Tonio, five years old, is the youngest sibling in a trio that includes fourteen-year-old Jaimi and seventeen-year-old Suneko. The siblings struggle financially due to their father's hospitalization, urgently needing money for his medical treatment. This desperation drives their response when confronted with a critical choice involving a mysterious button.

A holder of the button presents Tonio and his siblings with a dilemma: pressing it grants 1 million yen (roughly US$7,415), but condemns the presser to endure 500 million years of absolute isolation in an empty void. After this immense timespan, the individual returns instantly to the moment the button was pressed, retaining no memory of the ordeal.

Tonio's extreme youth renders him the most vulnerable of the three as they grapple with this profound ethical and psychological challenge, born of immediate need and unfathomable long-term sacrifice.