Ar y Ffin follows experienced Magistrate Claire Lewis Jones as she faces personal turmoil while presiding over cases at Newport Magistrates Court. When Ned Humphries, a childhood friend of Beca, Claire's daughter, faces arson charges, Claire's loyalty to her community is put to the test. Matters are made worse when local criminal kingpin Saint Pete appears on the scene. Claire and Saint Pete go back a long way – will her past come back to threaten her present? As she delves deeper into the case, and suspicions grow around her daughter's new bad-boy boyfriend, Sonny Higgins, Claire uncovers a web of criminal activity that could put her and her family at risk.

As the truth emerges and consequences unfold, Claire must confront her own biases and decisions. The lines between right and wrong become blurred, as Claire's maternal instincts clash with her commitment to uphold the truth.