Reiko is a rich heiress to a zaibatsu, but she chose to work as a detective. She resents her superior, Kazamatsuri, who always seem to be making illogical deductions that causes murder cases to become cold cases. One day, her butler suddenly announces his retirement, and he chooses Kageyama as his replacement. Reiko realizes that Kageyama's deduction skills are very good after Kageyama solved a seemingly unsolvable mystery. After that, whenever Reiko meets an unsolvable case, she would lay out the details of the case for Kageyama, and he would explain to her how that particular case is solved after dinner. After every case, Kageyama also explains to Reikko some universal truth or human nature that led to each crime being committed.