Teru, a younger brother, heard about sex from a friend and became interested in it. Then, while eating with the family, Teru starts blurting out "sex, sex" without any shame, which makes his older sister Yuzu, who is of marriageable age, feel awkward. Yuzu tries to teach Teru about sex by gently bringing him to orgasm with her own hands, but this ends up igniting Teru's Kokoro instead...