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 Habersaats repeated requests. But during his forced retirement ceremony Christian kills himself sending Rose into a journey deep into her own traumatic past. Later, the newly engaged Carl arrives on the island where he, Rose and Assad are embroiled into an old cold case of a girl found dead hanging in a tree. The killing, officially written up as a traffic accident, has connections to both the local Police precinct and a cult of sun worshipers from where young women have vanished without trace for years. Once the case is reopened, and the island now dangerous, Carl must solve it and resolve his own personal dark past to be able to move on to a new life with his fiancée.