On her 60th birthday, a woman takes her own life. When the case lands on Detective Carl Mørck’s desk, he can’t imagine what this has to do with Department Q, Copenhagen’s cold cases division, since the cause of death seems apparent. However, his superior, Marcus Jacobsen, is convinced that this is related to an unsolved case that has been plaguing him since 1988. At Marcus' behest, Carl and the Department Q gang reluctantly begin to investigate. And they quickly discover that Marcus is onto something: Every two years for the past three decades, there have been unusual, impeccably timed deaths with connections between them that cannot be ignored. As the investigation goes deeper, it emerges that these "accidents" are in fact part of a sinister murder scheme.