Koyomi Takasaki’s life unfolds against the backdrop of his parents’ divorce, leading him to reside with his mother in his grandfather’s home. Childhood regret over his grandfather’s passing ignites a fascination with parallel universes, nurtured by visits to his father’s research institute. This obsession molds him into an academically ambitious yet introverted figure, prioritizing intellectual pursuits over personal relationships.

During adolescence, Kazune Takigawa enters his life, asserting she is his companion from the 85th parallel universe. Though skeptical, Koyomi slowly opens to her presence, their shared curiosity sparking collaborative academic pursuits and eventual romance. Their journey is marked by Koyomi’s struggles to articulate emotions and Kazune’s guarded hesitance, yet their bond deepens, culminating in marriage and parenthood.

A fracture in their reality emerges when an alternate Kazune arrives following their son’s tragic death. Confronted with overlapping timelines, Koyomi navigates the emotional weight of multiverse theory, assuring this displaced Kazune that her reality’s counterpart will stand by her—a testament to his evolving grasp of intertwined fates across dimensions.

Later, Koyomi joins his father’s institute, dedicating himself to studying temporal and interdimensional shifts. His work becomes a conduit to mend past regrets and safeguard his family’s stability. Decades on, an elderly Koyomi encounters a familiar stranger at a crossroads, her presence sparking contemplation of unchosen trajectories and the quiet resolve to cherish his lived path.

A divergent timeline reveals an alternate existence where Koyomi and Kazune forgo romance yet sustain a collaborative partnership. Their unyielding mutual respect and shared intellectual fervor demonstrate the persistent influence of their connection, transcending the boundaries of love and reality itself.