Astronauts Glenn and Fuji are sent to investigate a mysterious Planet X, which is beyond Jupiter. Back on Earth, Fuji’s sister Haruno is dating failed inventor Tetsuo Torii, who Fuji disapproves of. Tetsuo, meanwhile, is awarded a contract from toymaker World Education Corporation for one of his gadgets, the Ladyguard Alarm, which makes a high pitched wail – but his invention is put into development limbo.
Shortly after landing on Planet X, Glenn and Fuji are surprised by thunder and lightning. They discover a highly advanced civilization, that has been driven underground due to continual attacks by Monster Zero, known on Earth as King Ghidorah. The beast damages a water plant, causing the Controller and his staff to leave for a short time, but not before isolating the astronauts in a darkened room. When they return, the Controller asks to borrow monsters Zero-1 and Zero-2 Godzilla and Rodan to defeat Monster Zero. In exchange, Planet X will provide Earth with a miracle drug that will cure all disease.
Even though Earth officials agree to allow the aliens to capture Godzilla and Rodan, Glen and Fuji are cautious. They suspect Planet X is in a severe water shortage, and may have more malevolent plans. Glenn rushes off on a date with Miss Namikawa, the representative from World Education Corp. When he returns, he tells Fuji he suspects the woman of collaborating with the Controller. A short time later, they discover that the Controller and others from Planet X are already on Earth. The aliens encapsulate Godzilla and Rodan in giant bubbles and transport them, Glen, Fuji, and Dr Sakurai to Planet X. On the way, the humans discover the aliens are able to control the flying saucers, animals and planets with their minds.
Tetsuo follows Namikawa to a remote island and discovers the truth: she is an alien collaborator. He’s captured.
Planet X is under assault by Monster Zero. Godzilla and Rodan are released from their bubbles to battle the three headed space monster. After a short fight, they drive away Zero. Glenn and Fuji slip away during the battle and find an oddly beautiful gold park-like room. Namikawa enters the room, wearing the Planet X uniform, but she fails to recognize Glenn, then a second Namikawa enters. Glenn and Fuji are captured by a group of the aliens. The Controller decides not to press charges and releases the pair to Dr. Sakarai. They leave in a replica of their original ship with the medical cure, leaving Godzilla and Rodan behind.
Back on Earth, they discover the tape they brought back doesn’t have instructions on how to produce the miracle drug. Instead, it contains a demand by the Controller to place the Earth under the control of Planet X as a colony, or be destroyed. Riots breakout worldwide. In an abandoned and heavily damaged building, Glenn finds Namikawa wearing a Planet X uniform. She admits she is an alien, but has fallen in love with the astronaut. The two are surrounded by aliens. Before Glenn is led away, she secretly slips a note into his pocket. The aliens kill her for falling in love with a human.
The aliens arrive, and order the humans to surrender within 24 hours or face death. They have brought Godzilla, Rodan, and Ghidorah to Earth and are controlling them with magnetic waves. Glenn, who has been tossed into a cell with Tetsuo, finds the note from Namikawa. She writes that the aliens from Planet X can be defeated through the use of a certain sound. Tetsuo realizes his Ladyguard Alarm emits that sound. Pulling one out of his pocket, he turns it on, rendering their guards helpless. They escape and report their discovery to Dr. Sakurai.
The Xns release Godzilla, and Rodan to attack. The pair smash buildings and knock over bridges in Japan, and Godzilla sets the countryside on fire, while the aliens blow up land-based weapons. Ghidorah joins them as they press the attack into an urban area, devastating the city.
The Ladyguard Alarm signal is broadcast on radio and TV stations. As the aliens try to regain control over their saucers, the humans bring to bear a weapon that disrupts the magnetic waves controlling the monsters, causing them to collapse. They continue to fire the magnetic wave disruptors at the saucers, which are careening out of control. The Controller orders his minions “into the future, the dimension we have never seen.” He presses a button, blowing up the saucers and their island HQ.
Freed from the alien’s grip, Godzilla and Rodan turn on Ghidorah. While the wrestle, they fall over a cliff into the ocean. Ghidorah flies out of the water, but Godzilla and Rodan whereabouts are not known. But it’s not believed the earth monsters are dead. Glenn is advised he will be returning to Planet X as an ambassador to the remaining inhabitants.
Human Cast: Nick Adams (Astronaut Glenn), Akira Takarada (Astronaut Fuji), Kumi Mizuno (Miss Namikawa), Keiko Sawai (Haruno Fuji), Jun Tazaki (Dr. Sakurai), Yoshio Tsuchiya (Controller of Planet X), Akira Kubo (Tetsuo Torii)
Monster Cast: Godzilla, Rodan, King Ghidorah / Monster Zero
Notes: The original Japanese language version is known as Invasion Of Astro Monster, and was released in 1965, but even in the Japanese language version, the creature is called Monster Zero or King Ghidorah, not Astro Monster. Among the differences in the Japanese and English versions: a change in the opening theme, the Controller offers “only” a cure for cancer (rather than a cure for all disease), and instead of Glenn being appointed as Ambassador to Planet X, both Glenn and Fuji are dispatched to conduct a survey of Planet X in the original Japanese version.
LogBook entry by Robert Parson