"Kiku-chan to Okami" is an adaptation of "Grave of the Fireflies" author Akiyuki Nosaka’s short story "Toshi-Oi Mesu-Okami to Onna no Ko no Hanashi" (The Story of the Old Mother Wolf and the Girl), which follows a girl and the wolf in the fields of WWII Manchuria who watches over her like a mother.

Source: MyAnimeList