Don, John, Will and Dr. Smith are out blasting for ore in order to refine deutronium. Smith is too tired to walk all the way to a designated blasting site and tosses explosives into a nearby bog instead. As a result of the explosion an invisible force, disturbed by the explosion emerges from the bog that night.
Dr. Smith constructs a Ouija board so that he can converse with the ghost of his Uncle Thaddeus on the anniversary of his death. When he and Penny attempt to it nearby plants rattle, a glass breaks, and the Ouija board goes flying up into the air. The Robinsons think Smith was trying to play a trick, but the doctor insists it was his Uncle Thaddeus, returned from beyond.
Later that night, the Robot warns of an unknown force outside the spaceship. The force acts in a very destructive manner, throwing tables and chairs around and damaging the hydroponic garden. When daylight comes, the force immediately vanishes. Don and John go out to investigate and find a set of enormous alien footprints. Dr. Smith remains steadfast that they are being haunted by the spirit of his uncle who is angry that the Robinsons have slighted him with their disbelief. Don figures out that Smith threw explosives into the bog and believes that by doing so released the invisible force.
The next night, Smith attempts to placate his Uncle Thaddeus by holding a seance and getting everyone to sing. The invisible force arrives and beats Don and John soundly before the family retreats into the Jupiter 2. It pounds on the hatch and absorbs the ship’s energy until dawn when it disappears once again. Don and John set up a trap to catch the invisible force, which they manage to do that night. However, when they try to inspect what it is, they find that it is still invisible. They decide to wait until daylight, thinking that they may be able to see it then. Doctor Smith is outraged, thinking they have imprisoned his Uncle, and that the spirits will be angry. He decides to hold a ceremony to appease the spirits and gets Will to come along. Smith and Will go to the bog, where the doctor chants and throws garlic. As this is going on, the invisible force breaks its way out of the cage and returns to the bog.
As Will and Smith head back home, Will slips and falls into the bog. The force, now visible, pursues Smith. Smith runs back to the Jupiter 2, where he tells the Robinsons that Will has fallen into the bog. He, Don and John rush to the rescue, but Will has become invisible. The bog monster attempts to attack Will, but he escapes. When dawn comes, Will becomes visible again and the monster disappears back into the swamp. Don and John find Will and are glad to see that he is safe and sound. When Doctor Smith sees Will, he is startled, thinking Will is a ghost who has come to haunt him. Will reassures Smith that he is not a spirit.
- The Robinsons get new fatigues in this episode, since Maureen felt they were due for a change.
- The Robot plays a guitar for the second time.
- The irony of the teaser is that as Prof. Robinson is posting the sign declaring "Danger - Keep Away - Ionized Gas" he taps a pocket of gas that overwhelms him. There is nothing instrinically dangerous about ionized gas - humans breathe it all of the time as it is naturally generated by the strong electric fields in a lightning storm. Plus Will and Dr. Smith are breathing it later in the first act.
- Smith makes the Ouija board by hand but manages to find a monk's habit, an old fashioned lantern, a candle. and a candle holder somewhere on the ship.
- Jonathan Harris tries on his future Dr. Smith voice in the third act when he is trying to convince Will to help him, changing it from the more licentious tone to something more mewlful.
- The cage used to trap Smith in "The Keeper" is used here to trap the entity with solar batteries used as bait. And just to underscore the point the solar batteries are quite clearly labeled "Solar Battery" in large letters instead of just having a part number or other descriptor.
- Why would Doctor Smith assume his Uncle’s ghost had followed him to another planet?
- After Smith throws the explosives, why does he throw the pin?
- What happened with the bog monster? The problem surrounding him and the destruction he’s been causing was never actually solved! Won’t he just keep harassing them night after night?
- John has painted up a sign, which says; 'Danger Keep Away Ionized Gas -John Robinson.' He goes to the bog to post the sign. As he pounds the sign into the ground, the ionized gas bursts up and hits him in the face, knocking him unconscious!! After Will and Don come to his rescue, John’s only comment is “Looks like I forgot to read that sign.”
- This Episode was repeated on Thames Television on February 14th 1969
- This Episode was repeated on Granada Television on March 6th 1969
- All of the crew, at various times during this episode, are seen wearing new outfits, courtesy of biochemist Dr Maureen Robinson's skills with the intergallactic sewing machine.
- When Don rescues John from the gaseous bog, he was originally scripted to commence mouth to mouth resuscitation. The scene was cut as the CBS bi-wigs felt that the scene would prove distressing to younger viewers, (ooooh Don...)
- In the opening scene, Judy is seen modeling her new threads. When the family are later seated around the dinner table, she is still wearing her old outfit