Former Greater Manchester Police officer Claire Church's peaceful existence in the Western Isles is shattered when four bodies linked to her previous investigations are unearthed. DCI John Hind suspects the bodies are connected to a case that dates back to 1998 and the subsequent inquiry forces Claire to revisit her troubled past in law enforcement, and drags her back into a world she thought she had left behind for good.

Another body is discovered, which seems to have been left as a message for Claire when her old shoulder number is found. She decides to return to Manchester and work on the case, to John's delight but leaving Norrie devastated as he feels she is being dragged away from him - especially as more secrets come to light and professional and personal issues collide. Claire takes responsibility for determining the identities of the corpses, and tries to persuade the Fenton family to determine if their daughter is among the victims.

Claire flees to the safety of the Western Isles after a direct threat to her life, and is convinced she is being watched. Norrie feels her presence endangers their daughter, and insists she return to Manchester to finish things once and for all. John also comes under pressure to crack the case before his misconduct review, and the two come up with a reckless plan that could cost them both dearly if they fail.

Claire's confession about their shared tragedy sends John reeling, and both crumble in the aftermath of a disastrous attempt to nail the prime suspect. When John is suspended from duty, Claire with her direct connection to one of the killers must put aside her guilt and fight to prevent another murder. DSI Lola Keir takes the investigation back to square one; Claire exploits a suspect's obsession; and John risks professional annihilation in the race to catch a killer. For Claire, the final confrontation will test her emotions beyond anything she's endured so far. As she finally confronts her own role in the crimes, will she finally put an end to the darkness or has the true horror only just begun?