Prof. Robinson and Major West put the Jupiter 2 into orbit around a mysterious planet. Will is being confined to his quarters due to his poor performance on a history test and expresses his displeasure at the situation to the Robot. The Robot goes to the Flight Deck to offer his services as a tutor for Will but is interrupted by the Professor requesting an analysis of the planet. The Robot declares the atmosphere is composed of 50% methane and 50% of an unknown gas that is alive, evil, and hungry.
Suddenly the systems go dead and everyone is paralyzed. A mysterious blue fog appears at the viewport along with a giant claw. The hatch opens and some of the gas enters the spaceship. Will is resting in his bunk when a voice speaks to him, offering to grant him any wish he wants. Will only wants to be left alone and tells the voice so, but unfortunately Dr. Smith (who has been resting in his bunk) has overheard the same voice and thinks it was talking to him. Dr. Smith wishes that everyone on the ship would just disappear. He then vanishes and appears in a nightmarish void full of oversized child's toys. There he has a brief encounter with a disembodied, sinister childlike entity before being returned to the ship.
Soon other people begin to vanish but not return - Maureen vanishes while in the galley, Judy while operating the elevator. A thumping noise is heard on the upper hull and the Robot states this is "an object of great weight trying to break through." The Robot then says this is the entity he spoke of, and that it is trying to generate fear as that is what it feeds upon.
After Penny, Don, and Dr. Smith vanish only the Robot, WIll, and Prof. Robinson are left. Despite tying a rope between Will and himself, Prof. Robinson vanishes while trying to use equipment in the Space Pod to reverse the disappearances. The Robot then vanishes as well, leaving Will all alone.
Down on the planet everyone is being held in the void by the entity. The creature steals Dr. Smith's form and returns to the Jupiter 2 in this disguise. The imposter Smith confronts Will but is openly belligerent and mean - the complete opposite of the real Smith - even striking Will when Will resists his efforts to land the ship on the planet.
Will is pursued throughout the ship by the imposter Smith. Will makes it back to the Flight Deck to discover the Robot has returned - apparently Prof. Robinson's plan has worked. The Robot tells Will the situation, including the fact that he is being chased by the creature in Dr. Smith's form. The imposter disables the Robot by removing the power pack, and chases Will down into the Jupiter 2's Power Core.
Major West and Prof. Robinson rematerialize on the Flight Deck, reactivate the Robot, and learn that Will is running from the imposter Smith. They begin to quickly search the ship in order to find them both.
Meanwhile, Will is in the Power Core arguing with the imposter Smith, with the creature not understanding why Will doesn't like him or appreciate what he's done for him. When Will states the creature is merely copying everything he does, the creature becomes agitated and releases Dr. Smith from his control. The creature now appears as a ghost-like being who retreats when Will confronts him with his determination not to take the dark path the creature wants him to take. The creature weakens, falls into the Power Core, and is destroyed.
Background Information[]
At one point "Dr. Smith" yells out "I AM YOUR ID", to which Will replies, "Don't try to scare me with your big words."
In perhaps the cheapest costume to appear in LIS, the monster's final apparition is an actor with a bedsheet thrown over him.
Ron Gans provided the distinctive voice for the evil alien Armus in the Star Trek:The Next Generation episode "Skin of Evil." It is mere coincidence that Armus looks like a bedsheet, albeit one covered in black tar.
The Jupiter 2 seems to grow an extra deck in order to accommodate the Power Core. Also interesting is that despite the fact the Power Core functions from a material "made by concentrating certain radioactive ores" Will can stand right next to it without any protection and suffers no ill effects.
The storage area just before the last Power Core door has a lot of boxes labeled "Food Concentrate". Maybe the radioactivity helps perserve it?
