When Detective Sergeant Jeff Evans takes Lowri Davies, a new, young Chief Detective Inspector under his wing, he foresees a turbulent time. After a young girl's body is found on a rocky beach near Glan Morfa, the two have to co-operate on the case, leading them far beyond the boundaries of north Wales.

Pan gaiff Ditectif Sarjant Jeff Evans gyfarwyddyd i gymryd Lowri Davies, y Ditectif Brif Arolygydd newydd, ifanc, o dan ei adain, mae'n rhagweld cyfnod cythryblus. Ond pan gaiff corff merch ifanc ei ddarganfod ar draeth creigiog ger Glan Morfa, gorfodir y ddau i gydweithio ar achos sy'n eu harwain yn llawer pellach na ffiniau gogledd Cymru.