Penny isn't attracted to sharing the surface issues of Judy and her mother, she wants to spend time with Will and he goes about the chores of Don and his father. Alone and misunderstood, she enters a cave where she finds an invisible but friendly being, with whom she can express her feelings, nobody believes her in her surroundings and they assume that it is a game, but when Dr. Smith discovers that in said cave There is a diamond mine, he begins to manipulate it to seize the precious stones, he convinces Don to drill the cave, assuring him that there is plutonium there to make the fuel they need for the Jupiter II, when the explosion happens with Penny inside, she is left unconscious. That provokes the wrath of the invisible being that comes out of the cave unleashing a storm that destroys everything in its path. This fantastic story is developed to perfection, the cast contributes equally to the story, highlighting as always the great performance of Jonathan Harris when he was still a nefarious and vile man and had not yet become the buffoonish character of season 2, and also Angela Cartwrith in a leading role, the music and effects are also perfect.