Koichi lives with Yumi, who became his stepmother after his father's remarriage. She is young, beautiful, kind, and approachable, and Koichi quickly grew comfortable with her. However, one day, he catches her intimately interacting with a friend at home while not wearing any underwear, completely braless. Shocked by the scene, Koichi develops intense anger and hatred toward his stepmother. He snatches her underwear and orders her to go out to the convenience store without it. The film depicts Koichi mocking his trembling, humiliated stepmother as he sexually dominates her voluptuous body. This work deeply explores familial tension, emotional shifts, and the psychological impact of unexpected events.