Cathal (James Nesbitt) is a killer who is pursuing his former lover Mary (Kate Dickie). Mary, a woman who comes from an ancient and magical Celtic race, and her son Fergal (Niall Bruton) hide in an outlying district of Edinburgh and use magic to protect themselves, but Cathal is determined to outsmart them. Local residents begin to die at the hands of an unknown force, but is Cathal the killer, or is he trying to destroy the beast?