After a brutal Tokyo job destroys his mental health, a man returns to his quiet hometown for the first time in years. There he unexpectedly reunites with Mahiro, the older childhood friend he always called his big sister. She looks almost exactly as he remembers, though she married and moved back to the countryside several years ago. He may now be taller than her, but she still treats him like a child—and their familiar bond begins turning into a forbidden affair.