Astronaut Frank Chapman has been sent out into space with Navigator Ray Makonnen on a search for another pair of astronauts who died. After some odd philosophy from Makonnen, the two are separated when a meteor storm hits the ship and Makonnen is propelled into space while Frank is left barely conscious.
Luckily, Frank's rocketship lands on what he claims to be a wandering planet, the "Phantom Planet" of the title, where Frank encounters a race of tiny people who are six inches tall. After the planet's atmosphere causes him to shrink to their size, Frank is forcibly integrated into the society of the planet Rheton. Over the course of the film, he deals with a love triangle between an aggressive woman named named Liara (not that one), her own suitor Herron, and Cute Mute Zetha, whom Frank really falls for. Over the course of the rest of the film, Frank learns the history of the strange world, duels with Herron, and helps fight off an invasion of the Solarites. Though the invasion leads to Zetha finally gaining the ability to speak, Frank learns of a way to leave Rheton and return to his original size. Zetha gives him a rock to remember her by and the film ends with Frank being rescued by astronauts from earth while contemplating the rock and trying to figure out if the whole film was a dream.
- All Just a Dream: It's left up in the air whether the whole Rheton thing was a vivid dream. Frank seems to think the rock Zetha gave him confirms it was real, but...it's just a rock.Frank: Now they'll never believe me...
- All There in the Manual: In the comic book that came out when Phantom Planet was released, it's explained that Ray Makonnen was formerly a priest, explaining his speech, which is a paraphrasing of a quote from clergyman Richard Cecil, and his reciting The Lord's Prayer as he drifts off into space.
- Artistic License Biology: Apparently, Rheton's atmosphere will cause you to shrink. But thankfully all you need to get back to normal is air from your homeworld. The only effect this has on the plot is to explain why the planet Rheton is so difficult to locate. There are a hundred ways they would have explained that without shrinking people to 6" tall.
- Artistic License Chemistry: Apparently, the size of people, whether on Rheton or on Earth, is determined by the air they breathe.Crow: So humans are just big balloons?
- And very small people can survive without eating?!
- Betty and Veronica: With Zetha as the Betty and Liara as the Veronica. Crow even comments on it.
- Bittersweet Ending: Chapman escapes, but can't be with Zetha, who he loves. It was worse in the comic book, The Phantom Planet is completely destroyed when Chapman escapes, and any inhabitants killed.
- Contemplate Our Navels: Ray Makonnen's oft-mocked speech.Ray: You know, Captain, every year of my life, I grow more and more convinced that the wisest and best is to fix our attention on the good and the beautiful... if you just take the time to look at it.
- Cute Mute: Zetha, though she's able to talk at the end.
- Defeat Means Friendship: Herron and Frank hate each other until Frank beats Herron at Combat Rod. Shortly afterwards they're good pals. While Herron is still eager to see Frank off to regain his status as alpha male, he thinks of it in terms of helping Frank. Frank is convinced that he and Herron would've become buddies on any planet.
- Dissonant Serenity: "But I am plagued with regret when I am forced to destroy." Said by Sessom with a warm, avuncular tone and a broad smile.
- Does Not Like Shoes: The women on Rheton go barefoot, the men on Rheton don't.
- Duel to the Death: The Duel of Rheton, which is a kind of inverse Tug of War (two people hold a rod and try to push the other onto a high gravity plate). Herron challenges Chapman out of jealousy when Liara shows an interest in the Earthman. Chapman wins, but refuses to kill Herron, which marks Defeat Means Friendship.
- Dull Surprise: Frank seems to be barely awake throughout the entire movie.
- The End... Or Is It?: In a subversion of the very common "The End" titles, the movie ends with "THE BEGINNING" (in context with the movie's theme that space exploration is always advancing). In the MST version of the episode, Mike and bots lament that this means they have to watch the movie over again.
- Flashback Cut: By the end of the movie, there is a quick montage recapping Frank's experiences throughout the movie, including scenes from just a few minutes ago.Crow: Come on, we didn't like these scenes the first time!
- Full-Name Basis: Liara initially has trouble with the idea of someone having two names, and calls Frank "Frankchapman"
- Incredible Shrinking Man: "See!! A 6 ft. astronaut shrink to 6 inches before your very eyes!"
- Jerkass: Frank Chapman. Par for the course for a 1950's white male lead in a B-movie, for the most part.
- Kangaroo Court: While we don't really learn much about the Rheton justice system, it's pretty obvious and apparent that Frank Chapman's trial is a complete sham. No evidence is considered, no testimony taken, no prosecution or defense arguments heard; just a declaration of charges, a summary "guilty" verdict and a punishment which amounts basically to being naturalized as "a free citizen of Rheton." The sole and entire purpose of the trial, as Sesson hints, is to manufacture an excuse to prevent Chapman from leaving lest he tell others about the Phantom Planet.
- Love Makes You Evil: Liara appears to be plotting some dark revenge after Chapman refuses her, but we later discover... no, she wasn't. She just gets over it offscreen and hooks up with Herron.
- Magic Pants: Averted. Our hero is starkers upon being shrunk within his space suit. If it weren't for the conveniently large crowd of guys pulling him out of his suit he'd probably be very cold and embarrassed.
- No OSHA Compliance: The high-gravity plates used in the duel just sit there in the middle of the main room with absolutely nothing keeping anyone from stepping on them and being vaporized.
Mike: "...c-could you just give a message to my wife and kids if you have the tim- well, nevermind."
- Made even worse by the fact that when we first see the plates in operation, they're clearly turned on by someone. Meaning it's possible for them to be turned off. Which means that rather than these plates being some weird tech that can't be turned off, the people of Rheton are just really stupid and/or lazy.
- Nothing to keep our space men from flying out into space while trying to pull repairs on their ship.
- This isn't forgetting what is apparently an auto-closing door that apparently can only be opened up from the inside. It seems OSHA doesn't exist in the future...
- The Pirates Who Don't Do Anything: Frank Chapman doesn't do anything remotely heroic, while the proud people of Rheton just... fly away. That's it.
- Space Does Not Work That Way:
- The astronauts are able to easily walk along the wing of their rocket... in outer space.
- At one point one of the Earth rockets just takes a sharp left... in space.
- Space Suits Are SCUBA Gear: This trope leads indirectly to Makonnen's death, to wit: a micrometeorite damages the exposed air line of Frank Chapman's space suit, leaving him unable to intervene when Makonnen loses his balance and falls off the rocket's hull... in space.
- Technobabble: Any technical dialogue is really nothing more than several inappropriately-used scientific terms strung together.
- Woobie, Destroyer of Worlds: The captured Solarite is supposed to be evil, but comes off as... cute. It doesn't help he was a prisoner and primarily acting in self-defense.
- Zeerust: The movie's prologue takes place on March 16th... 1980.note The rest of the movie, and technology, pretty much fits with early 1950s imaginations of the future, despite coming from 1961. And Frank was making ready for a mission to Mars before being sent out to look for the expendable Red Shirts from the opening.