The story began in a classroom of a middle school. Our main character, Kyu, when called by the guidance counselor, declared that he wanted to be a detective. He took the entrance exam to Dan Detective School where talented people from the whole country gather. He passed the difficult entrance exam for the so-called "Q Class" (Q = qualified), which is personally instructed by the founder and head master of the school, the legendary famous detective, Dan Morihiko. There he got to know Minami Megumi with her photographic memory ability, Touyama Kintarou - the descendant of Kin-san from Touyama, Narusawa Kazuma - a genius programmer, and Amakusa Ryu - the weird Tokyo University student. The five of them work together, as well as competing each other, heading towards graduation.
Q dreams of becoming a great detective, and what better way for the ninth grader to realise his dream than by enrolling into the Dan Detective School? Dan Detective School is Japan's First and only academy set up to groom the next Sherlock Holmes. And in "Class Q", a special class for the academy's brightest, student aren't just taught the basic know-how of detective work, but also the qualities that define a great detective, whether it's a curious mind, the ability to discern the smallest bit of evidence, or a devotion to righteousness. Kyu manages to qualify for Class Q, where he is joined by others: Megumi, Kintaro, Kazuma, and Ryu, who's highly regarded by Dan himself, but a mystery to others. Fellow rivals and friends in class, the fivesome will have to put their collective skills together if they are to solve, time and again, some of the most baffling cases involving everything from cold-blooded murder to restless ghosts. Will they make the grade?