The Robinson family are scheduled for a journey into space aboard the Jupiter 2, headed for Alpha Centauri. Just before the countdown commences, Dr. Zachery Smith, a government spy, climbs on board in an attempt to disable the guidance system. But his plan backfires when he is trapped as a stowaway on a ship that will be lost in space.
John detects a signal from another spacecraft which draws the Jupiter 2 onto the alien spacecraft. As John and Don search for any maps that can help them, Will and Dr. Smith search the craft and run into some giant bubble-like creatures.
While checking a planets air quality, John's parajet runs out of fuel and lands on the planet. Suspecting sabotage, the crew head out on the chariot for a rescue mission, while Dr. Smith stays back and schemes to steal the Jupiter 2 and reprogram the robot to kill anyone who tries to stop him.
Aboard a planet which seems to be headed away from the Sun, the crew head South in the chariot to fight the freezing temperatures. But their travels are impeded by an encounter with a carnivorous cyclops.
Forced to cross the sea of ice, back to the Jupiter 2, the crew now face tremendous heat because of the planets orbit and severe storms. Also, they have tracked a missile headed towards them on the radar screen.
The Robinsons discover another Earthling on a planet, a lost astronaut named Jimmy Hapgood. Having helped Jimmy fix up his ship for a return to Earth, the Robinsons asked him to bring Will and Penny back home with him. But Dr. Smith has plans of going home himself.
Penny discovers a cosmic force which takes the shape as an imaginary friend that she refers to as Mr. Nobody. But when Penny is accidentally injured by Dr. Smith, Mr. Nobody unleashes a cosmic storm which changes the planet's look.
A group of luminous aliens want to replace a burned out computer guidance system with Dr. Smith's brain. But he outwits the aliens by promising to give them Will's brain instead.
A piece of untested alien fruit turns Dr. Smith into a giant, and plans to kill the Robinsons who he believes had purposely left the food for him to eat.
A communication gap between the Robinsons and a group of aliens called Taurons, who speak in gibberish, seems to cause no trouble. That is, until Dr. Smith draws a gun on them.
Dr. Smith has been banned from living with the Robinsons aboard the Jupiter 2, and has taken up residence in an abandoned spaceship which contains a machine which can make anything materialize. As a means to get back on the Jupiter 2, Smith gives the machine to the Robinsons, who discover its drawback. The machine plays on people's greed and Smith is ordered to destroy it. Before he can, the machine's owner comes to retrieve it.
The Robinsons build a small spacecraft which is designed for two to head back to Earth. However, Dr. Smith cons Will into going aboard and then accidentally sets it off. But their destination isn't exactly Earth as expected.
Penny and Judy explore the crash site of a small craft and discover a small dog, which Judy brings back to the Jupiter 2. When the dog runs off one night, Penny follows and she meets up with a hairy, mutant giant, only to be saved by the dog.
A giant plant is discovered which can create a duplicate of anything it has been fed. Dr. Smith believes if he feeds it the fuel from the Jupiter 2, it will yield enough to get back to Earth.
Nobody believes Will Robinson when he sends himself back to Earth with an alien transporter, since the world believes they all had died once they realized their ship had gone off course.
An alien zoo keeper whose job it to collect human specimens, wants to get a hold of Will and Penny for his collection. Meanwhile, Dr. Smith plans to steal the alien's ship and, along with the robot, return back to Earth. But instead, the robot releases all of the keeper's specimens.
With all his creatures released, the Keeper demands the Robinsons hand over Will and Penny or he will allow the animals to take over their planet.
The Robinson's meet up with another Earthling named Alonzo Tucker, who was once abducted from Earth by aliens and now lives as a space pirate. Will befriends Tucker, unaware that he has him kidnapped while being chased by his own abductors who threatens the lives of the Robinsons.
A spirit is raised when Dr. Smith throw an explosive into a gaseous area. He believes the creature is really the ghost of his uncle Thaddeus and wants to hold a seance while the Robinson wants to catch it before it destroys their camp site.
A discovery of an alien robot which is more powerful than their own is working on gaining the trust of the Robinsons. But instead of helping them, he plans on sending the family back to his planet of an advanced race of aliens as their slaves.
A broken mirror leads to a new dimension when Penny's pet jumps into it and she heads off to recover it. While inside, Penny and Judy discover the occupant of the mirror, a lonely little boy, seeking companionship.
Will is unknowingly drawn into a battle to the death with a young boy who must prove his superiority over the Earthling in order to validate being crowned a prince. When John discovers the plan, he takes Will's place against the alien ruler.
After the Robinsons food supply is destroyed by a wily space merchant, he offers his own food for a price. Dr. Smith signs away the robot for food which he has hoarded away. But when his scheme is discovered by the Robinsons, they demand the robot back, which means that the trader can take Smith's body to use as a slave.
Dr. Smith feels proud when he is named king of an alien civilization, only to find out that the race choose the most useless person available and will be executed when an even more useless person can be found.
Dr. Smith hopes that dating the mother of a group of space farmers will get him sent back to Earth. But soon he learns that their crop is a flesh-eating plant that feeds on human flesh.
A fleeing thief gives Penny a ring which turns anything the wearer touches into platinum. But when Dr. Smith puts on the ring, he accidentally turns Penny into the metallic substance.
A search uncovers a civilization stuck in suspended animation. When Will kisses the Princess, she awakens and wants him to remain behind and rule along side her. Their plan is to conquer the universe, starting with the Earth.
When an intergalactic delivery service lands on Priplanus, Dr. Smith decides to have Will give their ship a test run. When he arrives back to home base, Will suddenly has increased intelligence while Dr. Smith becomes an aged man. Noticing his new-found smartness, Dr. Smith gives the spacecraft a drive expecting the same results, but winds up even older.
John Robinson's body and mind has been taken over by the spirit of alien warrior when he is knocked unconscious by a falling rock in a cave. The new "John" commands that the family prepares for a return to his home planet, which would require a sacrifice of Will to keep his secret safe.
After a series of earthquakes indicate the planet they are staying on is about to break up, the Robinsons race against time to blast off into space.
After the castaways blast into space, Dr. Smith plots to set a course back to Earth instead of Alpha Centauri.
Because of Dr. Smith, the castaways are forced to land their spaceship on a planet that is run by cyborgs.
The Jupiter 2 is forced to crash land on a planet that is controlled by an eccentric alien.
Will is tricked into joining a space traveling circus and it's up to the others to rescue him.
The castaways are imprisoned in their camp and charged with space crimes but as things proceed it becomes obvious that Dr. Smith is responsible for most of the crimes.
Will and Dr. Smith discover a machine from which the good doctor accidentally orders a female android whom he uses as his personal slave. Then the owner of the machine turns up and demands payment.
A group of galactic gladiators challenges John Robinson to compete in their games. When he declines Dr. Smith takes up the challenge when he's offered a return to Earth should he win. Unknown to Smith, if he loses Earth will be destroyed.
An alien known as the thief gets Will to help him rescue a princess who was kidnapped years ago by an evil vizier.
Dr. Smith's cousin, Jeremiah, somehow follows him into space and pays the castaways a visit but the doctor wants nothing whatsoever to do with his kinsman.
Dr. Smith is confronted by his outlaw lookalike named Zeno who forces him to trade places. Arrested for Zeno's crimes, Smith confesses out of fear. It's up to Will to save the day.
Dr. Smith, Will, and the Robot discover an old harp which leads them to unlocking passage to an underground world occupied by an alien named Morbus. Morbus is imprisoned there and Judy is the only one who can set him free.
A group of aliens wants to trade gold for the castaways' robot.
Penny encounters a giant, golden man who needs only human emotions to be complete. Dr. Smith, in return for being returned to Earth, agrees to steal the emotions from the Robinsons.
Dr. Smith and the Robinson women are confronted by a mysterious golden man who claims to be at war with a group of green aliens. Penny then meets one of the green aliens and takes a liking to him.
Athena, the girl from the green dimension, returns to bedevil Dr. Smith and this time her jealous boyfriend follows her.
Will and Dr. Smith confront an eccentric knight named Sagramonte who says he's pursuing a creature known as Gundemar. Will soon becomes the knight's pageboy but Penny befriends the beast who's really female.
Dr. Smith discovers another android machine but when he tries to order something he finds himself trapped inside the machine. Will later gets trapped inside too and both he and Smith are mistaken for toys.
When Dr. Smith finds an alien spaceship he wants to use it to return to Earth but the owner of the craft soon arrives on the scene and puts a stop to his plans.
The castaways are forced to do battle against the mighty Viking god Thor.
Dr. Smith gets a magician named Zalto to conjure up a spaceship with which to return the space castaways to Earth.
The Robinsons face a deadline to leave the planet but must first rescue Dr. Smith from a cave.
Space pirate Alonzo P. Tucker returns for another adventure with the castaways. This time he's looking for the treasure of Billy Bones and he's using Bones' head, which he keeps in a box, to help him find it.
Penny encounters Verda, the android Dr. Smith once ordered. She's become more humanlike but is now on the run from another super android.
The castaways' communications system is knocked out by a female warrior who plans to turn the planet into a colony for her people.
Will and Dr. Smith want to re-charge the power of the Robot but he's grown to a tremendous size so they end up going inside him.
Returning to camp following a storm, Will encounters android versions of his fellow castaways and must teach them to be like their human counterparts in order to save the rest of his family and friends from captivity.
The camp of the Robinsons is surrounded by the mechanical men of Industro who claim they want a leader--the Robot. If he's not turned over to them they threaten to attack.
After being trapped in a cave with Dr. Smith during a storm, Will gets sent back to Earth through a cosmic time warp and winds up at a spooky Scottish castle.
After saving an alien's life, Dr. Smith and Penny are given an amulet as a reward. Soon, two other aliens who want the amulet arrive on the scene.
Launching into space to avoid a rogue comet, the Robinsons encounter the Vera Castle, a floating prison full of frozen criminals. On a whim, Doctor Smith frees Phanzig, because how dangerous could a man playing Cat's Cradle with a bit of twine be? Then Phanzig forces Doctor Smith to take his place! When Phanzig discovers that the clock that regulates length of sentences stopped long ago, he frees other prisoners to launch a rebellion. In the midst of all this, the robot guard mistakes Don for a prisoner and freezes him solid!
Travelling in space, the Jupiter 2 encounters a warp that returns them to Earth. When they cannot contact Alpha Control, they elect to land, and then discover the warp sent them back in time as well as through space! It's 1947, and the people of Manitou Junction (the nearest town) think they're invading aliens called Voltones (one of them read a story about such creatures). And then Doctor Smith complicates matters by disguising himself as a small town fire chief and taking charge of the effort to imprison the Robinsons!
Smith answers an alien distress call requesting the help of a doctor. He almost ignores it, but the alien voice promises great wealth... Stealing the pod, he travels to the alien probe, there discovering that the aliens are robots, and their leader a complex computer. Smith, completely unqualified, refuses to help until the aliens imprison Will! Worse, when their leader is repaired, the aliens intend to conquer the universe! And since they can control the flow of time, they might just manage it!
The Jupiter 2 must find a landing spot so John and Don can repair her engines. Scouting an inhospitable world, the space pod encounters a force field and must land. Outside, a monster attacks, and John destroys it. Unfortunately, it was the quarry of Megazor, who must prove his skills before assuming the throne of the Zon civilization. As punishment, Megazor makes John battle an invisible alien, and when John wins, Megazor's computer rates him at just enough points to secure the throne for Zon. Now John is the prey, equipped with a defective survival suit, while Megazor the hunter carries the dangerous Zon Destructo-blade!
Don and Dr. Smith try to cap a volcano that threatens the Jupiter 2: if it stays, lava will overrun it, and if it takes off, the explosive and corrosive gases from the fuming stack will damage it, perhaps severely! But the territory around the volcano harbors primitives who worship the great Protineus, a machine they claim will evolve them. Don and Smith have contaminated and damaged this, so Protineus decrees that they must die!
While exploring, Will and Doctor Smith chance across a cave containing a control panel. Smith, unable to leave anything dangerous alone, operates several controls and actives an android creating machine. It begins churning out android copies of Smith bent on galactic conquest. Worse, when trying to stop him Will falls into the machine and emerges changed! And that's when Smith learns something about who was really in charge the entire time...
Chased from their planet by a space storm, the Robinsons take an alien boy named J-5 with them. They encounter a ship that serves as a kind of space lighthouse. The Robinsons are ecstatic with hope that they can obtain fuel and charts. But Colonel Fogey may not be who he appears to be, and J-5 may be more than who he appears to be. And then there's J-5's mysterious pet, the Zaybo, which is and isn't there...
Will and the Robot are checking the pod for readiness when Dr. Smith enters, determined to get a back massage from the Robot. He accidentally launches the pod, which lands on a strange jungle-covered world where fruit explodes and invisible birds cast shadows. The Jupiter 2 lands nearby, but can't find the jungle - they see only rock! Then Will meets people who claim it is 2270 and they are the descendants of the Robinsons! To reach his family, Will will have to figure out what, exactly, is going on.
An alien civilization dispatches four "social burdens" to a planet whose eccentric orbit threatens their world. These four people can become productive citizens by destroying this world. The aliens warn the "earth colony" there - but the Robinsons need at least a week to get their ship spaceworthy, and the aliens have given them exactly "one half of one diurnal cycle." Compounding the matter is Smith's accidental inhalation of alien fumes that give him the strength of dozens of men - for short periods of time.
Locked in orbit around a planet whose inhospitable atmosphere is partially composed of an unknown element, the Robinsons find themselves picked off, one by one, by a malevolent force. An errant wish from young Will brought this force into being - it is a part of him given form and evil power by the strange mist. Now Will has to face his worst nightmares if he wants to see his family again!
The Robinsons must land to repair their ship. As the pass, space capsule Alpha-784 latches onto the Jupiter 2 magnetically and follows them in. It contains a disassembled female robot. The Robot helps reassemble her, and even forces the Robinsons to give him a crucial part to re-energize her! But she's evil, and her escape has not gone unnoticed by aliens willing to destroy the Robinsons if they will not help find and recapture the robot. Re-energized, she proves a match for her pursuers - so how will the technologically less capable Robinsons deal with her?
Don, Judy and Penny venture out to collect rock and plant specimens. Penny, on her own, stumbles into a net trap! There she learns she's been trapped by Farnum B, owner of an intergalactic zoo, who intends to capture all of the Robinsons and put them on display. For Doctor Smith, this is an improvement, since Farnum's technology gives him everything he wants! But when Farnum's assistant Oggo strands Farnum on a distant planet, Smith takes over. He plans to run the zoo, still using the Robinsons as exhibits!
A spaceship claiming to be from Earth requests a landing beam, but John doesn't trust them and uses a shield to keep them away. A good thing, too, because the crew is four criminals who robbed a deuterium bank and are now fleeing the Space Patrol. They chanced across Zumdish, who has quit the Celestial Department Store and now operates a tour agency. Using this cover and playing on Smith's greed, they find a way to land. And they decide this planet will make a nice hiding place, once they get rid of the people who are already there.
Dr. Smith, bungling yet again, manages to open a frozen cave and reveal its occupant - a creature sealed inside frozen fluid. Ordered to guard the find, Smith becomes angry and casually tosses his "malfunctioning" thermal blanket onto the ice block before storming off. The blanket melts the ice and frees its occupant - an ice princess! Then an alien named Chavo appears, and he's very interested in the ice princess - so interested, he captures Will and demands the Robinsons hand her over in exchange for Will's safe release!
Duplicates of John and Don, prisoners of a shadow world, plot their escape. As John uses an atomizing unit to investigate engine repairs, they sense their chance. The evil copy of John breaks through and drags the real John back to his own dark world! He plans to make the switch permanent, unless Will and the Robot can stop him!
Doctor Smith detaches the robot's tracks from his torso, and accidentally launches them and the space pod towards a nearby planet. The aliens there invite the Robinsons to retrieve their missing pod. After they land, Doctor Smith encounters one of the aliens, who looks much like a melted candle with eyes. Smith reveals that humans all look different, prompting one of the aliens, Gitt Proto, to hatch a scheme to capture the Robinsons and replace them with reshaped aliens! They'll then take the Jupiter 2 back to Earth to learn enough to conquer it! Will and Smith evade capture and sneak aboard, but it's still two against five! Fortunately, Will realizes something about the aliens that gives him an edge...
Captain Kraspo, his steward Fedor and his crew of robots have been in space for five long years, searching for the missing princess of Beta. She, and only she, can wield the scepter that keeps the rebellions robots and computers of Beta under control. Kraspo, desperate for his reward, concocts a scheme to pass Penny off as the princess. But Fedor has plans of his own, and when the crew mutinies, and Penny can't control them, Don will have to decipher the garbled message send by the real princess's nurse years earlier - it's the key to finding the only person who can save everyone from the machines!
Will performs an experiment designed to capture subatomic particles during a cosmic storm, but he nets something quite different: Dr. Chronos! The doctor claims he controls the "time tapes" of everyone, everywhere, and administers their use of time - even going so far as to destroy a planet that has "used up" its time! Smith uses Chronos's equipment to send himself back to Earth, before he boarded the Jupiter 2, in the hopes of rewriting his own history. But if he does this, the ship won't go off course - and a rogue asteroid will destroy it, four months into its trip! John, Will, and the Robot have to find a way to persuade Dr. Smith onto the ship over Smith's and Chronos's objections.
When The Robot announces that he has detected Alpha Centauri, the Robinsons are overjoyed! They have finally reached their real destination, and they prepare to land. But when they arrive, they discover a planet run by teenagers, who quickly sequester Will and Penny. Attempts to indoctrinate Will and Penny partially succeed, but they break away only to see their parents depart, claiming to have only one daughter, Judy! Who really runs this planet, and why do they need Will and Penny so badly?
Creech, a hard case inmate of Prison Planet Destructon, flees custody and makes it to the icy world on which the Robinson's have landed. There, he forces Smith to trade his warm jacket for a prisoner's shirt. Seconds after Creech flees, guards and the Warden from Destructon appear, and accuse Smith of abetting Creech's escape. Smith drags Don into it, and before long both men find themselves sentenced to Destructon for the rest of their natural lives in a rigged trial designed to help the warden achieve his own goals...
Farnum B, galactic showman, has a new client. He's looking for women to enter a beauty contest. At first, afraid of his client, he refuses to enroll Judy. But the client sees her and wants her in the contest - and he has a way of ensuring that Farnum does as asked. But when Farnum cannot sign her, and forgery does not work, he turns to Doctor Smith, whose subtle chicanery soon has Judy on the runway. But when Judy sees just what's inside the mysterious sponsor's armor, her desire to win the contest evaporates...
Violating orders, Doctor Smith has grown an alien plant in his cabin. Space radiation turns it into a monstrous mutant that thinks he's its “mother!” Ordered to dispose of it into the jets, Smith instead lazily flushes it out an airlock, and it promptly attaches to the ship, causing engine problems! To remove the plant, John proposes a close passage through the dense atmosphere of a nearby planet – but that planet is inhabited, and the residents do not want visitors. The Jupiter 2 lands anyway, and then discovers that the planetary inhabitants demand a high price for their safe departure!
Doctor Smith realizes he may one day need the Robot's good will. So he commandeers the space pod and descends to a planet home to vegetation, but no animals. His goal: a few flowers for the Robot's birthday party! But on this planet, ruled by Tybo, vegetables are king, and harming them is a capital crime. For his crimes, Tybo intends to turn Smith into a plant! And when the Robinsons descend looking for their errant passanger, and begin hacking their way through the dense foliage, well, Tybo soon devises new forms for them, too!
The Jupiter must land, to repair a fuel tank that is causing fires. But first, someone must explore a cosmic cloud with the space pod to verify the Jupiter can pass through it. Smith draws the short screwdriver, but talks the Robot into taking his place. Then a magnetic force captures the Robot and hauls him to a space "junk yard" run by a bizarre robot junk man. Then a strange form of rust plague infects the Robinsons' food supply, forcing Smith to trade parts from the robot to the junk man in exchange for meals. But when the junk man has what he wants, he plans to cut off the food and steal the Jupiter, stranding the Robinsons on his own wretched world!