"Ōyukiumi no Kaina" is a science fiction manga series set in a distant future where humanity faces environmental collapse. The story unfolds in a world where the surface of the Earth has become uninhabitable due to extreme cold, forcing the remnants of humanity to live in towering, biomechanical structures known as "Orbs." These Orbs are controlled by a ruling class that maintains strict control over resources and population, while the lower classes struggle to survive in harsh conditions.

The protagonist, Kaina, is a young boy who has spent his entire life in the upper reaches of the Orb, isolated from the rest of humanity. His existence is relatively peaceful until he encounters a mysterious girl named Ririha, who is the princess of the "Land of the Sea," a mythical place believed to hold the key to humanity's survival. Ririha seeks Kaina's help to reach the surface and uncover the truth about their world. Together, they embark on a perilous journey that challenges their understanding of their society and the environment.

As Kaina and Ririha descend through the layers of the Orb, they encounter various factions, each with its own agenda. The ruling class, represented by the "Atland" faction, seeks to maintain control over the Orbs and suppress any dissent. Meanwhile, rebel groups and outcasts struggle to survive and dream of a better future. The narrative explores themes of survival, class disparity, and the consequences of environmental degradation.

Kaina's journey is marked by encounters with allies and adversaries, including Liliha, a skilled warrior who becomes a key ally, and Yaona, a scientist who holds critical knowledge about the Orbs and the world's history. The story delves into the origins of the Orbs, the true nature of the "Land of the Sea," and the possibility of restoring the Earth's environment.

The manga balances action, mystery, and character development, with Kaina's growth as a central focus. His initial naivety and curiosity give way to a deeper understanding of the world's complexities and his role in shaping its future. The narrative builds toward a climactic confrontation that challenges the status quo and raises questions about humanity's resilience and capacity for change.