My little sister Yui is dominated by her twisted stepbrother. Through repeated training, she gradually transforms into a masochistic sex slave, enduring brutal acts such as spanking, deep throat, and swallowing cum. Yielding to her brother's intense demands, she experiences ecstasy while bound, craving his cock and even finding pleasure in gang rape. Slowly evolving into a depraved woman, she develops an extreme dependency, yearning for creampie after creampie. This is the story of her transformation.