In the middle of summer…. Kurisuno Sakura, along with her fellow Mystery Club Members (and the teachers supervising), decide on a trip to visit Utsugi Mansion – a place hidden in the woods where 10 years ago was known for a forced group suicide to have occured. Soon after arrival, Sakura and co learn that the previos owner of the mansion, Utsugi Masagi, had been involved in the research of a very special flower – one who’s pollen would carry poison while displaying impressive regenerating ability. A person suffering from it would be gradually covered in red marks and then die, as some cases develop an extreme tendency for violence towards others in the meantime. On top of that, the poison itself is also contagious by blood. Naturally it doesn’t take too long till one of them is also infected, so the battle for survival begins…