"To Your Eternity" centers on an immortal being known as Fushi, created by a mysterious entity to observe and adapt to the world. Initially taking the form of a rock, Fushi evolves into a wolf after encountering a lone boy in a snowy wilderness. The boy, who lives in isolation, names the wolf-form Fushi and forms a bond with it. When the boy dies, Fushi absorbs his form and memories, gaining the ability to transform into him. This marks the beginning of Fushi's journey to understand humanity, mortality, and the complexities of existence.

As Fushi travels, it encounters various individuals, each leaving a profound impact on its development. Fushi's ability to take on the forms of those it encounters, along with their memories and emotions, allows it to grow and adapt. Key characters include March, a young girl from a remote village; Parona, March's protective older sister figure; and Gugu, a boy with a disfigured face who becomes a close companion. Each interaction deepens Fushi's understanding of human emotions, relationships, and suffering.

The narrative explores themes of loss, identity, and the passage of time. Fushi's immortality contrasts sharply with the fleeting lives of the humans it meets, emphasizing the transient nature of existence. The series also introduces antagonists known as the Nokkers, mysterious entities that seek to destroy Fushi, adding a layer of conflict and urgency to its journey. As Fushi continues to evolve, it grapples with the weight of its immortality and the responsibility of preserving the memories of those it has lost.

The story is structured in arcs, each focusing on Fushi's interactions with different characters and communities. These arcs highlight the diversity of human experiences and the emotional connections that define life. Through its encounters, Fushi gradually develops a sense of purpose, striving to protect and honor the lives it has touched. The series balances moments of profound sadness with hope, creating a narrative that resonates with themes of resilience and the enduring impact of human connections.