From Lilyka:

High schooler, Hitsuka, is unhappy and dissatisfied with her life and projects her discontent towards fellow classmate, Chisako. She is envious of Chisako, who is at the top of her class in terms of popularity. And while Chisako seems perfect in every way, that all comes crashing down when she has an accident while on a trip. She now has to live with an incurable disease known as “Nekushi Illness.” “It affects the mind, body and spirit, causing fatigue and overall weakness.” While one girl has struggled all her life to be happy, the other girl will finally see what life is like feeling the same way.