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 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'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, including mysterious piles of salt at the scenes. As the investigation goes deeper, it emerges that these "accidents" are in fact part of a sinister murder scheme.

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.