The film begins with Sophie Wynsfield being jolted awake by her mother, Helena, who is screaming in horror after apparently committing murder. Kate Fuller, played by Olga Kurylenko, is a criminal psychologist who is consulted by Detective McCarthy to determine if Helena should be institutionalized following this traumatic incident. During her interviews with both Helena and Sophie, they both claim that the tragic fate of Mr. Wynsfield is the work of a sleep demon named Mara. Consequently, Kate secures Helena's commitment into a mental health facility.
As the story unfolds, Kate herself suffers from sleep paralysis and encounters a mysterious figure stalking her in her own apartment. The next day, she seeks out a man named Takahashi, only to discover his decayed body, reminiscent of Mr. Wynsfield's fate, which leaves her unnerved. Skeptical about the legend of Mara, McCarthy remains unconvinced of its authenticity. A visit to Helena in the facility reveals crucial information; Helena describes how she and her husband were connected to a man named Dougie through a support group for sleep paralysis victims. Desperately pleading to see her daughter, Helena warns Kate that Sophie too may fall victim to Mara, revealing a sinister mark in her left eye that indicates her impending doom.
Motivated by a growing sense of urgency, Kate attends a Sleep Paralysis support group led by Dr. Ellis, where she encounters Dougie, who is vehement about the reality of Mara and warns that she intends to kill Saul, another member of the group. When Saul's eyes are uncovered, revealing a spectral red hue, things take a darker turn, and his life ends tragically that same night in a horrifying plunge into self-immolation. Kate, afflicted by her own experiences of sleep paralysis, struggles with the manifestations of fear.
When Kate attempts to have Helena released, she finds to her horror that Helena has died under mysterious circumstances, her eyes unnaturally red. As the film progresses, Kate's encounters with Mara intensify, culminating in chilling visions and physical confrontations. Presenting a grim history, Dougie shares the harrowing legacy of Mara as he acknowledges the pattern of tragedy that signals her return. He recounts the distinct stages of Mara’s manifestation—paralysis, marking, contact, and finally waking death if one succumbs to sleep once more.
Skeptical of the supernatural, Dr. Ellis attributes these experiences to cultural phenomena surrounding sleep paralysis. However, supernatural occurrences spiral further out of control as Dougie's alarms fail, closing him in on the terror of Mara's approach. In a desperate attempt to keep his sleep at bay, Dougie reaches the brink of insanity, only for Mara to claim him as her next victim.
With a new urgency, Kate races against time to save Sophie, who spirals dangerously close to Mara's grip. Realizing the connection among the victims—the weight of guilt—they all share—Kate acknowledges the depth of their respective tragedies. As she nears Sophie's side in the hospital, she fights against her own guilt, ultimately experiencing a disheartening reunion with Helena’s accusatory spirit.
The film builds toward a climax as Kate's struggle with guilt manifests in tangible threats, leading her to a final confrontation with Mara, suspended in the air between consciousness and the nightmare reality that awaits, leaving viewers with a haunting sense of unresolved dread.
