When Itsuki's parents die he inherits their mansion and business. Shortly after he is reintroduced to his elder sister Yuri Kimishiro and her younger sister Ran. Although they have different mothers their father is the same making them half siblings.

So Itsuki ties up his sister Yuri and rapes her then moves Yuri and Ran to the basement to live their lives as his sex slaves. There will be no challenge to his fortune.