Giant cockroaches from a dying Earth-like planet in the Space Hunter Nebula M plot to colonize the Earth and destroy all cities to make it more "peaceful" (peace and technology being the themes of this film). They inhabit the bodies of recently deceased humans, thus resembling them, and work as the staff of the Japan branch of the peace-themed theme park World Children's Land (based in Switzerland), the centerpiece being the Godzilla-shaped "Godzilla Tower". The plan of the Nebula M aliens is to use the space monsters King Ghidorah and Gigan (guided by two "Action Signal Tapes") to wipe out civilization. A cartoonist named Gengo Kotaka stumbles onto their plan after being hired as a concept artist for them. When Gengo and his friends play one of the incomprehensible Action Signal Tapes (which he obtained by accident) on their tape player, only Godzilla and Anguirus hear it from afar and catch on to this evil plot as well. Godzilla sends Anguirus to the source of the sound to make sure nothing's wrong, but once Anguirus arrives at Tokyo Bay, the Japanese military, having no clue on the monster's intentions, drives him away. Anguirus goes back to Monster Island, and Godzilla then follows him back to the city. Both monsters try to save the Earth from King Ghidorah and Gigan, though the Nebula M aliens plan to lure Godzilla into a shocking fatal trap via placing an extremely powerful laser cannon inside the Godzilla Tower's mouth and firing it at Godzilla. Once the tower is destroyed by the main human characters, Godzilla and Anguirus drive Gigan and King Ghidorah into a retreat back into space and saved the world.
Stock Footage Goofs
Because the movie used many stock footage clips, some major goofs occurred. The first is very obvious: the scene switches from day to night, and back again. Another major goof was that when King Ghidorah picks up Anguirus and drops him, the scene is suddenly on Monster Island. During the climactic final battle, Larva Mothra can clearly be seen as Godzilla approaches King Ghidorah for an attack. This is clearly stock footage from Ghidorah, the Three Headed Monster. These are some very noticeable mistakes and are not as well-flowing as other stock footage movies (like Godzilla vs. Megalon).
- Gigan is shown in the theatrical posters to have a laser he can emit from his forehead. This power is never actually used in the film, although in a brief scene you can see the spot where the laser is flash very brightly. This laser has been used in subsequent video games and Godzilla Final Wars though.
- The music used in this film was entirely stock music from previous Godzilla and other kaiju films.
- This was Haruo Nakajima's final time playing the role of Godzilla, a role he had played since 1954.
- One of the Space Nebula M Hunters bears a strong resembelance to Henry Chan from the Hanna-Barbera cartoon The Amazing Chan And The Chan Clan another 1972 program.
- Kengo Nakayama, who had played Hedorah the previous film, here portrays Gigan, and would reprise the role in the next year's Godzilla vs. Megalon, as well as in an episode of Toho's television series Zone Fighter. Nakayama, under the name Kenpachiro Satsuma, would take over the role of Godzilla in 1984, and play the monster untill 1995.
- The Godzilla suit used in this film, nicknamed the 'Shoshingekigoji" suit, was used in the previous three Godzilla movies. Durring the shooting of the film, the suit's scales fell off, and it is easy to see that the right hand is sewn on (as Godzilla's hand was intentionally removed for the filming of Godzilla vs. Hedorah). There is even a scene, where Godzilla is on top of Gigan and punching him, that pieces of the suit come flying off.
- Gigan is the first monster to make Godzilla bleed. The previous special effects director, Eiji Tsuburaya, had been extremely opposed to having the monsters bleed in the films, as he did not wish for the series' younger viewers to see such graphic images. After Tsuburaya's death, Taruyoshi Nakano took over as the head of the special effects department, and many of the Godzilla films he worked on, including Godzilla vs. Gigan, included scenes of monster bloodshed.
- This is the second time Anguirus bleeds. The first time is when he fights Godzilla back in 1955 which was his first appearance and the third is when he fights Mechagodzilla in 1974.
- Due to the slashed budget for this film, special effects director Teruyoshi Nakano used stock footage from previous Godzilla films (as well as some other Toho sci-fi films) for many of the special effects sequences. Clips from the Godzilla films Ghidorah the Three-Headed Monster, Invasion of the Astro Monster, Destroy All Monsters, and Godzilla vs. Hedorah, as well as Rodan and War of the Gargantuas, can be seen in this film.
- This film had gained popular recognization as Godzilla and Anguirus talking is translated in the Dubbed Version while in the Japanese version the translations are word bubbles. Below is the dubbed version.
Godzilla: Hey. Anguirus!
Anguirus: What do you want? (crawls to Godzilla)
Godzilla: Somethin' funny going on. You better check. (Motions his left arm forward)
Anguirus: Oh, yeah! (crawls away)
Scene Two (Godzilla and Anguirus swimming towards Japan.)
Godzilla: Hey, Anguirus, come on! There's a lot of trouble ahead. (Points his hand forward in a straight direction) That way!