Best in the World.In the past, a 14-year-old sadistic necrophiliac named Kōsuke Kira was arrested for murdering three women. Back in the present, a lawyer named Sōichi Natsume discovers Kōsuke near the shore, skewering frogs for fun. Sōichi tries to explain that it is wrong to kill living things out of amusement, but Kōsuke rejects the explanation. After a food supply drops nearby, the two investigate it with Kōsuke's abusive father, Yoshihisa Kira. Yoshihisa, after forcing his son to use an Implosion BIM to retrieve the food supply from a tree, decides to take the case for himself, going on about the survival of the fittest. Kōsuke, tired of the abuse, activates an Implosion BIM on Yoshihisa and kills him, feeling a rush of euphoria. Sōichi runs away frightened, saying Kōsuke is akin to a monster. At this point, Kōsuke is discovered by Ryōta and Kiyoshi, who retreat after failing to reason with him. As Kōsuke chases after them with the intent to kill, Ryōta decides to fight in order to protect Kiyoshi. Ryōta and Kōsuke soon recognize each other as Btooom! players and engage. After Kōsuke taunts and attacks him, Ryōta hits his head against a rock, which causes a personality shift where he perceives reality as being the game world. Ryōta defeats Kōsuke by hiding his movements with advanced sonar cancelling. Although Ryōta cannot bring himself to kill Kōsuke, Ryōta binds him to a tree, taking his food supply and BIMs. When Ryōta leaves, Kōsuke is thrilled to learn Ryōta is actually the top ranked Japanese player of Btooom!.