Chief Inspector Boyd and his team are back to reopen the files of more cold cases. A spectral leader, calling himself "The Shepherd" is inciting vulnerable women to commit murder; a sadomasochistic killer is counting off his victims, leaving the word "sorry" carved into their skin; and the discovery of an unexploded bomb threatens to bring unforeseen upheaval to the team. Meanwhile, Boyd's visiting a shrink to deal with his anger management, while an apparently mild-mannered ex-con starts to reveal signs that he's actually well connected in a sinister crime ring. Whatever happens, whatever the result, Boyd is determined to get to the bottom of every crime, even if he must examine his own ways of working to unravel the truth. But the final case of the season brings a shocking turn of events that shakes the team to its core.