- Godzilla, the King of the Monsters.
- Rodan, a flying monster.
- King Ghidorah, a space dragon.
In 196X, the two-man spacecraft, crewed by one Japanese and one American (Fuji and Glenn), is approaching Jupiter to visit the newly-discovered Planet X, which maintains a position directly behind Jupiter. The planet is rather dark, but still lit up enough to be visible and for it to be possible to navigate its surface. The spacecraft lands, and the astronauts disembark.
One astronaut vanishes, and the other wonders where he, and the spacecraft, have gone, and then a flat voice intones to him, instructing him where to go. The astronauts are led through subterranean corridors to the office of the Controller of Planet X.
The spacecraft is safe, he assures them, and indicates that they are about to be attacked. The astronauts recognize the attacking monster is King Ghidorah, the three-headed space dragon. After a brief black-out, when the astronauts are cut off from being able to see and hear the Controller, they are assured that Ghidorah, known to the Xians as Monster Zero, has left.
The Controller says that they want Earth's help: to be allowed to capture Godzilla and Rodan, known to the Xians as Monster Zero-One and Monster Zero-Two. In return, Planet X will gift humanity with a wonder drug that cures all diseases (in the original Japanese version, it is simply the cure for cancer). The astronauts agree to return to Earth with the proposal. As they lift off, they say on the radio to the Controller, "We're glad we found friends on Planet X."
Meanwhile, Fuji's sister's boyfriend, Tetsuo, who's invented a personal alarm he thinks women could use to summon assistance if they're endangered. It creates a horrific loud noise that can be heard blocks away, and wonders why nobody's interested in buying it, but then a Miss Namikawa comes and makes an offer to buy it as an educational device, but keeps putting Tetsuo off on completing the deal.
Fuji and Glenn arrive home, and tell their superiors of the offer by Planet X. Scientists begin searching for Godzilla and Rodan. Then, the Controller of Planet X makes mysterious appearances on Earth, and Glenn becomes suspicious about Planet X. The Controller finally makes his appearance overt, "apologizes" for his unannounced presence, and offers to help locate the two monsters. Two Planet X spacecraft rapidly fly off and extract the two monsters.
Glenn, Fuji, and Dr. Sakurai are invited aboard a spacecraft to accompany the Controller back to Planet X, a trip that takes only a few hours; the Controller says that soon they'll be able to travel as fast as light. On reaching Planet X, there is almost immediately an attack by Ghidorah, and the two from Earth are released to battle it. Ghidorah is driven off, and the Controller exults about that. He presents a box that, he says, contains information about the miracle drug, and presents the three men with a faithful duplicate of their spaceship so they can fly home.
On arrival, the box is taken to a special international meeting and is opened to find a reel-to-reel tape. It is loaded onto a machine and set up to play. When the speakers remain silent for a long period, some wonder if the systems are compatible, but others say their system is exactly the same as ours. Finally, there is a beep, and the voice begins. "This is the Controller of Planet X. You will listen to my instructions..." It is an ultimatum to surrender to Planet X.
The Xians arrive, destroy the spacecraft, and threaten to release King Ghidorah, Godzilla, and Rodan, which are now all under Planet X's control. In a show of confidence, the Xians even betray the method of control: magnetic waves. The Earth scientists realize that they could exploit this information, and work rapidly to find a way to disrupt those magnetic waves, while in the meantime, Earth's armies fight nearly in futility with conventional weapons as the monsters wage most of their destruction against Japan.
Tetsuo, meanwhile, is dissatisfied with the lack of progress on his device, and his inability to get Miss Namikawa to tell him what's happening. He gets imprisoned by the Planet X soldiers. In the course of pursuing his interest in Miss Namikawa, Glenn learns that she's from Planet X and all their women are virtually identical. Fearful of what he knows, the Xians arrest him and place into the same cell as Tetsuo. However, this proves to be their undoing as he and Tetsuo put their clues together. Before being disintegrated by a Planet X soldier, Namikawa gave Glenn a letter in which she told him the weakness of the people of Planet X: the sound made by Tetsuo's lady guard alarm. Tetsuo, still in possession of the prototype, takes it out and sets it off. It upsets and paralyzes the Planet X soldiers, enabling Glenn and Tetsuo to escape.
They reach the space center scientists and explain about the device. Arrangements are made to broadcast it on all radio and television stations, a tactic that will be employed when the magnetic disruption devices are deployed.
The three monsters are removed from Planet X control, as Planet X spacecraft explode and personnel escape the noise by blowing up ships. Planet X withdraws its attempts to conquer Earth. Meanwhile, Godzilla attacks Ghidorah with the aid of Rodan, forcing Ghidorah into a retreat. Godzilla and Rodan never come out leading to humans believing that King Ghidorah killed them.
Glenn and Fuji are to be sent to Planet X again as ambassadors to seek peaceful relations.
- Dialog was dubbed to English.
- In the scene where Godzilla dances, there are stomping sounds added in the USA Version.
- Several shots of Godzilla stepping on houses and huts were deleted.
- Some short shots of flying saucers were also deleted.
- Another scene that was deleted was when Rodan blew away tanks on top of a hill.
- Several scenes where the Aliens of Planet X speak in their own language.
Invasion of Astro-Monster sold approximately 3,780,000 tickets in Japan.
- This film originally intended to feature Mothra, but was scrapped due to a heavy budget.
- The scene where Godzilla dances was protested by Ishiro Honda and Tomyuki Tanaka, but Eiji Tsuburaya insisted that "it would keep the children happy."
- The Godzilla suit used in this film was called "Daisensogoji."
- The first and only Godzilla film to show on-screen kissing.