Shunsuke Kobayashi live with his imouto Mayu at their parents home, while his elder sister Natsumi live with his husband, the rich heir Kimishima Yohey and Yohey younger sister Yoko. Indebted after the death of both parents in an accident, Shunsuke and Mayu where asked to move to Yohei mansion, Natsumi seem to be worried for that but Shunsuke accept and move in with the help of his girlfriend and classmate, the fatherless Yuki Kawai; at the dinner Yohei tell everyone "Never enter to the room at the ground", but it's clearly a invite to enter. And in the evening Shunsuke hear moaning from that room; "That's normal because Yohei and Natsume are married" but he can't restrain to peek and see Yohei having sex with his own sister Yoko. In that moment Natsume came from the back and introduce Shunsuke to the room and undress; but Yohei treat Natsume like a toy, having her masturbate, drink his pee and give a blowjob to the terrified Shunsuke. It seem that Natsume in past gave sex services in a soapland to pay same debt, and there killed a man that was threatening her, Yohei blackmailed and married her, and turned her in to a masochist, also that man she killed was.... Now Yohei want to involve Shunsuke as his companion in this crazy sex life.