It has been half a year since her husband left for a work assignment, and mother Shinobu Horii is gradually getting used to living alone with her son Tooru. Lately, however, she has noticed something strange about Tooru's behavior. Whenever she's in the bath, she senses someone lurking in the changing area, rummaging through her bras. One day, while Shinobu is bathing, Tooru sneaks up to the bathroom despite knowing she's inside. Upon seeing her large breasts, he becomes visibly excited, lunges at her, groping and hungrily sucking on them. Though shocked at first, Shinobu unexpectedly begins to feel pleasure. Their relationship starts evolving into something entirely new.