While in Wales visiting her husband James (Sean Bean), Adèlle (Maria Bello) tries to fix her relationship with her daughter Sarah (Sophie Stuckey). By the side of a cliff, they see a strange memorial with evidence of a plate missing and with the name "Annwyn" marked on it. A local man Dafydd (Maurice Roëves) explains that, according to traditional Welsh mythology, Annwyn is a sort of afterlife.
Later, Sarah vanishes on the beach, and another similar looking girl, named Ebrill (Welsh for "April"), appears in her place. Ebrill is the long-dead daughter of a local shepherd who also served as the town's pastor fifty years prior. When Ebrill, who was a sickly child, died, her father gave her to the ocean, sending her to Annwyn. He then convinced his followers to throw themselves into the ocean, claiming that it was the way to Paradise, while he privately hoped that their sacrifice would return Ebrill to him from Annwyn. Ebrill did come back, but, something came back with her. Her father tried to draw the evil out of her, through trepanning and locking her in her room. Dafydd was one of the followers who did not throw himself off the cliff, though both his parents did. Ebrill's father took him in, and when Dafydd could no longer bear witnessing the shepherd hurting Ebrill, he set her free, which in turn allowed the evil within her to lash out and shove her father over the cliff.
Adèlle makes the connection that Ebrill is back once more because she has found a living substitute in Sarah. In an attempt to rescue her daughter, Adèlle throws both herself and Ebrill over the cliffs, despite James' protests, and sends them both to Annwyn, a sepia-toned, misty version of reality. While in Annwyn, the film reveals that (prior to the events of the film) Sarah attempted suicide following an argument with her mother, resulting in their trip to Wales. Adèlle begs for a second chance with her daughter. Ebrill informs her that the dead don't get second chances. Ebrill and her father perform trepanation on Adèlle, to draw out the evil within her. Adèlle eventually escapes her bonds and rushes to find Sarah, who is locked behind a door. Adèlle finds a key and tearfully apologizes for being so selfish. In unlocking the door, Adèlle is able to rescue Sarah from Annwyn, though, in doing so, Adèlle sacrificed herself, only to realize too late that the Sarah she brought back was tainted by the same evil that had tainted Ebrill all those years ago.
