Dr. Smith, Will and the Robot are left alone at the spaceship when the rest of the family goes on a camping trip. An alien ship tries to convince John to allow them to land, but John refuses, which turns out to be a wise move on his part, as the aliens turn out to be bank robbers.
When John refuses to help them, the aliens contact the Jupiter 2. They speak to Dr. Smith and claim that they are wealthy vacationers who are looking for a place to stay. Smith thinks he can make a tidy sum of money, and despite Will’s protestations, he offers up the Jupiter 2 as a resort hotel.
The four aliens (two male, two female) transport down to the planet in clever human disguises along with a brain-washed Mr. Zumdish as their travel director. Zumdish speaks to Smith, insisting that his guests be shown a wonderful time, and threatening to leave without paying if things are not satisfactory. Dr. Smith assures Zumdish that “Happy Acres” is the best hotel there is.
One of the alien women, Non, starts to flirt very heavily with Smith, which he seems to enjoy. Will and the Robot are not enjoying themselves at all, as the doctor is working them day and night and they are both exhausted.
When night comes, the aliens all revert back to their natural forms and wander around outside the spaceship, plotting. The male aliens hatch a plan. They are going to get Non to give her matter transformation ring to Dr. Smith. That will cause him to turn into an alien at night. Then they will kill him and plant his body where it will be mistaken for one of them. That way the authorities will think the bank robbers have died and will call off the search.
The next day, Non gives her matter transformation ring to Dr. Smith. He is very happy because he thinks it is actually a giant diamond. Non continues her flirting and arranges for Smith to meet her outside at midnight for a romantic date.
When midnight comes, Smith slips outside for his rendezvous and the ring causes him to shift into the form of an alien. Of course, he is not at all aware of the change. When he meets up with his lady friend, she too is in alien form. Smith does not recognize her, and thinking she is some kind of monster, he screams in horror and runs away.
Meanwhile, the evil male aliens decide to kill all the humans. They disable the Robot and head to Will’s cabin, intending to do away with him. Will hears the commotion and peeks out the door of his cabin. When he sees the aliens heading his way, he conceals himself. The aliens are unable to find him and they leave the ship.
When morning comes, all the aliens as well as Dr. Smith himself revert back to their human forms. Smith is confused, having no memory of why he was running around outdoors all night howling at the moon. He happens on the chariot and the returning members of the party and he screams for help, telling John and Don that monsters have taken over the Jupiter 2.
Don and John grabs their lasers and rush to the rescue, but when they get to the ship there are no alien monsters in sight. They are very confused and suspicious when they see the hotel set-up, and they question Smith about it. Smith feigns ignorance, but Will explains everything to his father. Mr. Zumdish snaps out of the trance the aliens put him in and informs John that the aliens are bank robbers and must be stopped.
Dr. Smith runs back to the chariot where the women are waiting. Maureen asks him what s going on, but he refuses to explain anything. The male aliens burst upon them and capture the women, taking them away to a cave where they plan to kill them by throwing them into a lava pool.
John and Don arrive at the cave in time to rescue the women. They fight the aliens and shove them into the lava pool where they are destroyed. Mr. Zumdish and the two female aliens prepare to depart the planet shortly afterwards, but Non wants to say goodbye to Dr. Smith first. When the Robot tells Smith that the ring Non gave him is not a diamond and is not at all valuable, he gives it back to her and breaks off their romance.
- Apparently a cash register is included in the standard Alpha Control equipment for interstellar exploration.
- this is the only episode where the force field is shown to cover the entire planet and prevent an entire spacecraft from nearing it. this is perhaps due to modifications with alien technology
- what is a landing beam and how come in the past spacecraft were able to land on planets without the beam activated
- Smith claims that at happy acres they have rocket testing and "meteor riding". how would "meteor riding" be possible? most likely he was lying.
- Carroll Roebke did not have any dialog for the episode. Edy Williams had all the female character alien interaction. This may have been done to save money as non-speaking parts pay less.
- Most of the trouble and strange things that happen are due to the actions of either Smith, Will or the Robot. So why would these three be left alone together to care for the spaceship?
- Where did Doctor Smith get all the patio furniture and sports equipment?
- Why would Doctor Smith think that people were trying to sneak into his hotel? It had only been open for a few hours, and it wasn’t even really a hotel!
- When NON is coming on to Smith so heavily, why does he practically ignore her advances? Does he really find a gaudy ring that much more appealing then a beautiful woman?
- Before Zumdish leaves, he commends Smith on his excellent hotel and says he will bring more clients by soon. If Zumdish is pleased, I assume he paid the hotel bill. If he did pay the hotel bill (which was to be paid in Deutronium fuel), then why was John so annoyed with Smith? The doctor would have just earned them enough fuel to get back to Earth!
- The alien masks were also used in an episode of 'Land of the Giants.'