Helpless and struggling for cash, 20-year-old Kusunoki sells the last of his possessions to buy food. Noticing his poverty, an old shop owner directs him to a store that supposedly purchases lifespan, time, and health. While not completely believing the man's words, Kusunoki nevertheless finds himself at the address out of desperation and curiosity.

Kusunoki is crushed when he finds out the true monetary value of his lifespan—totaling a meager three hundred thousand yen. Deciding to sell the next 30 years of his life for ten thousand yen per year, Kusunoki is left with only three months to live. After heading home with the money, he is greeted by an unexpected visitor: the same store clerk he sold his lifespan to. She introduces herself as Miyagi, the one tasked with the job of observing him until the last three days of his life.

Jumyou wo Kaitotte Moratta. Ichinen ni Tsuki, Ichimanen de. follows the remaining three months of Kusunoki's life as he confronts lingering regrets from the past and discovers what truly gives life value.