Shinshoku 

 浸蝕  

Toshiya is an orphan. He is adopted in the kitagawa family which consists of Kitagawa Kureha, her daughter Kitagawa Misaki and her husband who is never home. Toshiya was warmly welcomed into the family and treated like a real son however the trauma he suffered with his real family - his mother always had sex with men other than his father and he witnessed it all in numerous occasions, and when his father died his mother deserted him thus he ended up in an orphanage- was never cured and grew up with a twisted mentality.  

One day he gets a perfect chance to carry out revenge on his new stepmother Kureha, who became a caring mother for him but regardless his trauma leads him to believe Kureha is like her real mother was, so getting his hands on a photo showing his stepsister shoplifting he starts blackmailing both his stepmom and stepsister...