A cosmic radiation storm approaches the planet, but Don is confident because Dr. Smith had been assigned the task of installing cosmic arrestors. However, Judy tells him she has seen them still lying on a pile of equipment. Don confronts Smith and insists he begin the job before it is too late. Meanwhile, Penny is missing and the adults fan out to find her before the storm strikes. Penny, out for a walk, has come across a large, ornate mirror. Smith finds her just as the storm strikes and they take shelter underneath, unaware that the cosmic radiation is having some effect on the mirror. After the storm has passed, John finds the pair at the mirror, but Smith insists he not investigate, saying it is only worthless trash. In fact, Smith suspects the mirror’s frame is made of platinum.
The next day, Smith sets out to strip the mirror of its platinum coating, but Don insists he finish installing the cosmic arrestors. Smith complies but quickly abandons the task. Meanwhile, Penny and Judy have gone to the mirror and have set it up to view their reflections. Smith sees them in the distance and mutters for them to get away from his mirror—it is his platinum. The girls are examining themselves in the mirror but end up quarreling over Judy’s comment that Penny is not interested in the things a girl her age should be. Penny ends up very upset and both girls say things they regret. Penny tearfully insists that she will stay the way she is forever.
Later, Penny goes back to the mirror in search of Debbie who has discovered a small bell somewhere. Penny asks Debbie to show her where she found the bell, so Debbie walks to the mirror and points, but Penny doesn't understand. Soon, however, Penny falls through the mirror and finds herself in a dark, mysterious world. There she meets a nameless boy about her own age who invites her to stay and have fun. He tells her he heard what she said about staying as she was forever, and entices her by promising that if she stays with him, she will never grow up or have to change. Penny tries to leave but is unable to do so. She eventually resigns herself to her fate but she soon discovers that the boy's idea of having fun—playing hide and seek with a monster and spying on the outer world through mirrors—does not interest her at all. Penny insists on returning home, but the boy says there is no way back, even when Penny asserts there must be, for the bloop entered and returned.
Dr. Smith actually witnessed Penny enter the mirror but disbelieved his own senses, sure that he was suffering from hallucinations. Later, though, he returns to destroy the mirror, the root of his problems, but he too falls through into the other world. He refuses to believe any of what he sees is real until he is attacked by the mirror monster. He grabs a rifle and flees toward the exit. There he lets off a random shot that hits a pool of water where his image is reflected and he is able to pass through to the outer world. Penny quickly understands what happened and begs the boy to go with her as the monster approaches closer and closer. At the last second, she shoots at her own image in the pool and passes back into the real world. Left behind, the boy only says that he cannot follow her… he has no reflection. Outside the mirror, as Penny stands in horror while Smith destroys the mirror, we hear the tinkling of a bell.
- Angela Cartwright posted online about this episode: "I always thought this episode 'The Magic Mirror' had a creepy quality to it... To this day, I have to admit, it occasionally crosses my mind."
- The title card shows the Jupiter 2 surrounded on practically all sides by outcroppings of rock. How the ship managed to crash land in this position strains credibility.
- Michael J. Pollard played the part of the "boy."
- Although Pollard looked young he was actually in his mid-twenties when he played the part. He was born on May 30, 1939, in Passaic, New Jersey, USA.
- Pollard was one of the few actors to have appeared on both classic Sci -Fi shows "Lost In Space" and "Star Trek". Pollard starred in the Star Trek episode, "Miri," and played Jahn.
- This is one of the few episodes where Will and the Robot are NEVER seen.
- This is one of the few times Dr. Smith wears this particular suit. Later, in "All That Glitters" this suit is given to Ohan, and is never seen again.
- OOPS MOMENT! Dr. Smith marches off from the Jupiter 2 to deal with the alien mirror, leaving Debbie the Bloop behind. When he arrives at the mirror, Debbie is there ahead of him and not out of breath.
- We are not told what happened to the young man inside the mirror.
- This Episode was repeated on Thames Television on February 28th 1969.
- This Episode was repeated on Granada Television on March 20th 1969.
- One reviewer said that this was a bittersweet episode of Lost in Space that has the always watchable Michael J Pollard, especially adept at portraying weirdos, guest starring as a mischievous boy who lives in a dimension on the other side of an alien mirror found by Penny. Pollard is so youthful and playful here, he really plays his part like a child stuck in the body of a young man, eternally trapped in the body of a teenager, never to grow old but longing for companionship.
- This episode mirrors 'Alice Through the Looking Glass.' At the beginning, the child Penny, doesn't want to have to mess with all that feminine 'goop' and try to be a young lady. However, at the end of the episode, Penny has transformed, and has decided to take upon the feminine "goop" and become a young lady.
- Why do Penny and Doctor Smith take shelter from an electrical storm by hiding under a large metal object?
- At one point, John refers to Debbie as a “he.”
- Why aren’t John and Maureen more concerned when Smith declares he’s going to GET PENNY in such an evil, vindictive tone of voice?
- Where is Will throughout this entire episode? Billy Mummy was filming "War of the Robots," while Angela Cartwright filmed "The Magic Mirror."
- Where is the Robot throughout this entire episode? The Robot was filming "War of the Robots," with Billy Mumy.
- Without the Robot to abuse, Doctor Smith turns his aggressions on Penny’s pet, Debbie. Smith piles insults on the poor little Bloop and even smacks her hand a few times.