My sister is defenseless in the house and keeps flashing her breasts and panties without caring about me. I've been watching her since I was little so I didn't think anything of it, but as a virgin I have a stronger libido than most people and I'm turned on by my sister's body, which I had never paid attention to before. When I couldn't help but stare at her, she suddenly became embarrassed and blushed, saying, "Big brother, you were looking at my boobs, weren't you!?" This is a record of the erotic incestuous relationship between four small-breasted sisters, Kanon, Kanon, Mai, and Hina, who are starting to become interested in sex.