The series stars Sian Reese-Williams as DI Cadi John, an experienced investigator whose love of policing stems from her father, Huw, a long-serving officer with Dyfed–Powys Police. Cadi and her partner, DS Owen Vaughan (Sion Alun Davies) are called to investigate when the body of a girl who disappeared five years ago is found in a lake. The investigation leads to the reopening of a cold case and identifies a possible miscarriage of justice. Aside from Reese-Williams and Davies, Rhodri Meilir, Gillian Elisa and Gwyneth Keyworth are also credited as principal members of the cast.