John, Don, Will and the Robot leave the campsite in the chariot to set up microwave relay stations. A meteor storm strikes soon after and the men are barely able to reach shelter in time. Back at the Jupiter 2, Dr. Smith believes that the meteor storm was in fact a barrage to weaken them prior to an invasion by aliens, but Maureen dismisses his worries as unfounded. She is more concerned that the men were safe.
Maureen, Penny, Judy and Smith notice a fresh crater on their way to find the men. Nearby the crater is a wrecked human spacecraft, but it is empty. After they leave, a horned beast climbs from the crater and watches them. That night, the force field is turned on for protection as everyone is beginning to feel a bit jittery, especially when they discover some of their food is missing. After being awoken in the night by strange howling, Maureen and Judy set out armed the next morning to investigate and find a small dog in the wilderness.
Although they do not know where the dog came from, they allow Penny to adopt it. The only reasonable explanation they have is that it was an test animal sent out into space many years before, but Dr. Smith is certain it is a hostile alien in disguise, his opinion strengthen when the dog growls and snaps at him. The women also find some fur caught on a rock; when Judy analyzes it, she determines that it could not have come from the dog and they conclude that there is still something lurking out there beyond the camp. They suspect that the weird ecology of the planet may be responsible for one of the local life forms growing large enough to become a danger to them.
That night the find that the force field will not function because its batteries have run down. The dog has also run off and Penny has gone to look for it, narrowly missing an encounter with the alien creature instead. Maureen is furious to find that Dr. Smith has disassembled all of their weapons in order to clean them, and they are now utterly defenceless. Penny finally manages to find the lost dog, but it is clearly agitated and nips and struggles to escape when she attempts to pick it up. Penny is shocked, but does not realize the monster is now drawing closer to her and the dog sees it. Meanwhile, the men are able to establish communication with the Jupiter 2, and although she is uncertain and worried, Maureen reassures her husband that everything is all right at camp; after all, other than finding a dog, there has been no concrete evidence of any danger. John does not believe her and immediately orders the chariot to return to camp.
Once back, Will reassembles the weapons, and the family go out to find Penny (and Judy, who had gone out earlier to look for her sister). When they find them, the monster is upon them, and only through quick action are John and Don able to drive it away before the girls are harmed. Will quickly finds the dog, and dropping his gun picks it up and holds it. Dr. Smith insists John or Don kill it, but they laugh at his belief that it is an alien spy. As one last service, the dog leaps from Will's arms when the alien beast reappears and then brings a laser gun back to Will so he can shoot the alien.
- Although Judy inquires about early space flights being sent up with animals, the viewers probably assumed she was thinking of the earliest Soviet launches (Laika on a Sputnik) and American launches (chimps). The Alpha Control Reference Manual states, however, that another series of tests in the 1970s/80s used dogs as America tested the technology that sent Jimmy Hapgood into space.
- Another force field continuity gaffe occurs. When Maureen and Judy are turning on the force field, Judy stands next to the force field projector and throws a rock "away" from the Jupiter 2, and the rock is destroyed by the force field. But, according to a statement by John in "The Raft", the force field does not work on objects traveling away from the force field generator, only objects heading towards the force field generator. Also, in the previous episode "The Raft," the force field controls are on a panel inside the ship, but in this episode the force field controller is back to being the large gun-like instrument shown in earlier episodes. See also the reference to the robot turning off the force field in "There Were Giants In The Earth".
- The same creature will return in "Hunter's Moon" as the animal Megazor was tracking when John Robinson kills it after it threatens him.
- Dr. Smith's personality undergoes a slight transformation from master villian to effete scoundrel as he initially believes the dog is actually a highly intelligent and threatening form of alien life disguised as a dog.
- The title card shows that the Jupiter 2 has settled in the ground a few feet as compared to the first shows of the season.
- Why isn’t there some kind of technical manual on the Jupiter 2 that shows how to put the laser guns back together? If not, couldn’t they have asked the computer?
- What the heck happened to the dog? Everyone loves him, but after this episode we never see him or hear a single word about him ever again!
- In the cliff-hanger re-fresh, the Robot is seen aboard the Chariot with John, Don and Will. Only trouble is, there is no view of the automaton in the exterior shots.
- When Dr Smith, Maureen, Judy and Penny walks back to the spaceship, and past the crater, the alien mutant lets out a blood curdling roar. How the heck did they not hear it, since they are still mere yards away?
- The Robinson females have no grasp of security, as they exit the spaceship without closing the spaceship’s hatch. And they wonder why some creature has been pilfering their stocks upon their return???
- The horny mutant is seen opening and peering into the lower hatch view-ports of the spaceship. Presuming that the lower hatch is buried under the planet’s surface after the crash landing; wouldn’t the monster have been digging for weeks to uncover it?
- Don understands dog speech. “Woof woof!” Oh, how totally normal, a cute dog on this planet. “Hang on; he wants us to follow him!” Yeah, let’s go!
- During the climax, John fires the laser pistol at the monster as it scrambles pitifully back into its sand pit. The last few shots he fires results in the tip of the laser pistol lighting up, but no laser beam is emitted.
- The cliff-hanger credits are different: “Sorry! To be continued two weeks from tonight! Same time! Same channel!
Dr Smith: [as a meteor shower bombards the planet] "I knew it! Doomsday!"
Dr. Smith: “My dear child, your mind is too young to accept all the harsh realities of life, yet I believe you should know the truth.” Judy:“What truth?” Smith: “That was not a dog - at least not in the sense of which we think of a dog. It was an alien spy!”
[Maureen and Judy reading John's speculative notes on the planet's inhabitants] Prof Robinson: [voiceover] “My greatest concern for our safety is the possibility of encountering mutants - creatures with no exact counterpart in nature. The giants are probably one form of mutant. I fear there may be others. They seem to grow and evolve through a process of metamorphosis, taking on new forms by absorbing all types of organic matter. Now, if this wild theory is true, any mutant we encounter may try to absorb one of us.”
span style="font-family: Tahoma; mso-bidi-font-family: "Times New Roman";">Gallery: