During a manned mission to Planet X behind Jupiter (not to be confused with the hypothesized Planet X), the astronauts encounter inhabitants called Xiliens who ask to borrow the two Earth-bound Monsters Zero-One (Godzilla) and Zero-Two (Rodan) to combat their own terror, Monster Zero.
At the outset of the movie, 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 inexplicably 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 creature: Ghidorah, the three-headed monster. After a brief black-out, when the astronauts are cut off from being able to see and hear the Controller, they are assured that the monster, known as Monster Zero, has left.
The Controller says that they want Earth's help: to be allowed to capture Monsters Zero One and Zero Two, known to Earthmen as Godzilla and Rodan. In return, Planet X will gift humanity with a wonder drug that cures all diseases. 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 has a 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 one of their superiors 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 aliens arrive, destroy the spacecraft, and threaten to release the three creatures, which are now all under Planet X's control. In a show of confidence, the aliens 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. When Glenn, who was infatuated with Miss Namikawa, is also arrested after finding out something about her - she's from Planet X and all their women are virtually identical, he and Tetsuo put clues together. Namikawa gave Glenn, before she was disintegrated by a Planet X soldier, a letter, and in it she says that one simple sound will destroy the people of Planet X. It is the sound made by Tetsuo's "lady beware alarm". He still has the prototype, takes it out and sets it off. It upsets and paralyzes the Planet X soldiers and Glenn can immobilize them and they 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 King Ghidorah with the aid of Rodan, forcing Ghidorah into a retreat.
Glenn and Fuji are to be sent to Planet X again as ambassadors to seek peaceful relations.
English versionThe film was released in North America by UPA and distributed by Maron Films in 1970 under the title Monster Zero. It played on a double bill with War of the Gargantuas. There were several alterations made:
- Dialogue was dubbed to English.
- The title song was changed, and some of Akira Ifukube's score was re-arranged.
- Several sound effects were added.
- Deleted: several shots of Godzilla's foot stepping on houses and huts.
- Deleted: some short shots of flying saucers.
- Deleted: Rodan blowing away tanks from the top of a hill.
- Deleted: a scene of the Planet X leader speaking in the language of Planet X.
The American version runs 93 minutes, three minutes shorter than the Japanese version. In his book Japan's Favorite Mon-Star: An Unauthorized History of 'The Big G', Steve Ryfle says "The Americanization...is respectful to the original Japanese version."
In the original edited Japanese version of this movie, Nick Adams' original voice was dubbed over by the Japanese voice-actor. However in the re-edited American version (including the unedited International version), Nick Adams' voice wasn't dubbed over.
- A new Godzilla suit was used for this film called DaisensoGoji, It looked a lot like the previous version, but had larger eyes, to fit Godzilla's new "hero" personality and slightly reduced dorsal plates.
- During the scenes where Godzilla & Rodan are attacking, stock footage from the original Rodan is used, specifically from the scene where Rodan blows over a train and the scene where two soldiers are blown away.
- The Japanese Army's theme from the original Godzillais heard when the humans break the mind-control.
- Mothra was originally planned to make an appearance but was cut due to budget constraints.
- The kiss between Nick Adams and Kumi Mizuno's characters before the flying saucers depart for Planet X with Godzilla and Rodan, is the only kiss in the entire Godzilla film series.
- This film marks the very first time Aliens appear and control the monsters in a Godzilla film. This concept was greatly expanded upon in later film Destory All Monsters. It was also used in other Godzilla films such as Godzilla vs. Gigan and Godzilla: Final Wars.