Kai, Mei, and Yui are high school students. The three girls are also detectives and they solve cases using their reasoning abilities. It’s also a secret that these girls are detectives. One day, the three high school girls receive a call from Nanba Kazuhiro of the Metropolitan Police Department. A letter was received in which the writer threatened to kill the popular idol group Morning Musume. Kai, Mei, and Yui go to the Morning Musume concert to guard them. After the concert, Mei is with members of Morning Mutsume on the tour bus, but Mei suddenly disappears.