In 1920, rural Ireland, Anglo-Irish twins Rachel (Charlotte Vega) and Edward (Bill Milner) share an isolated existence in their crumbling family estate. Each night, the property becomes the domain of sinister watery presences ("the lodgers") which enforce three rules upon the twins: they must be in bed by midnight; they may not permit an outsider past the threshold; if one attempts to escape, the life of the other is placed in jeopardy.

They are reminded of these rules by way of a nursery rhyme: "Girl child, boy child, listen well. Be in bed by midnight's bell. Never let a stranger through your door. Never leave each other all alone. Good sister, good brother be, follow well these cautions three. Long as your blood be ours alone, we'll see you ever from below.