While in orbit around a planet, the Jupiter 2 becomes infiltrated with a mysterious vapor. The robot names it as “Unidentified gas comprising 50% of the planet’s atmosphere. But it is more than a gas. It is a living organism. A highly dangerous one. It is evil and hungry.” When asked, “Hungry for what?”, the robot replies “Still computing that.”
The ‘voice’ of the fumes/miasma speaks to both Dr Smith and Will. ‘It’ says they can have whatever they want. Will, who’s been getting a hard time of late because he’s been neglecting his studies, wishes for everyone to just leave him alone.
As if on cue, the Robinsons, beginning with Maureen, start disappearing. Penny, Judy and their mother are together in the galley one minute, the next Maureen is suddenly not there. Later Judy vanishes mid-journey on the ship’s elevator (‘glide-tube’). Will is then instructed by his father to search the Jupiter 2 top to bottom – including the lower deck Power Core – in an effort to find Maureen and Judy.
Later the malevolent mist descends again outside the viewport. The robot announces it has finally computed what the organism is hungry for. “Fear. It feeds on fear and hatred. Therefore no one must be afraid.” This time Penny is the one ‘removed’ when Will has his back turned.
We then see Penny, Judy and Maureen and the fog-shrouded, purgatory-type netherworld they have been transported to. They are huddled protectively together as a voice Penny describes as ‘creepy’ speaks to them about what their fate will be as well as the ‘game’ they are currently forced players in.
Dr Smith proposes the idea the malignant fog only causes females to disappear. This theory is proven false when Don vanishes while at the controls of the space pod.
Professor Robinson attaches a rope to Will so they are always connected. They go looking for Smith. John shares his theory that each person who has disappeared was in contact with a machine that had a built-in oscillator. His guess is if they reverse the polarity on one of those oscillators, everyone will be brought back.
Even though they are now connected by rope, John still disappears, followed by the robot. Will reverses the polarity of one of the machines and one by one the Robinsons reappear on the ship. When Dr Smith returns, he is not himself but rather is inhabited by the evil force responsible for all the disappearances.
Smith pursues Will, who runs to the Power Core – the place where the ship’s nuclear reactor is housed. When Will shows he is not afraid of Dr Smith, the evil presence leaves Smith’s body and assumes a ghostly form. When Will declares he is not afraid of the spirit apparition, it grows weaker and eventually falls back into the fiery furnace of the Jupiter 2’s nuclear reactor.
This episode, along with two others – Season Three’s SPACE DESTRUCTORS and Season Two’s REVOLT OF THE ANDROIDS – would be my equal favorite episodes of the entire series. Big call eh?