When one of Dr. Smith’s experiments almost injures the rest of the party, John is furious. Smith leaves the camp in a huff and sulks. He lies down for a nap, and when he awakens, he finds that he has been taken prisoner by an army of small robots, each of which is a minature duplicate of the Robinsons’ own Robot.
When Smith does not return by nightfall, the children are worried, but the adults think Smith is playing on their sympathy. There is a sudden explosion in the camp and the force field generator is destroyed. Don blames Smith, but Will defends him. The next day another explosion destroys the weather station. Don and John blame Smith, but this time the Robot tells them that another force is at work. When John asks what force that is, the Robot refuses to answer, saying that he cannot inform on his own kind.
That night a group of mechanical men approaching arrive at the camp. The purple robot demands that the Robinsons hand over their leader, the Robot. When John refuses, the mechanical men demonstrate their power by detonating several bombs. They then offer to exchange their prisoner, Dr. Smith, for the Robot. At the Robot’s insistence, John relents and Will accompanies him to the mechanical men’s cave.
The mechanical men soon discover that the Robot is completely unlike them; far from hating humans, he is peace-loving and recommends cooperation with the Robinsons. The purple Robot decides that exchanging the personalities of Dr. Smith and the Robot will give their leader the qualities they desire.
Will runs back to the Jupiter 2 to tell his family what has happened. They are about to go and rescue Smith when he suddenly arrives, acting and speaking much like the Robot did. Shortly afterwards, the Robot arrives and demands the Jupiter 2; if the Robinsons do not surrender it, his army will take it by force.
That night, the mechanical men stage a sneak attack. The Robinsons are prepared and fight back. When John manages to disable the Robot with a shot from his laser, he is badly damaged. At the same time, the proper personalities of Dr. Smith and the Robot are restored. With their leader gone and their plans in ruins, the mechanical men retreat and leave the planet.
- The tiny mechanical men are toy LIS robots put out by Remco at the time. These were then repainted to match the colors of the full size robot.
- As with most 2nd season LIS efforts, this one strains credibility almost to the breaking point in that regardless of how powerful the tiny robots are made to appear (and the worst they do is blow up a tree) the Robinsons could probably run them all over with the Chariot and repel the invasion that way.
- The chemical lab of the tiny robots is used again as Dr. Smth's wine-making equipment in "The Prisoners of Space."
- Why does the laser blast cause Smith and the Robot to regain their original personalities?
- John claims they have no deutronium. In the episode “The Girl From the Green Dimension,” they had several canisters of fuel, and they have not flown anywhere since then. So where did the deutronium go?
- When the mechanical men attack, why does John send Maureen and Judy inside, but allow Will to remain outside?
- How did the mechanical men manage to tie Doctor Smith onto that large piece of wood without waking him up?
- Will and the Robot walk together, discussing fond memories and “good times.” Robot mentions one such memory as being “the day we met, when we left Earth, heading for outer space.” However, the day the Jupiter 2 left Earth, the Robot was programmed for sabotage and was trying to destroy the ship and everyone on it! Ah, good times.
- Smith and the Robinsons find the mechanical men to be scary and “horrible.” Why would that be? They look exactly like smaller versions of the family’s own Robot. They’re cute as buttons!
- One mechanical man moves twice as fast as all the rest.