From Rayns&Lele Scans:

A devastating disease called Melancholia spreads across certain areas of the city, causing the unfortunate person to lose their memories of their most important person. Kazanami Toki is unaware that he has caught the incurable disease, but "by chance" he meets his forgotten lover, Hanamiya Yuna. After their fateful encounter, he is left with a strange feeling he is unable to understand.