When builders discover a human skeleton in the wall during a renovation of a café, they immediately call in DCI Harry Nelson and Dr. Ruth Galloway. The bones turn out to be modern—the remains of Katherine Sands, who went missing in the 90s. Right before she disappeared, Katherine attended a spiritual wellness course led by none other than Ruth’s dear friend Cathbad.

The police start asking questions and discover that there are other missing women connected to the wellness group. Suspicion soon falls on Cathbad, but believing the druid is innocent, Nelson follows a trail to the famous Neolithic flint mines in Grimes Graves and to an archaeology club. Ruth agrees to infiltrate their ranks to try to save her friend and while there she meets a pivotal figure from her own past. When he realizes that Ruth is in danger, Nelson is finally forced to face his feelings for her. What can the future hold for them after all this time?