Ireland, 1924. Seamus Óg McGrinna (Dominic Cooper) is a lighthouse keeper whose elaborate rituals keep him at a distance from the outside world as he awaits the fabled Each Uisce (Water-Horse) to take him. His carefully constructed prison is blown open by three people who change the course of his life: an American war widow whose quest for closure ignites something in him; a housekeeper who fiercely protects him; and the hard-line priest who starts a battle of wills.