Shortly before the Jupiter 2 prepares for lift-off, Penny goes to get her scarf from a cave. Once there, she encounters an alien boy by the name of J-5 who begs her to take him with her family when they go. He says that if they leave him behind he will be eaten by a Zaybo. Penny asks what a Zaybo is, and a lion suddenly appears nearby. Frightened, Penny runs to her father for help. In reality, the Zaybo is J-5's shape-shifting pet created from his mind, but he keeps that a secret from Penny and the others. Penny finds her father and tells him about how J-5 saved her from the lion. All three then hurry back to the Jupiter 2 and the ship takes off for outer space.
Everyone likes J-5, especially Penny. Will feels a little jealous of all the attention the new boy is getting, and scoffs at some of the stories J-5 tells. J-5 decides to teach Will a lesson by scaring him with the Zaybo, but when the Robinsons promise to take J-5 back to his own planet, he changes his mind and puts the Zaybo away.
The Jupiter 2 develops some mechanical trouble and must land to make some repairs. Luckily, a space lightship is nearby, so they stop there. John gets out to take a look around, and finds the place to be rather messy, but safe. He is greeted by a very friendly man named Colonel Fogey. Since Fogey has not seen any other human beings in over a decade, he is thrilled to meet the Robinsons. Colonel Fogey prepares a banquet for them and everyone has a lovely time.
Fogey says he can give the Jupiter 2 enough fuel to get back home to Earth which makes everyone very happy. However, it seems they don’t have enough fuel to stop over at J-5's planet. J-5 is angry and heartbroken and runs away to plot his revenge. Penny tries her best to comfort him, and she eventually convinces him to come back down to dinner. J-5 secretly tells the Zaybo to start making mischief.
A few moments later, alarm bells start to go off. Everyone rushes to see what the matter is except for Dr. Smith, who stays behind to drink. The Zaybo, disguised as a pretty lady appears and tries to tempt Smith into going to J-5's planet. J-5 then tries to trick Smith and Penny into helping him steal the Jupiter 2.
In the control room of the lightship Colonel Fogey has started to cry, confessing to the Robinsons that he isn’t really a colonel at all; he’s just the caretaker. He doesn’t know how to work any of the computers and he is very upset about it. He also admits he doesn’t actually have the fuel he promised the Robinsons. He only said he did because he wanted to impress them and hoped they would stay longer to keep him company.
Will leaves the bridge looking for Penny and ends up encountering the Zaybo in lion form again. He is frightened, and Penny rushes to the rescue. Afterwards, Penny tells J-5 that she has figured out that he controls the Zaybo and has the power to make people imagine things. J-5 apologizes, saying he only did those things because he’s been so lonely.
In the end, J-5 decides to stay behind on the lightship with Colonel Fogey. That way they can help one another and neither of them will be lonely. The Jupiter 2 departs, getting back on its course for Earth. It seems to be a happy ending, but Penny realizes that she will miss J-5 very much.
- When John puts on his pressure suit and leaves the Jupiter 2 to go over to the other spaceship, he is not wearing his gloves. Wouldn't he need them if he was going out into the vacuum of outer space?
- Where did Penny get the match from? Does she carry a box of matches on her?
- The computer voice over that greets the Jupiter 2 upon docking informs the visitors that, among other things, the ship launched September 18th, 1997. The true date is of course October 16th (strangely, in the later episode 'The Time Merchant' it is given correctly.).
- The computer voice also says the Jupiter 2 has been lost in space two years, five months, and six days, so the date should be February 24, 2000 if the September date for launch is used.
- The 'lighthouse' is the 'F-12 Fuel Barge' seen in the series 2 episode Wild Adventure.
- J-5's pointy ears are the same as used by Mr. Spock on 'Star Trek'.
- Since a rescue ship is on its way from Earth to the station, it seems odd that Dr. Smith gets back on the Jupiter 2 rather than simply wait it out for a guaranteed return.
- Why was this episode titled “The Haunted Lighthouse” when there were no ghosts or haunting of any kind?
- In the episode “Kidnapped in Space” it is stated that Doctor Smith is not a real medical doctor and has no business trying to practice medicine. So why was he asked to give J-5 a medical checkup?
- Why does Doctor Smith scream in horror upon seeing a beautiful woman?
- How did Doctor Smith get full technical knowledge of the Jupiter 2?
- When John first arrives at the space port, the docking computer begins to repeat information it has gathered about him and his ship. The computer states “Spaceship Jupiter 2 departed Earth September 18th, 1997. Mission; exploration.” The Robinsons hear this message and nods their heads, agreeing that it is correct. The Robinsons and the computer are all wrong. Spaceship Jupiter 2 actually departed Earth on October 16th, 1997. Its mission was colonization.
- In this episode, J-5 asks Penny to get Dr. Smith's glasses. Since when did Dr. Smith start wearing glasses. Penny says she can't find them, but Dr. Smith never wears them. If J-5 never knew or saw anything about the glasses, how did he know about them?
Uncredited co-stars (alphabetical)Edit
- Bob May as the Robot
- Dick Tufeld as the Robot (voice)
- Woodrow Parfrey as Colonel Fogey.
- Lou Wagner as J-5.