Following the defeat of Mechagodzilla Kazunari Mori and the Simeons by Godzilla Toru Kawai and the Okinawan guardian monster King Caesar, Interpol agents scour the bottom of the Okinawan Sea for Mechagodzilla’s remains in hopes of uncovering fresh clues about the Simeons’ plans. Their submarine is suddenly ambushed by a colossal marine dinosaur, Titanosaurus Tatsumi Nikamoto, and the crew vanishes, leaving the pursuers with a troubling mystery that will soon pull them deeper into a dangerous confrontation.
Interpol launches a full investigation into the incident. With the help of marine biologist Akira Ichinose Katsuhiko Sasaki, they trace Titanosaurus to a reclusive and unscrupulous genius, Shinzô Mafune Akihiko Hirata, who harbors a cruel motive: to eradicate humanity. As the investigators press Mafune’s home for answers, they meet Katsura Mafune, Mafune’s daughter, who has undergone surgery and become a cyborg after injuries sustained during her father’s experiments. Katsura insists that her father is dead and claims she burned his notes about Titanosaurus, complicating the team’s efforts to separate truth from deception.
The plot thickens when Mafune is visited by Tsuda Tôru Ibuki, an aide to the Simeons’ supreme leader Mugal Gorō Mutsumi. Mugal envisions a joint plan to rebuild Mechagodzilla into a more formidable form—Mechagodzilla 2—and offers the Simeons’ services to Mafune so their monsters can jointly target humanity. A crucial device for control over Mechagodzilla 2 is implanted into Katsura, tying her fate to the evolving threat.
Amid growing tensions, Ichinose’s growing affection for Katsura leads him to unwittingly share Interpol’s strategic information about the Simeons, Mechagodzilla, and Titanosaurus. The revelation only intensifies Mafune’s impatience, and he pushes Titanosaurus into action against Yokosuka without awaiting the Simeons’ approval. Interpol discovers Titanosaurus’ vulnerability to supersonic waves, but Katsura takes a desperate step and destroys the island’s supersonic wave oscillator, altering the balance of power.
The arrival of Godzilla signals a dramatic shift in the battle. He confronts Titanosaurus and defeats the creature decisively, forcing it to retreat. Yet the larger threat remains, as Mafune’s forces unleash Mechagodzilla 2 and Titanosaurus on Tokyo when Ichinose is captured by the Simeons while visiting Katsura. Interpol scrambles to repair the wave oscillator they had earlier used, and the Japanese armed forces rally to fend off the two intruding monsters. Godzilla enters the fray, but the odds are overwhelming until Interpol provides a crucial distraction—reusing the repaired wave oscillator to draw Titanosaurus away and let Godzilla concentrate on Mechagodzilla 2.
With the city under siege, Interpol operatives infiltrate the Simeons’ hideout, rescue Ichinose, and eliminate Mafune along with many of the Simeons. The remaining conspirators attempt an escape, but Godzilla disables their retreat by blasting their ships with atomic breath. Katsura, gravely wounded, makes a self-sacrificial move to disable Mechagodzilla 2’s control device, and Godzilla destroys Mechagodzilla 2. Katsura dies in Ichinose’s arms, a somber reminder of the collateral cost behind the clash of giants. In the aftermath, Godzilla, aided by Interpol, defeats Titanosaurus once more and returns to the ocean, leaving the world to pick up the pieces and wait for the next chapter in a long history of awe-inspiring and devastating encounters.
