Continuing after the end of Godzilla vs. Mechagodzilla, Interpol agents, led by Inspector Kusaka, search for the wreck of Mechagodzilla at the bottom of the Okinawan Sea in the submarine, Akatsuki, to gather information on its builders, the aliens of the Third Planet from the Black Hole. But, the Akatsuki is suddenly attacked by a giant aquatic dinosaur called Titanosaurus, and the crew is apparently lost.
In response to the incident, Interpol begins to investigate. With the help of marine biologist Akira Ichinose, they trace the incident and Titanosaurus to a reclusive, mad scientist named Shinzô Mafune, who was forced to leave the institute, and now wants to destroy them as well as all of mankind. When visiting his old house in the seaside forest of Manazuru, they meet Mafune's lone daughter Katsura, who tells them that not only is her father dead, but she also burned all of his notes on the giant dinosaur (at her father's request). But unbeknownst to them, Mafune himself is alive and well, visited by his scientist friend Tsuda, who turns out to be an aide to the new black hole alien leader Mugal, who is leading the project to quickly rebuild Mechagodzilla. Mugal offers their services to Mafune, so that his Titanosaurus and their Mechagodzilla 2 will be the ultimate weapons. The ultimate goal of this new wave of black hole aliens is to wipe out mankind and rebuild cities around the world as a high-tech dystopia.
But things are complicated for both factions when Ichinose falls in love with Katsura, and unwittingly giving her Interpol's secret information against Titanosaurus, the new Mechagodzilla, and the aliens. We also find that Katsura is actually a cyborg, and Mugal may have use for her.
In the course of the film, Interpol discovers Titanosaurus' weakness: Supersonic waves. But when they construct a Supersonic Wave Oscillator, Katsura sabotages the machine, prompting Interpol to hastily repair it before Mafune and the aliens unleash Mechagodzilla and Titanosaurus on Tokyo.
And when the situation gets desperate, Godzilla comes to the rescue. While Interpol distracts Titanosaurus with the Supersonic Wave Oscillator, Godzilla is able to destroy Mechagodzilla and then has a final battle with Titanosaurus while he is confused by the Supersonic Wave Oscillator. Godzilla blasts Titanosaurus with atomic ray and Titanosaurus then falls back into into the water, his fate is unknown.
Box OfficeIn Japan, the film sold 970,000 tickets. It remains the lowest grossing Godzilla film of all time in Japan, and is also one of only two Godzilla films to sell less than one million tickets. As a result, the series was put on hold until 1984. Despite being the lowest grossing Godzilla film, it is often considered a big fan favorite.
- This was the last Godzilla film directed by Ishirō Honda. He was slated to direct Godzilla vs Mechagodzilla II (1993) but died early that year.
- This was Akihiko Hirata's final appearance in a Godzilla film. He was slated to play Professor Hayashida in The Return of Godzilla (1984) but died of lung cancer before production began (Hirata was replaced by Yosuke Natsuki).
- This was Tomoko Ai's film debut. She was previously a semi-regular in the TV series Ultraman Leo.
- In Takayama's original script, the monster Titanosaurus was the singular, combined form of twin "Titan" dinosaurs, which were to meet and unite at some point in the story. (Compare to Hedorah and Destoroyah.) Due to budgetary constraints, only the singular form was used.
- This film, much unlike the films prior to it, had a much darker tone and returned to the original style of the series. It was much more serious and Titanosaurus was one of the more realistic beasts of the Showa series.
- This is the first Godzilla film to feature an original score by Akira Ifukube since 1968's Destroy All Monsters; although Ifukube's music had previously been used in 1972's Godzilla vs. Gigan, it was all stock music from previous scores.
- Current U.S. prints are severely edited for violent content (some important plot points removed in the process). This film also had the first shot of nudity in a Godzilla film (gone from all U.S. prints): Katsura's prosthetic breasts exposed while Planet 3 surgeons graphically operate on her lower heart area. It is seen in the movie poster and Godzilla Encyclopedia.
- The monster situation in this film is the opposite of the previous, where it was Godzilla and King Caesar against Mechagodzilla. Also, Mechagodzilla is not a melee-capable fighter like his first version. This is shown in how, when Godzilla does finally get in close to attack, Mechagodzilla is incapable of fighting him off. Perhaps since Titanosaurus was backing him up, the aliens designed Mechagodzailla as a long-ranged attacker to back up Titanosaurus' melee power.
- Humorously, Mechagodzilla's "new weapons" are his finger-launched missiles as before, only the hand spins several times very fast before firing them. What effect this has on the missiles' destructive ability is unknown. One character does mention them being a weapon with a faster rate of fire, but this is not enough to be noticeable.
- As this is the final film of the Showa series, one could say Godzilla was saying farewell to the audience in the final scene with him in the water as he was both looking at us and roaring at the same time.