On a windswept stretch of the Welsh coast, Adèlle [Maria Bello] visits her husband James [Sean Bean], and together they try to mend a strained bond with their daughter Sarah [Sophie Stuckey]. The family’s quiet reunion is interrupted by a rugged cliffside memorial that looks incomplete—one plate is missing and a name, Annwyn, is etched into the stone, a term locals associate with an afterlife. An old local man named Dafydd [Maurice Roëves] explains the belief that Annwyn marks a border to another realm where the living and the dead might brush against each other, a notion that casts a shadow over the frail family dynamic.
When Sarah disappears from the beach, a startling semblance of her reappears in the form of Ebrill [Abigail Stone], the long-dead daughter of a local shepherd who once served as the town’s pastor fifty years earlier. Ebrill’s history is chillingly entwined with the town’s lore: as a sickly child, she died and was cast to the sea, from which her father—along with his followers—sought to draw her back through offerings to the ocean, claiming it would lead to Paradise. While Dafydd was among those who refused to leap, his family did; and when Ebrill’s father was unable to bear witness to the harm done to his daughter, Dafydd intervened by freeing Ebrill, an act that unleashed a darker force that eventually pushed her father to his doom at the cliff.
Adèlle begins to see a dangerous echo of the past in Sarah, sensing that Ebrill has returned in the girl she loves most. In a desperate bid to save her daughter, Adèlle and Ebrill push toward the edge of the cliff and tumble into Annwyn, a sepia-toned, mist-filled mirror of reality. Inside this other realm, it’s revealed that Sarah had attempted suicide after a heated argument with her mother prior to their trip, a truth that reframes the family’s entire journey. Adèlle pleads for a second chance, but Ebrill coldly warns that the dead do not grant second chances.
Within Annwyn, Ebrill and her father enact a disturbing ritual—trepanation on Adèlle—in an attempt to extract the evil they believe has taken root in her. Adèlle fights to free herself, races through the dreamlike corridors, and finally reaches a door where Sarah is held. Finding a key, she frees her daughter and offers a heartfelt apology for her selfishness. The moment she unlocks the door, she brings Sarah back to the living world, yet the returned girl bears the taint of the same evil that haunted Ebrill all those years. In the end, Adèlle pays the ultimate price, sacrificing herself to rescue Sarah, only to confront the painful truth that the danger lingers within the daughter she hoped to save.
