In 1994, two young twins are found brutally murdered in a summer cottage. A series of clues point to a group of young upper class students from a nearby boarding school, but the case is closed when a local outsider pleads guilty and is convicted for the murders. Twenty years later, the supposedly solved case lands on the desk of brilliant detective Carl Mørck in Department Q, and he quickly realises that something is terribly wrong. As Carl, his assistant Assad, and their new secretary Rose, start investigating, they are led to an old emergency call from a desperate girl who seems to know the secrets of the murders. Soon they are plunged into an intense search for the girl, who has been missing since the murders happened. But Carl and Assad are not the only ones trying to track her down as the girl's testimony is of great danger to a group of influential men at the top of society who will do all they can to keep her silent.