After Dr. Smith loses a laser pistol in a cave, he and John must go in to look for it. There is a cave in and John is trapped; Smith refuses to help and runs away. John searches for another exit and finds a chamber containing artefacts from an ancient civilization. A powerful alien voice calls him by name and demands that he surrender control of his body before rendering John unconscious. At the Jupiter 2, Smith tells the Robinsons that John is trapped in the cave. They take with them digging tools and go to the cave. They rush to the cave to rescue him, working all day to try to unseal the entrance of the cave. When night comes, the family is exhausted. Despite their unwillingness to to stop, the darkness forces them to return to the ship.
Everyone is surprised but overjoyed when John arrives at the Jupiter 2 on his own. He is dirty and tired, but seems to be unhurt. The next morning, however, John shows violent mood swings and frightens everyone. Dr. Smith expresses his professional opinion that John must be having a nervous breakdown; after a while, the others start to wonder if he may be right.
When John Robinson sleeps, the alien voice takes a stronger hold over his mind. It gives him instructions, saying that he must begin preparations to leave the planet as quickly as possible. John does so, designing a machine that can synthesize deutronium fuel. He orders everyone to work day and night so they can lift-off in a week. When Don and Maureen question his orders, he becomes violently angry.
Dr. Smith offers to treat John as a psychiatrist, and discovers that the Professor is being controlled by the spirit of an alien being named Canto. Canto warns Smith that if he tells anyone else of his presence, he will be destroyed. Terrified, Smith agrees to keep quiet.
That night, John leaves the spaceship in secret and returns to the cave. Will follows him but John catches him and accuses him of spying. He angrily orders the boy back to the spaceship. After Will has gone, Canto takes over John’s mind completely.
In the morning, Maureen, Judy and Don go to the cave to find the missing professor. John tells them his name is Canto now, and that John Robinson is gone forever. Maureen and Judy try very hard to appeal to whatever of John may remain, but it is no use. Canto fights with Major West, and after defeating him, leaves the cave, sealing all three inside.
John, now wearing a golden mask representing Canto, arrives at the Jupiter 2, and tries to trick Smith and the children into thinking their parents, sister and Major West have been kidnapped. Canto tries to convince them to come on board the Jupiter 2 with him so they can fly away to rescue the others. Will refuses, knowing that Canto is lying, and that something strange is going on. Canto says he will show Will proof, and Will reluctantly goes along with him.
Canto takes Will to the top of a high crevasse where he asks the boy if he knows what he plans to do. Will answers that he plans to kill him, and asks to see his father’s face one last time before he dies. Canto agrees and removes the mask. Will looks into his father’s eyes and tells him he loves him. Will’s words reach what remains of the professor’s consciousness, and John throws the golden mask into the crevasse, freeing himself from the evil spirit of Canto. Father and son embrace, safe and reunited once again.
- This is said to be one of Guy William's best performances of the series. If you liked "Follow The Leader," here is a continuation of that story by a fan: http://www.bookscape.net/lis/unintended_consequences1.htm
- This episode is the last to be filmed in black and white.
- The cliffhanger is reused from the end of "One of Our Dogs Is Missing", leading into "Attack of the Monster Plants".
- During the fight in the cave between Professor Robinson and Major West, West's sword is pulled away by Professor Robinson's weapon. In the next shot it reappears in Major West's hand.
- When Professor Robinson is walking into the Jupiter 2 campsite wearing the alien uniform, his shadow appears on the background.
- This episode is the third time the Robinsons have found remnants of an ancient civilization on Priplanus with all of them stashed or hidden away in caves or underground. In "There Were Giants in the Earth" they find ruins inside of a cave like the ones here. Perhaps the inhabitants moved inside to avoid the wild climate outside. However it is not clear if the ruins are related to one another, which might be expected given that they are all certainly within a few hundred miles of each other.
- Gregory Morton will return to do additional voice-over work as Judge Iko in "The Prisoners of Space", as well as many other voices in the series
- Why did it take the Robot so long to detect that something was amiss with John?
- How did Don, Maureen and Judy get out of the tightly sealed ruins?
- The end of "Follow the Leader" is similar to the end of Return of the Jedi. John Robinson (mind-controlled by Canto) wants to sacrifice his son, but love frees him from the dark side, just as Darth Vader want to sacrifice his son, but love frees him from the dark side. John Robinson tosses the evil Canto helmet into an abyss, as Vader tosses the Emperor into an abyss.
- In the very next episode, “Blast Off into Space,” we see Don and Judy drilling for Deutronium again. Why would they be drilling for fuel? What happened to the amazing machine Canto invented that synthesizes Deutronium for them?