Dr. Smith orders the Robot to separate his upper and lower sections so Smith can use the lower section as a mobile serving tray. Moments later, Smith accidentaly launches the space pod with the lower half of the Robot trapped inside. John and Don are very displeased, especially when the gravitational pull of a nearby planet forces the pod to land on the planet’s surface.

A voice greets the Robinson family, inquires about the pod and invites them to land the Jupiter 2 on the planet so they can recover it. The crew agrees and Smith runs out waving a white flag.

Much to Smith’s horror, he is greeted by a tall misshapen alien. The alien, Gilt Proto, seems docile enough and asks the doctor about the rest of the crew. When Smith explains that every member of the Robinson party is a unique individual, the alien seems very intrigued.

Smith runs back to the Jupiter 2 where he informs his companions that hoards of evil alien monsters will surely kill them all. Maureen tries to soothe him and suggests he go to his cabin where he can rest and recover from the ordeal, but before he can do so, Gilt Proto suddenly appears in their midst.

Somehow, Gilt manages to “freeze” the entire crew temporarily, rendering them unable to move or speak. He then walks among them, studying them with great interest. It does indeed seem that the idea of a species consisting of different individuals is a foreign concept to him. Gilt then vanishes, taking the upper half of the Robot with him.

Once they are able to move again John and Don decide to leave the planet immediately, reluctantly leaving the robot behind, but find that they are unable to lift off as all of the Jupiter 2's power has been drained.

Upset about the kidnapped Robot, Will sets off to find him. Gilt Proto greets Will and informs the boy that he has reprogrammed the Robot and made copies of every crew member of the Jupiter 2. The clay aliens themselves all look and act alike and they hope to better their society by observing the Robinson duplicates, which might teach them how to be different from each other.

Will does not mind too much until Gilt Proto goes on to explain that the copies intend to steal the Jupiter 2 and fly it back to Earth where they will take over the entire planet. Confused and horrified by their reasoning, Will tries to convince Gilt Proto not to go forward with the plan, but the alien refuses to listen.

Will runs to the Robot to beg for his help, but the Robot, who has indeed been reprogrammed, will not listen. Will returns to the spaceship to warn his family of the impending danger, and Don and John go out to have a try at talking some sense into the aliens. Their efforts are in vain, and the pair is almost immediately captured and imprisoned.

Back on the Jupiter 2, Smith says everyone is going to die and looks for a place to hide. Will manages to persuade him to come along to find Don and John and they soon come across the gang of clones training for their mission. Cleverly, Smith and Will manage to disable their own copies, and then take their place in the group.

The alien clones trap Maureen, Judy and Penny and then take the Jupiter 2 up into space. The alien Smith clone wakes and manages to sneak aboard at the last minute, but real Smith beats him up and stuffs him into one of the cabins.

Dr. Smith distracts the clones while Will attempts to win the Robot back over to their side. Sadly, he is unsuccessful. Trouble brews when the clones discover their friend, the clone Smith, tied up in the cabin. The clones realize Smith and Will are not of their own kind and plan to eject them into space.

Luckily, a meteor storm hits and provides a useful distraction. Will uses the time to contact Alpha Control secretly. He warns them that the Jupiter 2 is a hostile ship and must not be allowed to land. In response, Alpha Control launches missiles, and the clones are forced to abort the mission and turn the ship around to return to their own world. Once back on their own planet, Gilt Proto releases the Robinson family. He tells them that he and his species have learned their lesson; civilization can only be made better through patience and hard work, not through violence.

Background information

• After almost three years of being lost in space, a group of aliens have no trouble getting the Jupiter 2 to travel to Earth. Why don't the Robinsons just repeat that feat as soon as they are freed and regain control of their spaceship? (For the same reason that The Professor on Gilligan's Island could make countless things but couldn't get them off the island, the show would then be over.)
• After the real Dr. Smith has been exposed on the trip to Earth, the alien Professor Robinson suddenly starts referring to Will as if he is the real Will Robinson, even though he has not yet been exposed as the real Will Robinson.
• The freezing tubes from the Jupiter 2 are seen as the props in the alien lab used to process and create the duplicates.
• The alien says his kind are all uniform, yet each of the aliens shown is a different color and slightly different shape or size.
• The aliens are tall and big, and yet all the chairs and equipment in their control room looks designed for human use.
• The alien tells John, "Do not resist, it is futile." This is the second time in the series in which the line spoken by the Borg in Star Trek: The Next Generation, "Resistance is Futile," is foreshadowed.
• Dr. Smith suggests the crew hide away in the "bowels" of the ship and then looks for such a place to hide; but the other crew members say there is no such place and Smith himself can't seem to find one. Yet in the LiS episode "Space Creature" the ship's power core is shown to be an entire hidden lower deck with plenty of space. Why don't they hide there? (I just finished re-watching it and they never say that there is no such place. They say that the duplicates will look everywhere on the ship and Don sardonically asks Smith if he's going to hide in a smokestack.)
• When Will and Dr. Smith go looking for John and Don in the building used by the aliens, Dr. Smith says he's looking for a sign. When Will asks what sign, Smith replies sardonically "Abandon hope all ye who enter here." That line is taken from Dante's Inferno, where it is the sign over the entrance to hell.
• Why does Will tell Dr. Smith to hide his duplicate in his quarters? It would make more sense to keep the duplicate off the ship.
• How do the duplicates know how to operate the controls on the Jupiter 2? And how do they know the personalities of their real counterparts? They haven't spent enough time around them to know what they're like.
• How did the duplicate crew members get the space pod to dock with the Jupiter 2 when all of the duplicates were on board the Jupiter 2? There was no one to pilot the space pod.
• Before the aliens assumed their human appearance they didn't seem rude or unpleasant, but in their human form they are petty, jealous, argumentative, and threatening. (There have actually been scientific studies showing negative effects of human beings in authoritative roles in charge of underlings. Why should alien life be different?)
• When the Robot says "Get thee behind me, Will Robinson," that phrase is borrowed from the King James Bible in which Jesus says "Get thee behind me, Satan" in Matthew 16:23. (Which Smith himself quoted in full in the episode "Space Creature".)
• How does Dr. Smith know about the missiles? He and Will were in the space pod when the missiles passed the ship. They weren't near a window and couldn't have seen what caused the disturbance.
• When the Jupiter 2 returns to the alien planet at the end of the episode, how is it that the space pod is shown in the same place where it was before they took off for Earth? The space pod was still inside the ship when they landed.
• Once the Smith double is discovered and the true identities of Will and the real Smith are revealed, the Smith double is not seen again.
• The status of The Space Pod is in doubt at the end of the episode. First, the aliens appear to load it into the Jupiter 2 before they depart for Earth. Then, it is back on the ground at the alien base. Then, when The Robot forces Dr. Smith to march into the Space Pod area, it appears to be in there.
• The Jupiter 2 demonstrates an ability to automatically dodge incoming objects (in this case, an Alpha Control Missile). It is possible this is the system mentioned by Major West in 'The Reluctant Stowaway' that would have allowed the ship to miss the incoming meteors were it not for Dr Smith's extra weight. (And yet, the Jupiter 2 did not dodge the meteors they first encountered in this episode.) {Which was from the pilot episode}
• Despite firing on the Jupiter 2, Alpha Control demonstrates no desire to communicate with the vessel first.
• The Jupiter 2 lands in Chicago and yet Will cannot spare the few seconds it would take to let Dr. Smith off the ship. The scene from the viewport is of a series of urban residential buildings so they are extremely lucky to have found an open space large enough to have accommodated the ship's landing. Also, the city seems to suffer no ill effects such as the power outage in "Visit to a Hostile Planet" due to the ship's landing and take-off.
• The alien leader is called "Gilt Proto" in the credits, although he is never called by name nor does he mention his name in the episode.
• In a minor continuity error when the Jupiter 2 lands on the alien planet the scene from the viewport is of a city (in fact Maureen comments "A nice, clean city instead of some dismal planet."). However when Gilt Proto opens the hatch the outside scene is that of the standard "dismal planet" landscape.
• The Robot tells Will he has no idea where his instructions to the Imposters will send the ship, only he hopes 'not back to Earth', which is exactly where they land! (But in fairness The Robot is an environmental robot not a pilot and Will himself acknowledges that what The Robot did wasn't enough to avoid Earth.)
• When Smith splits the Robot in two, how is it that the upper half is able to remain suspended in mid-air? (In universe? Anti Grav? Real world? Probably a poll holding it up out of camera range. Notice Bob May couldn't fit inside just the upper half so Robot's arms don't move at all while he's split in two.)

• How many times is Smith going to accidentally launch the Space Pod? Shouldn’t someone make a rule to prevent him from ever going near the damn thing?
• When the Robinson clones appear, they are encased in bubble-wrap. Why? Were they fragile?
• John Robinson becomes disoriented at the sight of his clone. He says he can’t get used to the idea of “talking to himself.” Considering he spent a great deal of time talking to his evil double, the Anti-John just last week, I would think he’s be quite used to that sort of thing by now!
• The Jupiter 2 lands on Earth. And Smith doesn’t get out. Why doesn't Will let Dr. Smith leave the ship while he (Will) has the chance? This is their opportunity to be rid of Smith and also to get him back to Earth where he wants to be anyway. (LOGICALLY I agree. But REALISTICALLY as much influence as Jonathan Harris had on the show at that point, he'd be the LAST person to want Smith back on Earth because back on Earth while the Robinsons go back to being lost in space would mean that Harris would no longer be on the show. Which many would have loved at least for a while. I suspect that even his harshest critics would eventually miss him if the show had ever gone on without him.)
• Great line: Smith: "Oh William, how do I communicate with alien doubles?" Will: "With double talk, I guess."

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