As Dr. Cara Harding (Julianne Moore), a dedicated psychologist, faces a deeply personal and unsettling case that challenges her understanding of reality and faith. One Christmas Eve, as she and her husband walk home from church, tragedy strikes when her husband is violently attacked and killed by unknown suspects, leaving Cara devastated and searching for answers amid her grief.
In her efforts to return to normal life, her father, played by Jeffrey DeMunn, introduces Cara to Adam (Jonathan Rhys Meyers), a patient suffering from multiple personality disorder. Her father believes that Adam's condition may be linked to unexplainable spiritual phenomena, and hopes that Cara will be open to exploring these unusual psychiatric theories. Adam exhibits remarkable and sometimes terrifying transformations, taking on physically impossible characteristics that reflect his different personalities.
Initially, Cara is told about just one of Adam’s personalities, David Burnberg, a man who needs a wheelchair and was a real person, long presumed dead after being murdered. Through her investigation, Cara contacts David’s mother, a deeply religious woman, and shares her concerns about Adam. David’s mother agrees to meet Adam in hopes of helping him, but her encounter with Adam as David reveals disturbing private details about her son, leaving her horrified and accusing Adam of evil.
As the story unfolds, Adam’s personalities become more aggressive and unpredictable. Among his various personas is Wes, a Satanist singer seemingly involved in dark rituals. Cara starts to believe that Adam has created these different identities as an escape from his pain or perhaps as part of a darker, supernatural influence. She visits Adam’s home and uncovers a disturbing scene: the house is filthy, cluttered with occult objects, and contains a decomposed body in the bathtub. Cara promptly contacts the police, but her worry deepens when Adam appears with her daughter, Sammy. During a tense confrontation, Adam, in the personality of a family friend, questions Sammy about her faith in God, revealing a darker, more sinister side of his character.
Soon, Cara’s investigations lead her to a mysterious figure known only as 'Granny' in the Witch Mountains. Despite her initial dismissiveness, Cara sneaks back and witnesses horrifying acts—Granny performing ritualistic exorcisms by sucking out souls and manipulating life and death through dark magic. Granny warns Cara that if she only believes in science, she may be missing something far more profound and supernatural.
The plot thickens when the old man Granny is caring for is revealed to have been a victim of the witch doctor’s rituals, with his soul being manipulated and kept in a container marked with a strange symbol. Granny’s explanation ties into a long history of folklore, where a faithless priest who disregarded divine power and relied on dark magic was cursed to become a shelter for wicked souls after he betrayed the town’s trust during a devastating influenza outbreak. When infected townspeople sought healing through faith, the priest, secretly an atheist, inoculated his own daughters against the virus and let others die, enraging the community.
Cara’s discovery leads her to the heartbreaking realization that Adam’s true identity is connected to that cursed priest. In a chilling revelation, she learns that Adam is the faithless priest from history, yet he is also a vessel for tormented souls cast out by faith. Her fears are confirmed when she finds her father’s soul being sucked away by Adam during a devastating ritual, and she understands that Adam has been killing her loved ones to feed the spirits he shelters.
The situation becomes increasingly desperate as Sammy begins to show signs of sickness, marked by a strange symbol burned onto her back—similar to the one on Granny’s container. When Cara’s brother takes Sammy to Granny for help, chaos erupts. Adam, now in the form of her father, appears, and her brother gets unintentionally injured. Meanwhile, Adam, or the priest’s spirit, kidnaps Sammy, turning her into a vessel for the forsaken souls. In a final confrontation, Cara fights to save her daughter and herself.
In a tense climax, Adam as Sammy betrays her, revealing that he does not just suffer from DID but is actually a dwelling for the spirits of those who have lost their faith. As he impales himself on a root and dies, all the tormented souls leave his body for good. Sammy’s lifeless body begins to recover as her spirit is restored, and she is returned to Cara, who holds her daughter close, grateful for her salvation. The film leaves a haunting reminder of the dark forces linked to faith, belief, and the unseen world beyond, ending with Sammy humming a tune written by David, hinting that his soul may have found peace within her.
