Following the catastrophic collapse of Mechagodzilla City, the remaining Bilusaludo aboard the Aratrum seek retribution against Haruo for eliminating what they considered essential for conquering Godzilla. Contrarily, the human crew perceives Haruo's actions as a revelation of the Bilusaludo's true ambition to dominate Earth. In response to the escalating tensions, the Bilusaludo stage a rebellion, cutting off power to the ship's engine room and forcing it to rely on auxiliary batteries for two days. Meanwhile, on Earth, Haruo discovers from Dr. Martin that Yuko has been left brain-dead, kept alive only by the nanometal coursing through her body. He also learns that the Houtua who survived the nanometal's attempts at assimilation were misled by Methphies, who duped them into believing their survival was a result of divine favor.
Confronted by Haruo, Methphies shares his ulterior motive to bring the Exifs' deity to Earth, seeking Haruo's assistance to fulfill this ambition. Dr. Martin advises him to remain hidden until the tumult subsides. Haruo is then escorted to a secluded camp by the Houtua twins, Maina and Miana. During a private conversation, Miana reveals that her people do not understand hatred; their existence is defined by either "winning" (survival and creating life) or "losing" (dying and vanishing). She tells Haruo he is "losing" and proposes to "connect life" with him, but he declines her offer. When Maina extends the same invitation later, Haruo realizes that it was she who rescued him, not Miana, and he accepts her proposal.
As events unfold, Miana discovers that Methphies is telepathically communicating with Endurph, who reveals his intentions before capturing her. She manages to reach out to Haruo and Maina using telepathy. Subsequently, Methphies performs a ceremony with his followers, in collaboration with Endurph, to summon their god, Ghidorah, to defeat Godzilla. Manifesting as a shadow on Earth, Ghidorah partially appears through singularities, consuming Methphies's followers and obliterating the Aratrum. Unhindered, Ghidorah attacks Godzilla, who finds himself powerless against the insubstantial entity, as its heads drain his energy. Dr. Martin concludes that Ghidorah's actual form exists in a different dimension and is controlled by an individual in their universe—Haruo deduces it to be Methphies, who has replaced his right eye with an amulet he reconstructed using nanometal.
Methphies divulges that his people have pledged allegiance to Ghidorah after realizing their universe is finite and doomed to destruction, having offered up planets for the monster's consumption. He then telepathically assaults Haruo, arguing that humanity's animosity towards Godzilla renders him a suitable offering. He insists that Haruo must submit to Ghidorah as its witness to facilitate its complete manifestation. However, Maina and Dr. Martin harness the Houtuan god's egg to psychically contact Haruo, revealing the means to halt Ghidorah. Meanwhile, Haruo recalls a charm he lost as a child when he fled Earth; the charm's floral design revitalizes the meaning of his name, "Spring," stirring the hope necessary to overcome despair. Fueled by determination, Haruo breaks free and shatters Methphies's amulet, rendering Ghidorah susceptible to Earth's physical laws, leading to its ultimate defeat at the hands of Godzilla. Methphies dies, warning Haruo that Ghidorah will perpetually keep an eye on him as long as he lives.
As time passes, the survivors bury their weapons and integrate into Houtua society, with Maina pregnant with Haruo's child. Dr. Martin informs Haruo that he has restored the last working Vulture mech, utilizing the nanometal within Yuko's body as a foundation to reconstruct civilization as it once was. Yet, Haruo feels a sting in his right eye, hearing Methphies's voice suggesting that this turn of events would inevitably invite Ghidorah's return. With Yuko by his side, Haruo provokes Godzilla to obliterate him and all traces of the living nanometal for the benefit of the Houtua. This ultimately prevents Ghidorah's reemergence, allowing the Houtua to coexist with Godzilla, treating the colossal creature as a natural disaster to be respected and avoided rather than battled against.
In a poignant post-credits scene occurring years later, an aging Maina observes a group of children as they partake in a ritual honoring Haruo, casting knotted strings representing their fears into a fire beneath a wooden effigy of a Vulture mech. Even in death, Haruo's tale serves as a cautionary story for the generations that follow.
