"Godzilla: The Planet Eater" is the final installment in the anime trilogy produced by Polygon Pictures and Toho, concluding the story arc that began with "Godzilla: Planet of the Monsters" and continued in "Godzilla: City on the Edge of Battle." The narrative picks up after humanity's failed attempts to defeat Godzilla, leaving the surviving human factions divided and desperate. The Exif, a religious alien race, manipulate the remaining humans into summoning their deity, Ghidorah, a metaphysical entity capable of consuming entire worlds. Ghidorah is portrayed as a god-like being with no physical form, existing in a higher dimension and feeding on the destruction of civilizations.

The protagonist, Haruo Sakaki, remains central to the story, grappling with his obsession to destroy Godzilla and his growing disillusionment with the Exif's motives. As Ghidorah is summoned, it becomes clear that the entity's power is beyond comprehension, capable of manipulating reality and time. Ghidorah's arrival triggers a series of catastrophic events, including the destruction of the Aratrum, the human colony ship, and the near-annihilation of the remaining human forces. The Exif's true agenda is revealed: they seek to use humanity as a sacrificial offering to Ghidorah, believing it will bring them closer to their god.

Amidst the chaos, Haruo is confronted by Metphies, the Exif priest who has been orchestrating events from the shadows. Metphies reveals his belief that humanity's suffering is a necessary step toward enlightenment, further challenging Haruo's resolve. Meanwhile, the Bilusaludo, another alien race, attempt to use their advanced technology to combat Ghidorah, but their efforts prove futile against the entity's otherworldly power. The story explores themes of faith, sacrifice, and the consequences of unchecked ambition, as Haruo is forced to confront the moral and existential implications of his quest for vengeance.

The climax of the film centers on a final confrontation between Godzilla and Ghidorah, with Haruo caught in the middle. Godzilla, representing the indomitable force of nature, faces off against Ghidorah, a symbol of destruction and chaos. The battle is depicted as a clash of titans, with Godzilla's physical might pitted against Ghidorah's reality-warping abilities. Ultimately, Haruo makes a pivotal decision to sacrifice himself to stop both Godzilla and Ghidorah, using a device to disrupt their energies and bring an end to the conflict. His actions result in the destruction of Godzilla and the banishment of Ghidorah, but at the cost of his own life.

The film concludes with humanity left to rebuild in the aftermath, reflecting on the lessons learned from their struggle against both Godzilla and the forces that sought to manipulate them. The trilogy's overarching narrative emphasizes the resilience of humanity in the face of overwhelming odds, while also questioning the cost of survival and the nature of power. "Godzilla: The Planet Eater" serves as a fitting conclusion to the trilogy, tying together its philosophical themes and delivering a resolution to the conflict between humanity, Godzilla, and the alien forces that sought to control their destiny.