When a police sergeant is sent on a punishment posting to a rural island with an eerie, hidden religion, the cold case of a missing boy threatens to unearth well-buried secrets.

A dead body discovered; Grace is convinced something sinister is at play - but mainland police disagree. Is Cai's death accidental? Or is there a murderer loose on St. Jory?

The Widow's Wail, an almighty, annual storm, brings the islanders to the village hall for fun and refuge. Trapped inside, Grace is on edge - could Cai's murderer be amongst them?

The word is out - the True Way were involved in Cai's death. As the island reels, an obvious suspect emerges... but are Grace's theories about to be turned on their head?

After ruling out the True Way, Grace turns her suspicions to Gruffudd. But the island seems to be galvanising for something else.

With Cai's killer caught, the True Way takes control of St Jory, threatening their own form of violent justice. Can Grace restore order or will she fall victim to the chaos?