On Sado Island, Kii works as a cleaner at a gold mine. One day, she finds a collapsed woman in the facility and takes the woman to her home. The woman doesn't remember her past or even her name. The woman is named Midori by the two other girls living with Kii. Kii gets permission from the director for Midori to work as a cleaner at the mine. One day, Midori is attracted by the presence of cats and this leads her to meet Ao, who works as a night guard at the gold mine.