Keiichirou Nagumo is a once famous high school detective who can solve cases instantly. But after more than 10 years, Keiichirou is just an old detective who cannot keep up with the modern and changing times, making him get less work. Until one day, a high school girl named Mashiro, who aims to be a detective, goes to Keiichirou's office and asks to be his assistant. The daily life of an old detective and his (self-proclaimed) beautiful and unique assistant begins.