As Christmas comes to the west cost of Iceland, a corpse is found in a fish farming pond. Detective Hildur Rúnarsdóttir and trainee Jakob Johanson barely have time to start their investigation before another body is discovered. And soon a third.

While investigating the case, Hildur's lost sister weighs heavy on her mind. Meanwhile, Jakob travels to Finland for the hearing of his fraught custody battle, that leaves him facing dire consequences. As the number of deaths continues to grow, Hildur and Jakob are desperate to catch the killer before they strike again.