Former Head of Department Q Carl Mørck has retired. That was the plan, at least. So when he comes across a disturbing audio recording, he turns it over to his former colleagues, who are struggling to keep the old Department up and running on their own.

That is, until a new transfer unexpectedly arrives - Helena Henry, a Frenchwoman who throws herself into the investigation with gusto, but who may also have secrets of her own.

As the team realizes that the audio recording might shed new light on a seemingly open-and-shut murder-suicide case from years earlier, suddenly everything they thought they knew about the case is called into question. And meanwhile, a spurt of seemingly random violence - an explosion off the coast of Jutland, a young woman shot dead in Copenhagen - sets the Department Q team investigating a possible connection to a different long-ago dark and sinister event, one that someone has never forgotten.