Flash Gordon meets Gulliver's Travels.
Captain Frank Chapman (Dean Fredericks) is on a mission to look for two vanished spaceships. He encounters a mysterious planet named Rheton, which can freely travel through space. (As the MST3K gang noted, it also looks like a piece of extra crispy KFC.) Chapman's ship is drawn to the planet, whose people are about six inches tall. As soon he breathes the air, Chapman shrinks down to the Rhetons' height. (Atmosphere controls your size. Don't question it!) While on Rheton, he helps defeat their enemies, the Solarites (who resemble a mix of a sad-eyed dog and a Star Wars Tusken Raider).
THE PHANTOM PLANET strikes me as made by people who weren't just looking to turn a quick profit. I'm thinking the filmmakers wanted to tell a story that was not mere formula. Alas, the end result fell short of that laudable goal. While the premise is intriguing, it was severely underused; the characters don't even qualify as one-dimensional; to call the acting “wooden” is an insult to trees; the visual effects are as half-baked as any I've seen in this type of film; the costumes are uninspired to the point of seeming like an afterthought; the Solarite costume (worn by a young Richard Kiel) is just pathetic; and the errors in physics are too numerous to count.
Item: When Chapman first appears on screen, he looks at his wrist as if checking the time; but he's not wearing a watch.
Item: When the astronauts exit their spaceship to make a repair, they are untethered and walking freely on the ship's wing—a neat trick in zero gravity!
Item: As Chapman's ship is pulled toward the asteroid, he is sitting in the left-hand seat; but once the ship has landed, he's in the right-hand seat.
Item: The Rhetons are able to translate all languages with voicetone waves. (Sure they are!)
Item: A mute woman suddenly becomes vocal when she gets the shit scared out of her. Speech therapists, take note.