A baffling series of crimes turns out to be the work of Shiriarty, a villain who seeks to steal a mysterious treasure known as the "O-PARTS". Oshiri Tantei along with his assistant, Brown, enlist the aid of Audrey, an investigator from the international police organization Wanterpol, in an effort to prevent the "O-PARTS" from falling into the wrong hands while Shiriarty strives to plunge the world into darkness.<br><br>The owner of Cafe Lucky Cat, located on the first floor of the Oshiri Tantei office, has a back injury and so he cannot do an interview and so Oshiri Tantei takes over for the day. During the interview, a huge order of food arrives by mistake. In order not to let the ingredients go to waste, Oshiri Tantei works everyone in town to make a super-sized sweet potato.