Yūta is a twelve-year-old boy carrying profound grief from his father's death in a traffic accident roughly a year earlier, symbolized by the old cell phone he keeps. During a summer trip to a deserted mountain dam site holding memories of his father, he encounters a mysterious elderly man and offers him a drink. A sudden thunderstorm causes Yūta to slip at the dam; the old man intervenes, inadvertently transporting Yūta thirty years into the past, to the Showa 52 era (1977).

Stranded in this past, Yūta finds himself in a rural village fated for submersion beneath the very dam from his own time. Presented as a cousin, he comes to live with a girl named Saeko and her grandmother. As a city dweller abruptly immersed in a pre-modern agrarian community, Yūta experiences a stark contrast to his urban life, participating in traditional farming, communal festivals, and firefly watching against the backdrop of the village's resigned acceptance of its impending doom.

Yūta forms a close bond with Saeko, recognizing her own experience of parental loss, and actively tries to lift her spirits. He also befriends a local boy, Kenjo, integrating into the village children's social circle. Throughout his time displaced in the past, Yūta grapples with memories of his father and his lingering grief. Living within a vanishing community and connecting deeply with Saeko fosters his emotional maturation, helping him confront his personal sorrow while gaining an appreciation for ephemeral beauty and human connection.