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.