Grace Farlo, an enigmatic ophthalmologist plagued by fragmented memories and troubling visions, receives a life-altering phone call from DCI Harris in Scotland. It is revealed that her brother Michael, a priest stationed at the isolated Mount Saviour Convent, has been discovered dead under suspicious circumstances, suggestively linked to a murder-suicide.
Driven by a desire to uncover the truth, Grace travels to the convent, where DCI Harris divulges the disturbing details of Michael’s demise; he is thought to have killed another priest, Father Carol, before taking a fatal plunge from the cliffs nearby. Harris paints a chilling picture of the convent, historically known for its battle against demonology and notorious for Sister Beth's grisly act of self-mutilation after claiming to have witnessed the devil himself. In a harrowing encounter, the convent's Mother Superior echoes this sentiment, attributing Father Carol’s death to demonic forces, while insisting that Michael’s tragic end was similarly orchestrated.
During her examination of Michael's corpse in the morgue, Grace experiences a fainting spell as she comes face to face with visions of her brother's ghost. When she regains consciousness in a hospital room, she briefly meets Father Romero, who has been dispatched by the Vatican to re-consecrate the grieving convent.
As Grace reveals to Father Romero the haunting backstory of her family—her father's imprisonment for the murder of her mother—she finds her nights plagued by vivid dreams of knights on horseback disrupting a dark occult gathering. These dreams escalate, depicting nuns willing to leap from the very cliffs that claimed Michael, thrusting Grace into an unsettling reality where she awakens clothed in eerie white robes within the convent.
Determined to confront the Mother Superior regarding the concealed truths of her brother's death, Grace is met with ambiguous responses about the nature of her disturbing visions. Mother Superior claims these visions could very well be fragments from both Grace's past and her yet-to-be-written future. During this turbulent journey, Father Romero enlightens Grace about the convent's ancient origins, tracing back to the 12th century when the Knights of the Morning Star established it to shield sacred relics. He presents her with a box of Michael's possessions, including a cryptic journal revealing Michael's belief that Grace's visions render her "special."
In a sequence of unsettling flashbacks, Grace recalls the traumatic childhood she shared with Michael, locked away in cages by their father. The family’s tragedy peaks when her father, in a fit of rage, ends her mother’s life while a preternatural force intervenes, sparing Grace from a similar fate. This pain is exacerbated when Grace learns of the convent's efforts to adopt them, leading to a traumatic encounter with Father Jacobs and Sister Dianne that ended in violence when something unseen caused their car to crash.
Grace's nightmare deepens as visions of her brother's torment at the hands of Father Carol surface, culminating in a shocking moment when Sister Matilda, driven by madness, confronts Grace with a knife only to be led to self-destruction by that same unseen force. Subsequently, DCI Harris takes both Mother Superior and Grace into custody for further inquiry about the string of bizarre deaths plaguing the convent. Grace, desperate to unveil the conspiracy, explains to Harris the convent's sinister exploitation of her brother's fate.
However, a web of deceit unfolds as Father Romero and the nuns concoct a narrative that forces Harris to release her, while Mother Superior chillingly states that Grace herself is the true relic they seek. In a haunting visit to her imprisoned father, he reveals that he took Grace in at the age of 10, but soon became convinced she was cursed, suggesting that she had conjured evil the night he killed her mother.
Returning to the convent, Grace finds Sister Beth poised to end her own life, only to be thwarted by an invisible force that compels her demise. Shocking visions lead Grace to a poignant discovery involving Sister Meg, culminating in the latter's tragic death mirroring Grace's ominous premonitions.
In the chapel, Grace encounters Father Romero and the nuns engaged in a ritual. Feeling ensnared by a heavy darkness, Grace desperately seeks Romero's aid to cleanse her soul. However, when he attempts to seal her within the subterranean crypt, the same malignant force takes his life, revealing in his final breaths that Grace is a fallen angel whose existence poses a profound threat to the divine.
As DCI Harris races to the convent, he glimpses Grace poised to replicate Michael’s tragic fate, yielding to the supernatural pull of her guardian angel—an unseen force responsible for a history of devastating deaths around her. Gripped by a blend of fear and intrigue, DCI Harris presses Mother Superior for the truth surrounding the events that transpired. Caught in a web of fate, it becomes clear that until she is subdued, Grace's latent powers may just bring about her own perilous end.
Now back in the mundane world, Grace shares lunch with her colleague, Dr. John Holmes, who reveals an astonishing miracle: an elderly patient, Valerie To, has inexplicably regained her sight after Grace referred her for treatment. A haunting flashback surfaces, depicting Grace’s doppelganger as the guiding force behind Valerie's miraculous recovery. Suddenly, Mother Superior confronts Grace on the street, weapon drawn, but before she can pull the trigger, an oncoming vehicle collides with her, setting the stage for a reckoning Grace never saw coming as she reflects on the powerful, enigmatic presence that has watched over her all her life.
