecting his face from that nasty alien sand. In he next shot, we see Zachary rising up off the ground with a spotlessly clean visage!