Shunsuke Kobayashi lives with his younger sister Mayu at their parents' home while his elder sister Natsumi lives with her husband, the rich heir Kimishima Yohei, together with Yohei's younger sister Yoko.

Indebted after the accidental death of both parents, Shunsuke and Mayu where asked to move to Yohei's mansion. Despite Natsumi's worries, Shunsuke accepts and moves in with the help of his girlfriend and classmate, Yuki Kawai. At the dinner Yohei tells everyone to "never enter the room at the ground", but it's clearly an invitation to enter.
In the evening Shunsuke hears moaning from that room, thinking it's normal because "Yohei and Natsumi are married", yet can't help but peek, seeing Yohei having sex with his own sister Youko.

In that moment Natsumi comes from the back and introduces Shunsuke to the room and undresses. Yohei treats Natsumi like a toy, having her masturbate, drink his pee and give a blowjob to the terrified Shunsuke.
Apparently, Natsumi worked in a soapland in the past to pay the same debt. When she killed a guest that was threatening her, Yohei blackmailed and married her, turning her into a masochist. Yet the man she killed was....

Now Yohei wants to involve Shunsuke as his companion in this crazy sex life.