Itsuki has lived an unlucky life. His parents passed away when he was young and his relationship with his adoptive parents did not last long. Now an university student, he took up a high-paying live-in part-time job in a mansion deep in the mountains despite being suspicious about it.
When he got there, he met a beautiful girl who wondered if he would become her older brother. Not knowing what she meant by this, he chased after her into the mansion where many other people had gathered, all but one of whom were around his age. Their task was explained to them there: to live there while playing their assigned role. As foreseen by the girl from earlier, Sui, he was given the ‘older brother’ role, while she was the ‘imouto’.
This strange life began without any problems, until one rainy night when the sound of glass breaking echoed through the mansion. They hurried towards the sound and found one of their own, Miho, had been brutally murdered.