- Godzilla, the protagonist of the film. The King of the Monsters fights his robotic clone, Mechagodzilla.
- Mechagodzilla, the robotic replica of Godzilla. A machine under the control of aliens, who plan to conquer Earth.
- King Caesar, a mystical guardian of Okinawa, who is awakened only by a prayer, and will come to vanquish evil.
- Anguirus, Godzilla's ally. When Mechagodzilla, disguised as Fake Godzilla, comes to Earth. Anguirus fights Fake Godzilla but is defeated.
- Fake Godzilla, a disguised Mechagodzilla. When the aliens released Mechagodzilla, they disguised him to be identical looking to Godzilla.
An Okinawa legend tells that when a black mountain appears in the skies, a monster will come to destroy Earth. However, if the divination comes true, a red moon will set, two suns will arise (one is an optical illusion from the west), and two monsters will fight evil to rescue the world. In a cave near the city of Okinawa, an engineer and an archaeologist find a statue of the protector of Okinawa, known as King Caesar. They believe him to be one of the two monsters to fight evil.
Later, a black mountain appears in the sky. Suddenly, Godzilla rises from Mount Fuji and starts a rampage. However, many people do not believe he is the evil monster. The reflection is reinforced when Anguirus fights Godzilla, and Godzilla rips his jaw open. Anguirus flees, as Godzilla continues his rampage. Then, when Godzilla approaches the city, another Godzilla appears. The 2nd Godzilla then reveals the Godzilla who fought Anguirus to be an impostor, a machine called Mechagodzilla, created by the ape-like aliens known as the Aliens of the Third Planet from the Black Hole.
Godzilla is nearly killed, and Mechagodzilla becomes mortally damaged. Godzilla flees to Monster Island, where he is struck by lightning repeatedly. Meanwhile, the aliens force humans to repair Mechagodzilla or they will die. The humans repair Mechagodzilla, who attacks an island. On the island, one of the humans pray to King Caesar, who is asleep in a giant cave, to aid Earth against Mechagodzilla and the aliens. King Caesar awakens and fights Mechagodzilla, but is quickly wounded. Godzilla, charged with the electricity from the lightning, aids King Caesar, who is quickly defeated. Mechagodzilla then shoots multiple darts at Godzilla and begins to fly away. Godzilla, with the power of electricity from the lightning, turns himself into a magnet and pulls Mechagodzilla towards him. Godzilla grabs it and twists it's head off until it comes off. Godzilla then walks away into the ocean, and King Caesar walks back into his cave, awaiting for his next calling.
The film sold approximately 1,330,000 tickets in Japan - modest business, but an improvement of about 350,000 over the previous Godzilla film, Godzilla vs. Megalon.
In 1977, CinemaShares originally released Godzilla vs. Mechagodzilla under the title Godzilla vs. The Bionic Monster. But due to the threat of a lawsuit against CinemaShares from Universal Studios on account of supposedly deriving the title from the TV shows, The Six Million Dollar Man, and The Bionic Woman, the film was retitled Godzilla vs. The Cosmic Monster. But both variations (the first variation is original artwork, having Godzilla and Mechagodzilla fight in front of a volcano, and the second being the Japanese artwork being surrounded in a blue rectangle) of the American Poster were called the films Bionic title. But after the threat of the lawsuit, both variations got the Cosmic Monster fix. But interestingly, official Godzilla merchandise like the Yamakatsu Mechagodzilla plastic kit and Toho's Perfect Collection soundtrack of Godzilla vs. Mechagodzilla still call the monster the BIONIC MONSTER.
For the theatrical release, scenes were trimmed to make a G-rating from the MPAA, mostly the aliens' fistfights were cut. The opening credits were altered as well. In 1988, New World Video released the film alongside The Return of Godzilla (Godzilla 1985), Godzilla vs. Gigan, Godzilla vs. Megalon, and Children of the Corn. The print of the film that was shown in America would later be released on VHS, twice.
- The special effects director Teruyoshi Nakano adopted Mechagodzilla's walk from the formal movements of Kabuki
- This film was made on Godzilla's 20th Anniversary
- The opening song "Miyarabi no Inori" and ending theme "Meka-Gojira wo Yattsukero" were both sung by Berubera Riin (Berbera Lynn), who plays Nami Kunigashira in the movie.
- In 2004, TriStar and Sony Entertainment released on DVD the original international cut and unedited version of the film, also featuring the original Japanese language track.
- The role of Anguirus was originally written for another kaiju called Baragon, but the suit was in such dire straits that the studio chose Anguirus to replace him.
- In the Anguirus and Mechagodzilla fight, you can see Anguirus do leaping and burrowing abilities, signaling that Baragon was meant for the role.
- The Fake Godzilla suit was used in the end of Terror of Mechagodzilla, to show Godzilla swimming away into the ocean.
- When the Azumi princess has her vision at the beginning of the film telling a monster to come and destroy the world, instead of Mechagodzilla, it was King Ghidorah, from the stills of the 1964 film Ghidorah, the three-headed monster, with flames overlaid on them. However, Ghidorah does not appear again in the film. This may be a foreshadowing, since Destroy All Monsters is set in 1999.