In seventh-century Ireland, a priest has a dream telling him to leave the sinful world behind. Taking two monks with him - young Trian and old Cormac - he travels down the Shannon in search of an isolated spot on which to found a new place of worship.

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. But in such a place, far from all other humanity, what will survival mean?