The first season of Lost In Space is widely regarded by fans as the best, with the first 5 often mentioned as the finest episodes of the show.
No Place To Hide (Unaired Pilot)
Earth's first family in space faces trials and tribulations as they become lost on their way to colonize a planet circling Alpha Centauri. Smith and the Robot are noticeably absent.
The Reluctant Stowaway
A saboteur reprograms the Robot to destroy the Jupiter 2, but gets trapped aboard at launch. The Jupiter 2 is damaged, but survives and becomes hopelessly lost in space.
The crew of the Jupiter 2 encounters a seemingly derelict space ship that dwarfs the Jupiter 2.
Island In The Sky
Professor Robinson attempts to scout a desolate planet, but his parajet rockets fail. Major West attempts to land on the planet to search for the Professor, but the Jupiter 2's rockets also fail and the ship crash lands.
There Were Giants In The Earth
The Jupiter 2 crew faces dangers on the planet they crashed on, including a gigantic cyclops, microbes in the soil that cause crops to grow out of control, and an erratic orbit that causes extreme heat and cold.
The Hungry Sea
The Robinsons and Major West head south in the Chariot to avoid the incredibly harsh cold. Meanwhile, Smith discovers that the erratic orbit of the planet around its sun will cause a sudden and intense temperature spike and sends the Robot out to warn them.
A fellow space traveler from Earth, Jimmy Hapgood, happens to land on the planet where the Robinsons are stranded. They help him repair his ship and John and Maureen ask Jimmy to take Will and Penny back to Earth. Smith, however, has other plans.
My Friend, Mr. Nobody
Penny makes friends with a mysterious invisible entity in a cave. It talks to and learns from her. But Dr. Smith draws the wrath of the entity when he destroys the cave in search of riches and, in the process, inadvertently injures Penny.
Invaders From The Fifth Dimension
Invaders From The Fifth Dimension, in the form of floating alien heads, search Preplanis for a humanoid brain to power their spaceship and take them home. When they capture Smith, they quickly find out that he's, by Smith's own admission, "a scoundrel, a thoroughly bad sort, hopelessly unreliable". So, Smith offers up a "small Robinson", one that will fit in the alien ship... Will.
When Smith uses all of the potable water for a shower, he forces the Robinsons to search for a new water source. They find it, in the form of fruit that grows in a brackish well. Unfortunately for Smith, he eats it before Professor Robinson has a chance to test it. Smith runs off, sure the Robinsons have purposely poisoned him, and takes the last of the working fuel cells with him. Unbeknownst to him, he turns into a paranoid giant overnight.
The Sky Is Falling
An alien probe precedes the arrival of an alien race of beings called the Taurons. The Robinsons believe these aliens are peaceful and looking to settle on Priplanus. Dr. Smith, however, thinks they are the first arrivals of an enormous invading army.
Wish Upon A Star
Smith and Will stumble on to a derelict alien ship and discover a machine that grants wishes. Use of the machine causes tensions in the group.
Will's attempts to build a probe to get a message back to Earth fail and leave the deutronium supply seriously depleted. Don and John's then develop an alternative fuel, but can't stabilize the output. The decision is made to pull the reactor chamber out of the Jupiter 2 and turn it into a small craft with which Don will attempt to find rescuers. Smith, however, has other plans.
One Of Our Dogs Is Missing
While Don, John, and Will are away from camp, an alien intruder invades the camp to steal food. While investigating the incident, Mrs. Robinson, Judy, and Penny discover a wrecked space capsule and the dog that had ridden in it. Dr. Smith thinks the dog is a spy.
Attack of the Monster Plants
After being exiled from camp, Dr. Smith discovers plants that can duplicate anything and feed off deutronium. Judy ends up being cloned and only Dr. Smith knows it's not her.
Return From Outer Space
When Will discovers a matter transfer machine left behind by the Taurons, he takes a trip back to Earth. He arrives in Hatfield Four Corners, Vermont. While there, he tries to contact Alpha Control to make them aware of the Robinson family's fate, but no one believes him.
The Keeper: Part 1
An alien animal keeper travels the universe looking for unique specimens to add to his menagerie. When he comes upon the Robinsons, he's determined to add a pair of humans to his "zoo".
The Keeper: Part 2
Dr. Smith angers the Keeper by accidentally releasing all his animals. The Keeper demands Will and Penny as recompense for his losses.
The Sky Pirate
Ghost In Space
War of the Robots
The Magic Mirror
The Space Trader
His Majesty Smith
The Space Croppers
All That Glitters
The Lost Civilization
A Change Of Space
Follow The Leader