In a moment of cowardice, Detective Carl Mørck sends Rose, his junior colleague in Department Q, to the remote Danish island of Bornholm to answer his old colleague Christian Habersaat's repeated requests. But during his forced retirement ceremony Christian kills himself sending Rose into a journey deep into her own traumatic past. Later, Department Q are embroiled into an old cold case of a girl found dead hanging in a tree.

Denmark, 2013. Police officers Carl Mørck and Hafez el-Assad, sole members of Department Q, which is focused on closing cold cases, investigate the disappearance of politician Merete Lynggaard, vanished when she and her brother were traveling aboard a ferry five years ago.

Denmark, 2016. A blurred note is found in a bottle that has traveled across the ocean for a long time. After deciphering the cryptic note, Department Q follow a sinister trail that leads them to investigate a case that occurred in 2008. At the same time, new tragic events take place that test their faith and deepest beliefs.

Denmark, 2014. A former police officer asks Carl Mørck, head of Department Q, to find out who brutally killed his young twins in 1994. Although a local inhabitant confessed and was convicted of murder, Carl and his partner Assad soon realize that there is something in the case resolution that is terribly wrong.

Copenhagen, 2018. A frightening discovery is made in an old apartment. The subsequent investigation of Department Q leads them to an infamous institution for girls that was suddenly closed in the early sixties.