On her sixtieth birthday, a woman takes her own life. When the case lands on Detective Carl M rck'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 to be clear. However, his superior, Marcus Jacobsen, is convinced that this is related to an unsolved case that has been plaguing him since 1988.

At Marcus's behest, Carl and the Department Q gang--Rose, Assad, and Gordon--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 murders by a very cunning and violent serial killer.

Faced with their toughest case yet, made only more difficult by COVID-19 restrictions and the challenges of their personal lives, the Department Q team must race to find the culprit before the next murder is committed, as it is becoming increasingly clear that the killer is far from finished.