John Robinson becomes possessed by an alien creature who forces the family to prepare to return to the creature's home world, even if it requires that he sacrifice Will to keep his secret from the rest of the family.
The Intergalactic Delivery Service comes to Priplanus. Once again Dr. Smith seeks transportation and has Will "test drive" the spaceship. Will returns much smarter than when he left. Smith, realizing the same increase of intellect could happen to him, boards the spaceship. His trip, however, ends in his rapid aging and subsequent impending death.moreless
Penny is given a gift by a thief on the run, who tells her to protect it for him. She and Dr. Smith discover that it leads them to a ring which, when worn, allows the person to turn anything to platinum, which Dr. Smith does. Unfortunately, the ring also turns their food into the metal too and food is always in short supply.moreless
An alien boy is sent to Priplanis to validate his prince status. Will Robinson is drawn into a battle to the death, unbeknown to his father. Once his real intentions are known, John Robinson substitutes himself in the challenge and the alien ruler substitutes himself for his son.
Penny and Judy stumble upon a mirror on the planet. But it's not an ordinary mirror as it seems that there is someone on the "other side". Before they know it they are transported to the mirror universe that is occupied by only one person, a boy who is lonesome for companionship.moreless
Will Robinson and Dr. Smith stumble upon another robot on the planet that is even more capable than their own. The alien robot sets out to gain the confidence of the Robinson family because, at a future critical point, he plans to transport them to his home planet as slaves.
Dr. Smith holds a seance to contact his Uncle Thaddeus. Instead a "spirit" creature is contacted and creates havoc for the Robinsons.
The Robinsons are given the option to turn Penny and Will over to The Keeper or face certain death.
A zoo keeper who travels the universe wants to take Will and Penny and add them to his vast collection of celestial animals.
Will uses an alien transporter to send himself back to Earth. However, many people believe the Robinson family to be dead after hearing the ship went off course. Nobody on Earth believes him, and they lock him up with the intention of sending him to a boys' home.
The Robinson family discover a large alien plant that can duplicate whatever it is fed. Dr. Smith steals the ship's remaining fuel with the intention of feeding it to the alien plant, hoping to double the fuel supply.
While the men are away, an alien creature stalks the Robinson women from the shadows. The girls just think it's a small dog that is to blame for raids on food and other mischief.
The Robinson family banishes Smith from the Jupiter 2. He takes refuge in an abandoned spaceship where he finds a machine that can materialize anything. In an attempt to get back into the Robinson's good standing, he gives them the machine, which plays on the family's greed.
A visiting family of aliens, called Taurons, have decided to use the Robinsons' planet as a new colony. Although the Robinsons cannot understand the language of a visiting family they welcome them to live in peace and harmony. However, as usual, Dr. Smith causes problems when he panics and draws a gun on the alien family.moreless
Dr. Smith eats some alien fruit, which turns him into a giant. He becomes convinced that giving him the fruit was the Robinsons trying to kill him. So he plots to take them out.
Luminous aliens capture Dr. Smith with the plan to replace their burnt out computer with his brain. Smith bargains with them, and they decide to use Will's brain instead.
Astronaut Jimmy Hapgood lands on the planet and the Robinsons befriend him. After Jimmy fixes his spaceship, the Robinsons try and convince him to take Will and Penny back to Earth with him. However, Dr. Smith tries to get aboard the ship to get back to Earth instead.
Professor Robinson discovers that the planet's orbit will bring immense heat on the spaceship. With that knowledge, the Robinsons take off in the Chariot and head south. Dr. Smith stays behind because he believes them to be lying, but soon realizes the orbit is odd. There are periods of extreme cold followed by equally extreme heat. The Robot is sent to tell the Robinsons and they try to return to the Jupiter 2. They must brave various geographical areas and the strange weather before returning to the Jupiter 2 where they are most safe.moreless
Professor Robinson discovers that the planet they are on will travel further from the sun. In an attempt to survive, the Robinsons pack up and head south. Dr. Smith stays at the Jupiter 2 site while the Robinson family encounter a terrifying Cyclops and an ancient deserted city.
Professor Robinson attempts to explore a planet that the family is planning to land on. Dr. Smith has sabotaged his para-jet propulsion system, causing it to fail. Smith has also sabotaged the propulsion system of the Jupiter 2. Major West crash lands the ship on the planet so that he and the family can rescue the Professor.moreless
The Jupiter 2 is pulled into a giant alien spaceship where there are advanced rock-like creatures. John and Don look for maps and equipment while Dr. Smith and Will look for trouble.
In the year 1997, the Robinson family is cryogenically suspended before their launch into space in the space-saucer Gemini 12 on man's first mission to colonize a planet orbiting Alpha Centauri.
Soon after leaving Earth, a meteor swarm sends the Gemini 12 spiraling out of control, ending in a crash landing on an unknown planet. The Robinsons survive the crash and set up camp, but their preparations to head south to avoid the sudden onset of the cold season are hampered by One-Eye, the giant cyclops, trapping John and Don in a cave.
After Will saves the men, and Penny is recovered after a turtle ride, the travellers journey to the tropics encountering along the way, the One-Eye again, fierce electrical storms, an underground city and a stormy sea crossing where they become caught in a whirlpool.