In seventh-century Ireland, a scholar and priest called Artt has a dream telling him to leave the sinful world behind. Taking two monks - young Trian and old Cormac - he rows down the river Shannon in search of an isolated spot on which to found a monastery. Drifting out into the Atlantic, the three men find an impossibly steep, bare island, inhabited by tens of thousands of birds, and claim it for God. In such a place, what will survival mean?