Gorou Hoozuki loves his older sister Haru very much. He tries his best not to make this fact obvious, but little does he know that his "secret" is known to many, including Haru! Due to this, Haru does all kinds of lewd and suggestive acts to her younger brother, which she takes much pleasure in performing.

Hoozuki-san Chi no Aneki is a tale about the everyday life of a perverted younger brother and a sadistic older sister filled with comedy, drama, and love.