Carl Morck, a troubled detective, is assigned to the newly created 'Department Q', a basement-bound job filing cold cases. He is allocated an assistant, Assad, and between them they review the case files to determine which ones can be closed. Always one to go against orders, Morck throws them headlong into the mystery of a politician's disappearance during a ferry crossing five years earlier.

In 1994, two teenage twins are found brutally murdered and a number of clues point to some pupils from a nearby boarding school, but the case is closed as a local outsider pleads guilty and is convicted for the murders. 20 years later, the case ends up on Carl Morck's desk and he soon realises that something is terribly wrong. Carl and Assad try to track down a key witness, but they are not the only ones looking for her as powerful people try to protect their secrets.

When an eight-year-old message in a bottle ends up at Department Q, Carl Morck and his assistant Assad are drawn into a horrific case involving a psychopathic murderer, religious fanaticism and abducted siblings never reported missing by their parents.

Two workmen make an alarming discovery in an older Copenhagen apartment: hidden behind a false wall are three mummified corpses, sitting around a dining table with one empty seat left. The case quickly ends up on the desk of Detective Superintendent CARL MØRCK and his assistant, ASSAD, and it is up to them to figure out, who the mummies are as well as who the fourth seat is meant for. Carl and Assad put all effort in to finding the tenant of the apartment and they end up finding clues tracing back to the notorious institution for 'wayward' girls on the island of Sprogø, where extreme neglect and medical experiments in forced sterilization are a part of everyday life. Even though the horrors of Sprogø are a closed chapter in Danish history, there are now people claiming the experiments have continued to go on up until today. Carl and Assad start a race against time to prevent new murders and attacks from happening.

One day Marco discovers a dead man. After learning that Marco may have invaluable information, Commissioner Mørck is determined to find and rescue him.