Penny is somewhat resentful of her brother Will when he is allowed to help their father and Don, but not she. Will is very blunt with her and says she does not even understand what he is doing, but her father is somewhat kinder even though he tells her to run along.
Left to her own devices, Penny goes off to play with her "friends," an animate bush that drops its leaves when she curtseys to it, and a fountain that gushes up out of a pool of water. She suddenly hears a voice, but mistakenly believes it is only an echo of her own. She gradually realizes that it is in fact a real voice and tracks it into a nearby cave. Although she finds no one there, it dawns on her that there must be some sort of intelligence dwelling in the darkness.
Everyone dismisses Penny's story as an imaginary friend, but Dr. Smith becomes interested when the girl returns to camp one day carrying some diamonds. Determined to find the source, he follows her to the cave and learns the location of the diamonds, although he knows nothing of Penny's friend inside.
As Smith enlists the Robot's aid in gaining further information, Penny's relationship with the disembodied voice grows in depth and quality. The Robot's only finding is that he senses danger. When Dr. Smith tries to trick Penny into believing he is her unseen friend, the real voice disappears and Penny is heartbroken.
Undeterred, Smith tricks Don into blasting over the cave in order to find some clay-bearing soil, apparently something which the Robinsons need. Heedless of Don's warning, Smith uses too much explosive, unaware that Penny has returned to the cave below to look for her friend. Penny is knocked unconscious in the catastrophic explosion, and when her friend fails to rouse her, it goes on a rampage, manifested in tremors, violent winds and lightning.
The Robinsons are defenceless in the face of the oncoming disaster. Only when Penny regains consciousness and struggles back to camp to beg for their lives does the creature's rage quieten. At that point something strange happens; having left its cave, the creature has begun some process of metamorphosis and begins to move out into outer space as it says good-bye to Penny.
- This is the episode in which Dr. Smith first utters his classic catchphrase "Oh, the pain."
- William Bramley provided the voice of Mr. Nobody.
- This is Angela Cartwright's favorite episode.
- The teaser has a great overhead shot that establishes the size of the set. Sadly, such shots were typically limited to the beginning of the first season.
- This is the third consecutive episode where the robot is disassembled in some manner. Smith seems to fix the robot in the space of a few hours. Over the course of the season, Smith would lose the ability to fix the robot.
- The delicate and lyrical music for this episode is the last Lost in Space score written by John Williams. However, he would return to the series one final time to compose new theme music for the third season.
- Writer Jackson Gillis had a unique talent for creating stories focusing on Penny. He would go on to contribute "Penny-centric" episodes such as "The Magic Mirror", "The Haunted Lighthouse" and "Princess of Space."
- This is perhaps the king of all Lost in Space bloopers… In this episode, the Jupiter II’s astrogator disappears!! All that’s left is the platform it was seated on. No mention of it being packed away where the Chariot is parked around the back, or perhaps in that strange storage cupboard to the right of the hatch, that interestingly looks big enough to hold a Space Pod...hmmm. It does eventually return, but not in this series. All is not lost, however, as the Robinsons replace it with a very nice table and chairs setting…
- In the cliff-hanger, John makes his incredible leap to save his daughter from the explosion, (actually Guy William’s stunt double). In this shot observe Penny from behind and you will notice the dumpier appearance of her stunt double being knocked out of harm’s way.
- Also,when Guy Williams’ stunt double rushes and tackles Angela Cartwright’s stunt double clear of the explosion, they end up prone on the ground alongside each other near a rock. The next shot shows Guy Williams actually on top of Angela Cartwright, instead of side by side.
- The Robot's elliptical sensors cease operating altogether. They are not viewed working again until "Blast Off Into Space" (ep 30).
- How To Rig An Explosive Robinson Style – firstly, drill a hole, then attach some wires randomly inside the hole; drop some pellets from your pellet bag into the hole, and carefully walk away and hide behind a paper mache rock. Oh, and do not check to ensure unauthorized crew members are wandering about aimlessly in the area.
- Shouldn't the men have maybe moved the drilling rig away from the hole before they started blasting?
- Judy wants a new hairdo! Of course, Maureen shall summon the Alpha Control Space Hair-Do Hat! Wait 5 minutes, and voila! Same Judy hairdo! Not that Judy realizes; she thinks it looks just grand!
- The Robot’s laser circuits used to combat the evil Mr Nobody look exactly the same as its electrical charges.
- Who can explain Mr Nobody’s astral transformation, and that it can now leave the planet?