The young man's name: Midou Ginnosuke. With his grandmother and her cat, Anzu, they should have lived a quiet life, but one morning the cat Anzu was suddenly able to change into a human! On her own, she was keeping a promise made with Grandmother, to work hard to become Ginnosuke's older sister, though she was always failing. However, Ginnosuke knew Anzu's sisterly heart, watching warmly over him. This is the start of "Ane-chan" and Ginnosuke's life together!