A cosplay contest is held a the hotel Shayoukan (Sunset Mansion) on a far-off island. A strange murder of the hotel owner occurs. The suspects come to be the various boys and girls who were present in the hall after the time of the murder, and among them is the singer Matsui Rena who has been called here as a judge for the contest. Suddenly, the Mousou Deka (Delusion Detective) Matsui Jurina appears. She starts to selfishly investigate on the suspects and how they could have killed the victim, coming to delusional conclusions. Can she find the real culprit...?