In a world covered with fine dust, all the 'C's, who have received the expensive antibody injection typically for newborns, are able to live until the age of 100 and the 'N's are living a completely different life according to their lifespan that ends at the age of 30. Io, a 26-year-old college student who lived and thought to be 'C' for life, discovers that she has not been given the antibody injection due to an error at the hospital when she was born. For the first time in her life, Io is curious about the lives of 'N's. In particular, Io is curious of Joan, the only 'N' in school.