A Challenge from the Grim Reaper! The Showdown with the Mastermind and Their Fateful Rivalry

Sasaki Rena and Minemori Kotoha visit a woman named Takahashi Rika. Rika's brother married Sawayanagi Nana and died eight days later under suspicious circumstances. The doubtful Rika hired the Abiru Detective Agency to investigate the matter. Rika shows Rena the investigative report written on Sawayagani. In it she finds a copy of her high school yearbook, which contains a photo of Sawayanagi.