Carwyn and Rhian – the last in a long line of sheep farmers – are living out a brutal year on their hillside farm, deep in the mountains of Eryri, North Wales.

When Carwyn discovers a buried prehistoric ruin in one of the fields on their land, his curiosity quickly descends into obsession. His wife, Rhian, meanwhile, is confronted with the growing realization that the man with whom she shares her life and home is becoming a frightening stranger.

As the harsh winter closes in, Rhian finds herself alone with her increasingly peculiar husband, and the mountains, and the looming megalithic stones.