While visiting Cornwall, Strike is approached by Anna Phipps, who seeks his help finding her missing mother, Margot Bamborough. Together, Strike and Robin tackle the nearly 40-year-old cold case by first sorting through the various players in Margot’s life at the time..

As they comb through old videos for clues, Strike and Robin meet with a number of Margot's former co-workers at the hospital. Then, while Strike's visit home grows increasingly difficult, a rogue Robin locates and goes after two key, and potentially dangerous, suspects.

With a narrowing list of unreachable suspects, Strike and Robin target the impenetrable Dennis Creed with help from the father of one of his suspected victims. Meanwhile, as his mother further declines, Strike's drunken antics cause a fissure between him and Robin..

With knowledge of a possible pattern in Steve Douthwaite's name changes, Strike combs through the internet for potential new aliases, while Robin navigates a misstep by Saul. As Strike and Robin inch closer to the truth, will they finally bring closure to the past?