Police inspector Carl Mørck is put in charge of a department of cold cases, joined only by his assistant, Assad. They dig into a case about a disappeared woman.

The murder of young twins initially implicates a group of upper class students as the killers, though the case takes a turn or two from its starting point.

An old note is found in a bottle in Jutland which has traveled very long across the ocean a long time ago. The note is hard to decipher, but when the Danish investigator from Section Q receives it, the odd personalities within the section find a new angle on it and try to unveil yet another old and diabolic case even though the lead is cold.

A series of mysterious disappearances in 2006 are eerily connected to the same institution.

Marco Jameson only wants to become a Danish citizen and go to school as a normal teenager. But his uncle Zola rules his pocket clan with an iron hand. One day Marco runs into a dead man and, realizing the extent of his uncle's criminal acts, decides to run away. But he soon realizes that besides his family, someone else wants to keep him quiet forever. In the meantime, Commissioner Carl Mørck is furious that his assistants Assad and Ross have taken on a new missing person case. Carl has other problems - his biggest enemy is already his boss and his first job is to make Special Department Q difficult. But after learning that a boy named Marco may have invaluable information about the case, Commissioner Mørck is determined to find and rescue him.

Department Q centres around Carl, a former top-rated detective in Edinburgh assigned to a new cold case whilst wracked with guilt following an attack that left his partner paralysed and another policeman dead.