The bus picks up the protagonist and his classmates as usual. He is talking to Riyo — a girl he has been paying attention to — when a car collides with the vehicle and everything goes dark. He wakes in a locked room inside a mansion he does not recognize, surrounded by classmates and two strangers who are also confused. Details accumulate: the room is locked from outside, people are dying under unclear circumstances, a blood-soaked memo has been found, and a photograph has turned up with no immediately apparent connection to anything. Whatever brought them to this mansion has not finished with them.