Sister Yui is responsible and takes care of all household duties, while her younger sister Karin is more timid and easily influenced—highlighting their contrasting personalities. The two live comfortably with their mother's new husband, their wealthy stepfather, without any worries. However, when their mother is suddenly hospitalized due to a chronic illness, everything changes. Their kind stepfather undergoes a shocking transformation, revealing a lustful and dominant nature. Caught in his sudden change, the sisters gradually fall under his control. Easily manipulated, Karin is seduced with words, while Yui is forcefully bred. Exploiting their differing personalities, the stepfather turns them into obedient female slaves. Over time, they transform into mere vessels for his pleasure, brainwashed into competing for value as his breeding tools. "Our worth exists only as our stepdad's pregnant womb—I must be first, first!" These thoughts invade their minds as they sink deeper into complete sexual enslavement.