One day, a mysterious package arrives addressed to his father...'Masaru''s friend says, "That's definitely an erotic video!" Masaru, who doesn't even know his existence, sneaks into his father's room, but there he finds his father masturbating. Surprised father, and his older sister 'Riko' who rushed to see what happened. Feeling pity for her younger brother who has no knowledge of her sexuality, Riko decides to do sex education with her own body.
