Sister Shio (23) notices a sudden change in her younger brother as he enters puberty, gradually realizing he has feelings for her. Surprised by his behavior, she secretly installs a hidden camera in his room to spy on him. While masturbating on his bed, inhaling his scent, her brother returns home tired, and she washes his body under the shower, touching his penis. Though he resists at first, his cock unexpectedly becomes erect, and Shio starts sucking it noisily with lewd sounds. Excited by her own actions, she rubs her pussy and aggressively pushes for sex. The next morning, the brother, now drawn to his sister's body, eagerly tries to insert his cock into her. In this way, the relationship between sister and brother takes on a new form.