Set in a near-future Earth facing imminent destruction by a colossal, mysterious asteroid named Thanatos, the narrative follows Kazuki Kamishina, a traumatized teenager who witnessed a deadly shuttle accident that killed his parents. Isolated and grieving, Kazuki’s mundane existence shatters when he encounters Toa, a quiet, enigmatic girl pursued by both the military and monstrous creatures known as Dragons. This fateful meeting awakens Kazuki’s latent ability to resonate with Dragons, transforming him into a "Dragonaut"—a human bonded with a shape-shifting extraterrestrial organism.Toa reveals herself to be a powerful Dragonaut partnered with Giil, a dragon who can assume human form. As Thanatos draws closer, it becomes apparent that the asteroid is not merely a space rock, but a sentient entity composed of the very same life-form as the Dragons. Concurrently, a shadowy international conglomerate called the ISDA seeks to exploit the Dragons and harvest their energy to ensure humanity's survival, regardless of the catastrophic ecological and ethical costs. Driven by a desire to
protect humanity and understand her own mysterious origins, Toa fights alongside Kazuki, whose protective feelings for her slowly blossom into romance.The plot thickens with the introduction of Rio Weber, a fierce ISDA pilot seeking vengeance, and Jin Kagurazaka, an enigmatic commander with hidden agendas regarding the true nature of Thanatos. As the narrative progresses, allegiances shift and shocking truths emerge. The Dragons are revealed to be guardians of cosmic balance, and Thanatos is actually awakening to purge a corrupted Earth. Humanity’s aggressive attempts to destroy the asteroid only accelerate its lethal trajectory.Midway through, the
emotional stakes heighten when Kazuki and Toa are forced to confront their inner demons. Kazuki must overcome the crippling survivor's guilt over his parents' deaths to achieve a deeper resonance with Toa, unlocking devastating new combat abilities. Meanwhile, the ISDA's reckless experiments spawn grotesque artificial dragon-human hybrids, unleashing chaos across major cities and turning the conflict into a desperate struggle for planetary survival.In the climactic finale, Kazuki and Toa lead a final, united assault against Thanatos. Utilizing the power of their pure resonance, they pierce the core of the sentient asteroid. Instead of destroying it, Toa communicates with Thanatos, bridging the gap between human emotion and alien consciousness. By sacrificing her physical form, Toa persuades Thanatos to halt its destructive course and spare Earth. The asteroid departs peacefully into the cosmos, leaving a scarred but surviving world. Kazuki is left mourning his lost love, yet he carries forward a profound message of unity, forever changed by the bond that saved humanity from the brink of annihilation.