After much thought, Ayumu determines that Rio is indeed the one that killed his teacher.  He determines that she intended to make the fact that she's missing a rib impossible to detect by blowing herself up.  He confronts her with Hiyono and is presented with a challenge, to drink water that may be tainted with poison.  Ayumu's pride is overcome by his lack of confidence but Hiyono reveals her secret tape and changes the rules.