Kyuu is a junior high school student who dreams of one day being a detective. Whilst out grocery shopping a chance encounter with a fellow student and an old construction worker leads Kyuu to investigate his first murder case.

Kyuu, Kinta and Meg arrive for the DDS entrance exam along with hundreds of other applicants. Their task is to solve a murder from limited clues and then follow the correct suspect. To make things even worse they have only a few hours to achieve their objective.

The four hopefuls now enter the next phase of their exam. As they chase a suspect up a hill they lose sight of him and soon become separated. Meanwhile, Kyuu and Meg come across an injured hiker. Kyuu beseeches Meg to go on without him while he helps the man.

Kyu and his friends have finally been accepted into the famous DDS. However, they immediately stumble across a threatening note, warning of three bombs placed within the school. Two of the bombs are quickly found along with the bomber. However, it is then up to Class Q to locate the final bomb before it is too late.

A woman's husband has died, but not before leaving behind an encrypted code, and it's up to Kuzuma to crack it.

A murder takes place and one of the suspects, a stage actress, has about 1000 witnesses who watched her performance when the murder took place.

When the class goes to visit an artist friend of Katagiri's, they arrive at the scene of his murder. The only clues are a completely blank jigsaw-puzzle and the victims photographic memory.

The DSS students are taken to an island where they meet a new student, Ryuu, who is a child prodigy. Things begin to get complicated when the students of class A are found murdered behind locked doors.

As more of class A die, Ryuu's talent is revealed and it appears that he is on par with if not better than Kyuu. Meanwhile, Kazuma's string theory is disproved and both Meg and Kyuu seem to have a strange fixation with Ryuu.

Ryuu appears to have solved the locked room mystery but all of the evidence points to Dan being the culprit.

Class Q finally figures out that Dan is the perpetrator of the "murders". Dan then reveals that class Q is not the remedial class but the school's elite, and that this was all a test to weed out the unworthy.

The class witnesses a séance, one in which a family will is revealed. When the psychic in charge is murdered while in an enclosed circle the class gets to work. Too bad that there are so many suspects.

The class gets closer to finding the murderer, but another death awaits as they get involved in insurance scams and feuds over the inheritance.

The identity of the murderer is revealed and there is another surprise regarding the psychic's true identity.

While on a vacation by the sea, class Q comes across a group of marine biology students whose teacher has gone missing. As such, the class gets involved with underwater graves, treacherous students and hidden treasure.

Nanami tricks class Q in to solving one of his cases for him by posing as Dan. When the real Dan learns of this he sends class Q off to investigate more thoroughly.

Class Q is sent to Gourd village to investigate the disappearances connected to the drawing. Upon arriving the class is greeted by many strange sights, such as people in masks and a group of TV journalists.

When a new murder takes place the local villagers believe that the local Gods are responsible. As such it is up to class Q to get to the bottom of the crime.

While Kyuu, Kinta and Kazuma try to unravel the mystery behind a cryptogram they found, Megu and Ryuu are summoned to the high priest.

Ryuu and Kyuu have solved the mystery including the tunnel, the murders, and the masks. Now as their signature accusatory meeting begins the truth will be revealed, as will the murderer.

With the facts laid bare the murderer flees and Kyuu's life is put in danger.

Whilst doing fieldwork on a plateau class Q come across a murder where the time of death doesn't fit with the statements of the local residents.

A cartoonist believes she has found the perfect way to commit murder on a train and get away scott free. Unfortunately she makes one mistake, sitting in the same compartment as Kyuu and Kinta.

When Kyuu gets interested in one of the cases sent to DDS he decides to use his vacation time to investigate. However, he is soon challenged to a deductive duel by Sakurako Yukihira of class A.

With some help from the rest of his class Kyuu is able to solve the case of the idol's curse. Meanwhile, hints of a sinister organisation begin to surface from the shadows.

Megumi is prompted by her sister to invite her classmates over for dinner. However, someone chooses that moment to break in to her house and the class begin to investigate.

Class Q has a new teacher, Hongo and his brash manner leads to Kyuu and Meg staying after class to clean the building. However, they soon discover a strange painting which leads to an even stranger murder.

With Kyuu and Meg still locked underground Hongo and the others try to find them. But time is running out for the poisoned Meg.

Since they are the only ones who can pass as college students, Meg and Ryuu are sent to a school where students are dying and videos of their deaths are being broadcasted on the internet. After Ryuu's intellect amazes the class, they make some friends and some enemies....+

As the case unravels new clues and suspects come to light, strange connections are found and the death count rises. Then Meg gets taken, and she too will be killed.

Meg is back safe, and the killer's identity is discovered. However, to catch him the class must set up a meeting with all of their suspects.

After Kyu and Kenta get caught sneaking around a girls dormitory whilst looking for a lost item, the school deploys an all girls team, consisting of Meg and two girls from class A.

One of Kazuma's old teachers is in danger, a mysterious dancing efigy is out to get her. When she takes one of its poisoned darts, Kazuma promises to find out who did it, but when he does, can he handle it?

Kazuma is closer to solving the mystery of who tried to kill his teacher.

Kyuu and Meg's holiday turns out to be a blessing in disguise when a bomb threat is issued.

When the class is called in to investigate a death threat sent to a prominent violinist, they become entangled in a fight for succession as each of the violinist's students vie for supremacy.

When one of the violinist's is poisoned, Ryuu's suspicions and the lack of evidence cause him to lose his self-restraint. However, things continue to get worse when another person dies.

Another death gives the class the break they need but the revelation of the killer's identity costs one of the students dearly.

While Kinta and Kyuu race to get Ryuu to a hospital Hongo tells students about the existence of the criminal organization, Pluto. To make matters worse one of their agents may be amongst them.

While staying with Kyuu, Ryuu gets involved in a mystery concerning a headless ghost.

The class is called in to investigate the strange occurrences and threats against a village whose residents oppose the building of a new dam. However, things turn deadly when a man is found decapitated and burnt behind a locked door. Meanwhile, at a local spa the boys discover that they are separated from the girls by only a thin, passable wall....+

The case becomes more complicated when a second murder is committed and a local legend comes to light leading to an abundance of suspects. Meanwhile, the police also cause problems when they propose a solution to the first murder.

Whilst conducting their wrap up the class bring many new clues to light and Ryuu realises that Pluto has something to do with the case.

Dan is kidnapped and the whole school is called upon to help search for him. In the process Kyuu finds a photo of Dan and a man who once saved his life, not to mention began his obsession with detection.

With Dan safe and sound, the truth about Kyuu's saviour and inspiration is revealed.