To trap the invisible creature, John and Don use the cage left behind by the Keeper.
The music's creepier moments make use of Bernard Herrmann's theremin cues for "The Day the Earth Stood Still". As "Day" was also a 20th Century Fox production, this would have caused no licensing problems.
The cast's costumes undergo a radical change starting with "Ghost in Space".
Although the crew's wardrobe goes through a change for everyone involved, Dr. Smith is only seen in his new clothes in this episode & two more to follow. By #1.22, The Challenge, he again dons his familiar turtleneck tunic for the remainder of the season. This continues in Season 2. When Season 3 begins, there is another change in wardrobe for all the cast. Dr. Smith has a new turtleneck & all the male cast join him in adopting turtleneck smocks as well, with the other three male cast member's shirts being more similar in nature & Smith's being more distinctive than the others.
First episode of the series directed by Don Richardson. According to Bill Mumy, he and Richardson did not like each other at all.