Tetsuo Yabusame is an extremely shy boy whose only friend is a giant robot named Yukio. Together they serve as humanity's saviors, fighting against colossal kaiju monsters that invade Earth from outer space. After a desperate final battle in which Yukio is destroyed, Tetsuo escapes in a pod and enters eight years of cryogenic sleep. When he finally returns to Earth, he finds the entire planet encased in ice and snow, a frozen wasteland now called Snowball Earth. Civilization has collapsed and human survivors are scattered in isolated settlements.

Tetsuo, still carrying the promise he made to Yukio, must learn to interact with other people for the first time in his life. He encounters survivors led by the pragmatic Ao Nogi, a blacksmith named Hagane Takimura, and various other refugees who have adapted to the frigid environment by hunting and harvesting kaiju for food, fuel, and materials. The survivors see Tetsuo as both a potential threat and a legendary hero. As he explores this world, Tetsuo discovers that the kaiju still roam the ice fields and that mysterious organizations, such as E-RDE's Light, have emerged among the ruins.

The narrative follows Tetsuo's personal journey from a socially withdrawn warrior to someone capable of forging bonds with others, while also unraveling the mystery of what caused Earth's global freezing and whether he can keep his last promise to Yukio. The story blends mecha action, kaiju battles, and post-apocalyptic survival with an intimate focus on loneliness, trauma, and the search for human connection in a world that has grown cold.