As Will, Penny and the Robot spend the day bird watching, they are interrupted by a spacecraft flying overhead. They witness a man and his luggage thrown overboard. Concerned, they hurry to him. He seems to be all right and acts pleasantly enough, but when Dr. Smith sees him, he screams in horror and runs away. The man then wakes and introduces himself as Jeremiah Smith. Ever the polite hosts, the Robinsons give Jeremiah dinner. Doctor Smith continues to hide in his room, and will not come out, even to eat. Jeremiah appears not to understand this puzzling behavior, but Don says that Dr. Smith acting strange is nothing new.
That night, Dr. Smith’s quarters fill with smoke. He wakes, and thinking there is a fire, runs outside. Jeremiah greets him and Smith screams and runs even faster. It turns out that there is no fire, but Dr. Smith is now lost in the wilderness. Jeremiah then announces he is actually Smith’s cousin, and moves into the doctor’s cabin.
The next day, Dr. Smith has still not returned. Jeremiah states that he and his cousin had a fight many years previously, and that is the reason for all the odd behavior. The Robot manages to find Smith, but refuses to give Jeremiah his location. Jeremiah is very frustrated. Will, Penny and the Robot pack a bag of the doctor’s things for him so he will not have to return to camp.
It turns out that Doctor Smith’s fears are justified; his cousin is in fact trying to kill him. Jeremiah tries to commit the deed with an exploding cream pie, but it doesn't work, as Smith sends the bomb back to kill him instead.
Jeremiah tries to get the women to gamble by tricking them with his cups and balls routine, but John foils him. The Robot appears to explain what is really going on; a feud over money. It dates back to Great Aunt Maude Smith, a rich old lady who stated in her will that her entire fortune would be left to the sole survivor of the Smith clan. Jeremiah and Dr. Smith are now the only ones left and Jeremiah has come to kill his cousin in the hopes of being able to claim the money afterwards.
Later that night, Jeremiah calls Little Joe, the head of a gambling syndicate, to whom he owes money. He tries to convince Joe to send a gambling machine, but Joe refuses. Jeremiah is frustrated and tries to shoot his cousin with a laser, but accidentally falls into a pit that Dr. Smith had dug to hide in in an attempt to kill Jeremiah.
Jeremiah again calls Little Joe and once more asks for a gambling machine, explaining that he will use it to trick his cousin into gambling away his claims to the inheritance. With Jeremiah the only eligible heir left, he will then pay Little Joe the money he owes. Little Joe agrees and sends a machine. Both Smiths gamble, and they both lose the inheritance, and Dr. Smith loses his portion of the Jupiter 2 as well. The gambling machine then pursues the cousins all the way back to the spaceship, trying to kill them. John comes up with a plan to save them. He gambles the rest of the spaceship and plays a game of cups and balls with the gambling machine and wins. The gambling machine is defeated and it explodes. Did John cheat? We never know.
- The closeup card manipulation and sleight of hand tricks were performed by Hollywood magician Jay Ose, who had just completed similar work in the 20th Century-Fox production of "The Flim-Flam Man."
- When the three parachutes fall from the spaceship, Will immediately identifies them as "luggage, trunks, and suitcases," though they are still too far up to be identified. How can he tell this when they are so difficult to see?
- John Robinson is usually completely fair and honest, practically to the point of being pathological about it. Therefore, if he did cheat, why would he do it for two scoundrels like the Smiths? There are actually two answers to that: First, John Robinson would never stand by and watch one of the Smith's being gunned down, even if they're both scoundrels. Second, John was also concerned with getting back the share of the spaceship that Smith had lost in the game.
- When Alonzo P. Tucker visited the Robinsons, he had quite a tale to tell about his kidnapping by aliens. And the Robinsons were eager to hear it. But Cousin Jeremiah had no such tale of alien kidnapping, just extensive traveling.
- The Gambling Machine and Little Joe are voiced by character actor Allan Melvin..
- The cave that Doctor Smith takes refuge in has some kind of massive stone door, which Smith somehow manages to easily push open and shut. The cave also happens to have three large torches on the walls. Where did Smith get these torches, and the fuel that is causing them to burn?
- Jeremiah Smith shows the women his cups and balls game, and then wants to make things “sporting” by getting them to gamble for money. Did he really think they would have any? They’re castaways obviously just struggling to survive.
- John says, in regards to Doctor Smith, “he’s done some strange things, but he’s never resorted to violence before.” Er... except for those first four episodes in the series where Smith was trying to murder the lot of you, right Professor?
- Jeremiah protects himself from an explosion by hiding under a napkin and ducking below the table. And it works!
- And of course, the most important pressing questions on our minds; who was driving that ship that dropped Jeremiah off? How did he get off Earth in the first place? How did he know his cousin was with the Robinsons? How did he know what planet they were on? How did he leave the planet after it was all over? Why didn’t he take anyone else with him? Why don’t the Robinsons wonder about any of these things themselves?
- Everyone in the galaxy (except Earth) seems to know where the Robinsons are, but none of them ever help them get back to Earth.