Dr. Alice Arnolds, Maggie Q, is a renowned sleep therapist and neurologist, whose calm, clinical exterior hides a traumatic memory from childhood: her younger brother died under mysterious circumstances while sleepwalking, found at the base of the stairs with his eyes wide open in terror. Though his death was ruled an accident, she has never fully accepted that explanation, and the fear has lingered like a shadow she can neither shake nor explain.
Now an adult, she works at the Whittingham Sleep Clinic, where she conducts overnight studies on patients suffering from parasomnias. She remains fiercely analytical, skeptical of anything that isn’t grounded in science, and she prides herself on maintaining professional detachment even when the cases brush up against the unnerving boundary of the unknown.
She is introduced to the Morgan family—Charlie Morgan, Sam Troughton, the father; Sarah Morgan, Kristen Bush, the mother; Emily Morgan, Honor Kneafsey, the teenage daughter; and Daniel Morgan, Lucas Bond, the younger son. Daniel has been plagued by night-time terrors: he wakes with screams, thrashes in his bed, and insists that something in his room at night is real and dangerous. The fear tightens its grip, and the family seeks relief and answers through the clinic’s monitoring.
During an overnight observation, Alice tracks Daniel’s vitals and brainwave patterns while the family watches with a mix of hope and dread. But as the night deepens, Charlie experiences a violent episode of his own: he sinks into a suffocating sleep, his breathing quickens, he convulses, and then, with his eyes still shut, he lunges and attacks Alice, leaving the room filled with tension and fear. It takes several staff members to restrain him, and when he’s sedated, his EEG appears flat, as if he no longer inhabits his own body. The incident unsettles Alice, who begins to suspect night terrors or sleepwalking, yet the circumstances feel increasingly disturbing and inexplicable.
Over the next twenty-four hours, Daniel’s condition deteriorates. He becomes paler, bruises appear around his chest and neck without any obvious injury, and he speaks more insistently of a “dark thing” that now also stalks him by day. Emily begins to see it too—a tall, skeletal figure with hollow eye sockets that lingers in the corners of rooms, vanishing whenever she approaches. Sarah reports that Charlie has grown withdrawn and erratic at night, mumbling in his sleep and waking in different parts of the house with bloody fingernails, as though he’s clawed at something unseen. The strain tears through the family, and Alice’s clinical calm starts to crack under the weight of these escalating phenomena.
Meanwhile, Alice herself begins to experience unsettling intrusions. She has vivid waking nightmares or perhaps lucid glimpses—she lies down to rest and suddenly finds herself paralyzed, a monstrous figure climbing onto her chest. Its face is contorted, its mouth filled with jagged, rotting teeth, and it hisses in an unknown language. When she comes to, she notices scratch marks on her body, a haunting reminder that the boundary between sleep and waking is leaking into her own life.
Desperate for answers, Alice seeks out Cam, the clinic’s janitor, who points her toward his uncle Amado, a once-renowned sleep expert who now lives off the grid after a mental breakdown. Cam, Vincent Andriano, introduces Amado, Sylvester McCoy, who speaks of an ancient sleep demon called the Nocnitsa, the Night Hag. This parasitic entity feeds on human fear, slips into the subconscious during REM sleep, and can bring about hallucinations, paralysis, and even death. It particularly targets the vulnerable—children and those bearing emotional trauma—and once inside a person’s dreams, it drains life and can possess its victims, turning them into carriers who spread the darkness through shared trauma.
Armed with this folklore, Alice returns to the clinic and persuades Sarah to help induce a deep REM sleep in Daniel while she engages in a controversial, experimental technique to enter a lucid dream state herself. The plan is to confront the demon inside Daniel’s dream by stepping into his nightmare alongside him. They perform the induced sleep protocol, and Alice, entering the shared dream, finds herself in a decaying replica of the Morgan house. Daniel is trapped in a nightmarish version of his room, where rot-streaked walls and mangled toys pulse with a malignant energy. The Nocnitsa appears—a towering, eyeless shadow with limbs that twist like tendrils, its voice a chilling blend of Daniel’s tones and the memory of her own dead brother.
Within the dream, Alice fights the creature while reliving visions of her brother’s death, each memory intensified by the Nocnitsa’s taunts. The entity whispers that she let him die, and it seems determined to consume her as she’s pulled toward an abyss. In a pivotal breakthrough, she accepts a painful truth: her brother’s death was not her fault, but rather the Nocnitsa’s design, something that latched onto her since childhood. This emotional reckoning becomes her weapon, lending her a strength she didn’t know she possessed.
Guided by this newfound resolve, she coordinates a risky move in the real world: at the precise moment, she administers a stimulant to Daniel, intending to punch through the nightmare and sever the demon’s hold. The shared dream erupts in a violent rupture—windows shatter, monitors explode, and Alice is hurled across the room. Daniel awakens, gasping for air, his bruises fading as if the nightmare itself was made of shadow rather than matter. The immediate danger passes for now, the family breathing a cautious sigh of relief, grateful to be together and intact.
Yet the final scene leaves a lingering chill. Exhausted and alone in her apartment, Alice begins to drift toward sleep once more. The camera closes in as the room grows quiet, and the Nocnitsa’s growl returns—the unmistakable sound of the Night Hag. Her body freezes, her eyes snap open, and the sense that the shadow has merely paused rather than left lingers, hinting that the battle is far from over and that the demon may return when sleep finally finds her again.
