Troubled by frequent episodes of sleep paralysis, Beth Bowman finds herself haunted by a demonic hag during these harrowing experiences. Initially, she feels agitated when her boyfriend, Evan, invites her estranged twin sister, Kate, to a surprise birthday party. Despite the initial tension, the occasion leads to a brief moment of connection through dancing. However, the mood darkens when Beth, prompted by her friend Linda Noble, awkwardly mentions her sleep paralysis, causing her to leave the party prematurely.
The following day, seeking comfort, Beth visits Kate and explains the terrifying sensation of being strangled in her sleep. In an attempt to understand her affliction better, the sisters consult sleep disorder specialist Dr. Sykes, who reassures them that sleep paralysis is typically harmless. That night, however, Kate experiences a vivid nightmare in which she observes her sister being tormented while paralyzed, which compels her to alert their parents, Dave and Claire. Tragically, upon discovering Beth in her bedroom, Mr. Bowman realizes that she has died under mysterious circumstances.
At Beth's funeral, Hassan Davies, another sleep disorder physician, introduces himself to Kate, sparking further inquiries into her sister's condition. Upon moving back in with her parents, Kate turns to Evan for answers about Beth’s troubling condition, leading them both back to Dr. Sykes, who dismisses Hassan’s theories about a demonic presence as unscientific. Yet, as Kate digs deeper, she learns that Dr. Sykes is also treating Linda, which raises her suspicions.
Feeling increasingly threatened, Kate reaches out to Hassan, who elaborates on the historical implications of sleep paralysis and warns her about the dangers she now faces after having witnessed the malevolent hag herself. The tension escalates when Kate almost drowns during an episode of sleep paralysis while in the bathtub. Seeking clarity, she reaches out to Linda to find out if she shares similar terrifying experiences. However, Linda remains evasive about her nightmares, intensifying Kate's worry.
Desiring to share her troubling experiences with Evan, Kate confides in him, and the pair conduct online research into their doctor, Hassan Davies. Their exhaustion catches up to them as they fall asleep on Evan’s couch, but Evan experiences a nightmare where Kate turns into the very hag that haunts her, and he wakes without revealing this frightening encounter to her. Meanwhile, Kate’s family expresses their concern, emphasizing their desire to keep her safe.
Eventually, Linda opens up to Kate about her own terrifying sleep paralysis incidents. Together, they return to Dr. Sykes to ask more about the “sitting ghost” that plagues them, but once again, he rebuffs their inquiries, further distancing them from his guidance. The urgency heightens when Kate visits Evan again and discovers him creating a disturbing painting of the demonic hag he encountered in his dreams. Their investigation leads them to John Pang, an Austin resident who has documented his experiences with sleep paralysis in a series of video diaries. However, they soon find that his insights are abruptly cut off as he suggests he may have developed a way to combat the sleep demon, but his current whereabouts are unknown.
Meanwhile, Linda’s fears regarding her husband, Daryl, manifest in a nightmare that leads to her tragic demise. Kate finds herself at Linda’s home the following morning, where Dr. Sykes inadvertently implies that she may carry some blame for Linda’s fate due to her encouragement of supernatural beliefs. Kate and Evan then set out on a journey to Austin to seek out John Pang, only to discover that he has descended into madness from his year-long battle against the sleep demon.
In a race against time, they return to Hassan, who divulges his struggle with sleep paralysis, having remained awake for months. He theorizes that the demon exists on the threshold between dreams and reality and proposes an adrenaline shot to weaken its grip on a victim. During a tense night, Hassan and Evan prepare to monitor Kate as she sleeps, ready to administer the adrenaline at the first sign of danger. However, a frantic race ensues when Kate realizes she left crucial research material at her parents' house, fearing they could also fall victim if they entertain the idea of the sleep demon.
After ensuring her parents are safe, Kate finds herself back at Evan’s loft. While she takes a shower, Evan unknowingly drifts into a coma following a nightmarish encounter with the hag. With hope dwindling, Hassan reminds Kate that while they’ve weakened the demon temporarily, she must confront the hag herself to overcome the terror. Tragically, during an attempt to monitor Kate's sleep, Hassan sacrifices himself to save her and tragically dies.
In her nightmare, Kate faces a haunting vision of Beth, leading to a pivotal confrontation where she ultimately stabs Beth, who morphs into the hag. In a fierce struggle, Kate fights back, eventually succeeding in strangling the demon to its death. Later, she documents her progress through a video diary, revealing that while Evan is improving, she has begun to experience sleep paralysis again but has not yet seen the hag. Resolute, she pledges to continue Hassan’s research and issues a warning to her viewers about the dangers of believing in the demon.
In an unexpected twist, Dr. Sykes’ unsettling fate unfolds when she experiences sleep paralysis in her own apartment, facing the terrifying night hag herself.
