In a last-ditch attempt to salvage her family, tough New Yorker, Adelle (Maria Bello) takes her daughter Sarah (Sophie Stuckey) to coastal Wales to visit her estranged husband James (Sean Bean). But the idyllic dream soon turns to a hellish nightmare when, during an innocent trip to the beach, Sarah is swept under the waves - her body un-recovered. While a frantic James desperately searches for Sarah, Adelle is haunted by mysterious visions and clues - hinting that Sarah is trapped somewhere in the house.
What's more, Adelle learns of a horrific, wicked event that happened at this site: decades ago a pagan religious colony, under the instruction of their leader, jumped to their deaths over a nearby cliff, and only one survived - Daffyd the local farmer and handyman. Through Daffyd, Adelle learns of a mythical underworld, "The Dark", an ethereal land of the dead, where one of the dead can be exchanged for one of the living.
When Adelle finds a traumatised girl Ebrill (Abigail Stone) in Sarah's bed, she soon makes the deadly connection between Sarah, Ebrill and "The Dark": how far will Adelle go to get Sarah back?