Ibuki decides to ask Shima about a rumor going around that he killed his former partner, Kosaka, but Shima refuses to talk about it. Troubled, Ibuki approaches Kokonoe, who tells him that Shima’s former partner died under suspicious circumstances. Enlisting Kokonoe’s help, Ibuki decides to reopen an investigation into Kosaka’s death and its possible connection to a serial poisoning case that occurred on the same day.