Born in 1936, Morihiko Dan is the founder and principal of Dan Detective School, and a famous detective who has solved many cases, and is the only private detective in Japan who is licensed to hold a gun.

Among many criminals he fought, the most powerful one was the crime organization known as Pluto. Although Pluto was finally thought to apparently have been destroyed, it wasn't without cost. Mr. Dan was forced to be in a wheelchair for the rest of his life.

Years later, there are signs that Pluto is being resurrected. Realizing that his days were numbered due to his old age, Morihiko Dan decided to open a new, special class called Class Q, consisting of several extremely talented persons. He would train them, analyze each one of them, and appoint the best student of this class to be his successor.