While John and Maureen are away researching nearby volcanic activity, the rest of the crew endure a series of meteor showers. After the latest one, Don and Smith go outside the ship to check on their force field. Dr. Smith is struck by a mysterious ray. Gradually, the remaining members of the party are each hit by the same ray in turn. In each case, the original is replaced by an emotionless duplicate.
Will and the Robot were away at a relay station while this was happening. After they return and notice how odd everyone’s behaviour is, they find out that the alien Lemnoc has created the duplicates in order to learn as much of the Robinson’s incredible instinct for survival so that they might pass that knowledge on to his own dying race. In the meantime, the original crew are being held hostage in suspended animation. Will is thus forced into educating the copies within the time limit given by Lemnoc.
Will and the Robot set about instructing the duplicates in as wide a range of skills as possible, given the short time available. Lemnoc is pleased with the results and releases Don, Judy and Penny. Less pleased with what the Dr. Smith duplicate has learned, he refuses to release the original.
When John and Maureen return from their expedition, Will explains what has happened. John and Don go to Lemnoc’s cave in an attempt to rescue Smith, but Lemnoc informs them that he will only release Smith if they give him the Jupiter 2. Since John knows they would stand little chance of surviving without the spaceship, he cannot make a deal, even for Smith’s life.
Heartbroken at the prospect of losing his friend, Will confronts the Smith’s duplicate when he tries to comfort him, raging that he isn’t real and that they can never have the same relationship that Will and the real Smith had. The duplicate Smith is so moved that it goes to Lemnoc and insists the real Smith be released. When Lemnoc refuses, the duplicate releases Smith himself and is destroyed in the process. Lemnoc realizes that he could have learned so much more through friendship than through intimidation, but now it is too late.
- Lemnoc is wearing the same costume that was worn by Morbut in A Visit to Hades.
- The suspended animation devices that Lemnoc uses look an awful lot like the freezing tubesin the Jupiter 2.
- After John and Don fight with Lemnoc, they leave his cave to return to the Jupiter 2.. When they are leaving, Mark Goddard bumps his head pretty hard against one of Lemnoc's pieces of hardware. One of the best bloopers of the entire series.
- In one scene, the Robot flashes but doesn't say anything. Which may be just as well, since a moment before that, in the same scene, the Robot calls Lemnoc "Lennock". In any case, Lemnoc's name is spelled explicitly in a legend displayed above his chair.
- If they are essentially made of cosmic dust, how are the clones able to eat?
- Why did Lemnoc write his own name in huge red letters on the wall of the cave?
- John and Don decide to let Smith die rather then hand over the Jupiter 2. However in the episode “The Mechanical Men,” they are prepared to hand over the spaceship with virtually no fight.