A quartet of space hippies have been recruited to blow up the planet the Robinsons are living on in order to prove their worth to their government.
Meanwhile, the Robinsons are experiencing a bout of very strange weather. They try to prepare the Jupiter 2 for lift-off as soon as possible, but there are many systems that still need repair. Suddenly, a loud voice speaks to the Robinsons and orders them to leave the planet immediately as it is slated for destruction. The radar then detects visitors nearby, so Don and John go out to investigate.
Will, Dr. Smith and the Robot find a strange silver box that is very heavy. Although the Robot strongly urges him not to, Smith looks in the box and inhales a green gas that issues out of the box. Smith gasps and falls down, apparently dead.
John and Don encounter the gang of space hippies who rudely tell them to get off the planet. John tries to explain that they cannot leave until the Jupiter 2 is repaired, but the hippies just laugh at them and refuse to delay the destruction of the planet.
Meanwhile, Dr. Smith revives and gets up, which surprises Will and the Robot. They are doubly shocked when they note that Smith’s hair is green and his muscles are huge. Smith grabs the heavy silver box and carries it back to the spaceship. Everyone is confused over what has happened to him.
The space hippies discover Smith stole their box of demolition supplies and they go to get it back. Smith uses his mighty strength to beat up the hippies but they trick him and make him lose his strength. The hippies take back their silver box and Smith runs whimpering back to camp.
The Robot urges Smith to exercise, and when he does, his green hair and muscles return. Dr. Smith goes to steal the silver box back from the hippies and Will and the Robot follow along. John attempts to talk reason into the hippies but he ends up fighting with them instead and they knock him down. The hippies then retreat to a cave where Smith finds them. He tries to take the box from them, but the female hippie flirts with Smith and talks him into coming away with her. Once she has him alone, she chops off his green hair, and he loses his strength once again. The space hippies tie up Will, Smith and the Robot in the cave and run away.
The Robinsons are nearly finished their repair work on the Jupiter 2 but cannot leave until they find Will, the Robot and Smith. Concerned, John and Don go out to search for them while Maureen and the girls finish preparing for launch.
Back in the cave, the Robot tells Dr. Smith to breathe deeply. He does so, eventually regrowing his muscles and green hair. He breaks his chains and frees his companions. They all rush out of the cave just as the roof collapses.
John finds the space hippies as they are about to plant the final explosive to destroy the planet. John asks them one last time to wait until the Jupiter 2 has flown to safety, but the hippies laugh at him and refuse. John tells them they have left him no choice and he destroys their space bikes, stranding them on the planet. Since they now have repairs to make as well, the detonation is delayed and the Jupiter 2 will have all the time it needs. The final preparations are made and the ship blasts off into space, setting course back for Earth.The misfits are able to repair their vehicles using spare parts left behind by the Robinsons. Then they destroy the planet and try to run the Jupiter 2 "off the road" when they return to their home planet. Everyone is happy except Dr. Smith, who has lost his green hair and strength for good.
- There is a cut away from a far away shot to a close up when the gas comes out of the box to disable Dr. Smith. You can see that a stunt man wearing a Dr. Smith mask has taken Jonathan Harris' place.
- The second time "Smith the Mighty" drops the box at the Jupiter 2 campsite, Mark Goddard's reaction to the ground shaking takes place before the box hits the ground.
- Continuity - never a LiS strong point - is a bit confusing here. In the episode immediately before this one in the production order ("The Haunted Lighthouse"), the Jupiter 2 is in full flight in deep space. The Robinsons have apparently crash-landed again in the interim - perhaps that's why they changed the run order so that "Flight in the Future" could show them actually making a planetfall. meaning that this world is the Emerald Planet from that show.
- Planetary orbits are not permanently fixed and will increase slowly over time. A contributing factor is loss of their sun's mass through radiation and stellar wind. This affects all planets in a system with the effect greater the further the planet is away from the star. This generally takes hundreds of millions of years so the alien masters must really be looking ahead.
- The Robot failed to warn his family of a cosmic storm because he was busy. Busy singing.
- It's always surprising at how calm Will remains after Doctor Smith “drops dead” right in front of him.
- When Doctor Smith regains consciousness with green hair and muscles for the first time, he doesn’t seem to really be aware that a change has occurred. If this is true, why would he pick up the heavy silver box? And why would he blow on a boulder to move it?
- The green gas is from the packagecontaining explosives to destroy the planet. How do explosives make Dr. Smith so strong?
- Doctor Smith asks a cave-in to “wait a moment!” And it does!
- When explaining Smith’s condition to him, the Robot says, “I have computed that all your strength came from your hair. Vigorous exercise may restore it to you.” Bald men of the world, start jogging! Apparently exercise re-grows hair!!
- The female hippie is drawn to Don and constantly flirts with him. But as always, Don rebuffs her.
- The voice that gives the space hippies their assignment at the beginning of the episode identifies the planet the Robinsons are stranded on by name: Kromah; this is only the second planet they've settled on that has been named in an episode (the other was Priplanus in season one).