As the Jupiter 2 travels through space, Smith accidently launches the space pod with himself, Will and the Robot inside it. They are drawn into the gravitational pull of a nearby planet and forced to land. The Jupiter 2 tries to track them, but the pod seems to vanish off the radar scope. When they attempt to follow, they too are pulled into a crash landing on the planet. John contacts Will on the radio and tells the boy to stay put in the space pod while they come to get him.
Despite John’s orders, Dr. Smith insists they all get out of the pod, and when they do so, they find themselves in a beautiful jungle. Will and Smith decide to lie down and take a nap. When they wake up, they find the Robot covered in dirt and rust. After cleaning him off, they begin to walk to see if they can meet up with Don and John. They don’t find them, but they do end up finding the Jupiter 2 in a terrible condition and covered in vines. When they go in to have a look around, they don’t find a single member of their family.
Outside the wrecked Jupiter 2 is a statue dedicated to the Robot. When Will reads the plaque at the base of the statue, they are shocked to read that it was erected in the year 2270. Somehow, they had been asleep for over 270 years.
Will is very depressed and concerned over the whereabouts of his family. His attentions are directed elsewhere when a raging army appears, followed by a giant cyclops. However, both of these things vanish before they cause any real harm. Two American astronauts arrive, claiming they are historians. They ask Will a few questions, and then they depart again.
Meanwhile, John and Don continue to search the planet for their missing crew mates. They arrive at the area where the scanner picked up the space pod’s signal, but they find nothing there. Confused, they have little option but to continue to look.
Will, Smith and the Robot explore a cave where they encounter an oddly dressed woman who looks exactly like Judy. Will greets her as such, but she claims she isn’t Judy at all. Rather, she is Judy’s great-great-great-grand-daughter. She takes Will aside to have a chat with him. A young astronaut appears and smacks Dr. Smith. He claims to be Smith’s great-great-great-grandson, but is not proud of it. He claims the name “Smith” is shameful to carry due to Dr. Smith’s disgraceful act of sabotage on the Jupiter 2. Dr. Smith, of course, feigns innocence and offers to do what is necessary to clear the family name. The young astronaut says Smith can board the mother-ship at dawn. Then he can testify before the Galactic Tribunal to clear himself of the charges. Dr. Smith agrees to do so, and the young astronaut leaves.
Judy’s descendant also tries to convince Will that the best course of action would be to get onto the mother-ship and leave the planet. Will refuses as he still holds out hope of finding his family. He leaves the cave, and Smith and Robot follow him. They go back to the jungle where they left the space pod but it is no longer there. They walk for a while, and eventually find the pod in an entirely different location. Will is very suspicious, having the feeling something very strange is going on around them.
John and Don also find what they believe to be the space pod, rusted and covered in vines. John refuses to be fooled, stating with conviction that it is only an illusion. With those words, the vines vanish and the space pod looks normal again.
Dr. Smith tries to convince Will to make the rendezvous with the mother-ship so they can leave the planet. Will firmly says there is no mother-ship. There was no statue, no rusted Jupiter 2, no astronauts. The whole ordeal has been an illusion, right from the moment they landed.
As Will speaks, a huge computer reveals itself nearby. The computer accuses Will of being an evil intruder, and demands that he leave. Will knows the computer is responsible for all the illusions. It is afraid and has been trying to scare them away from its planet. When Will refuses to go away, the computer creates another illusion: a monster and terrible flames. The Robot blasts the computer, and it explodes. The Robinsons are reunited and the strange illusions are gone for good.
- This episode has a short color clip of the cyclops giant, seen for the first time since The Keeper, Part II.
- Dr Smith is referred to by his 'descendant' as the "Fifth Columnist" of the Jupiter 2 expedition.
- Dr. Smith attempts to eat a peach, which springs out of his hand and explodes. He then states that "All this fruitless activity has done me in; I think I'll take a little nap.".
- The space suits worn by the "Space Historians" were created originally for the classic SF film Destination Moon.
- What is the point of the space historians wearing space suits if will and dr. smith can breath fine. not only that,the spacesuits have large holes in their visors, which makes them pointless even if the atmosphere was toxic
- The Emerald Planet external visual is a re-use of the Sun visual from "Wild Adventure", with green tint added.
- Logic, never a strong point for the show, is particularly weak here. In the last episode the Robinsons were at a Alpha Control lighthouse where they had a clear fix on Earth, making a clear fix on Alpha Centauri possible. Assuming that's where they were headed, it would have also been the first star system they came upon. Instead they enter another strange star system that somehow has inserted itself in the way.
- Didn’t Smith already accidently launch the Space Pod a few episodes ago in “Kidnapped in Space”? Why don’t they make a policy of forbidding him to go in there?
- Doctor Smith becomes indignant and insulted over the idea of having to give the Robot a rub-down. However, in the episode “War of the Robots” Smith willingly rubbed the Robot daily for two weeks.