Trapped in a toxic and dangerous relationship with her sinister and violent husband, Adrian Griffin, an optics scientist named Cecilia Kass makes a daring escape in the dead of night with assistance from her sister, Emily. In a desperate act, Cecilia drugs Adrian with Diazepam, but as she attempts to flee, she inadvertently sets off the car alarm while freeing their dog, waking the furious Adrian who pursues them. In the chaos, she leaves behind the drug bottle, which Adrian discovers.
Fast forward two weeks, Cecilia is in hiding at the home of her supportive friend James Lanier and his teenage daughter, Sydney. Struggling with her fear, Cecilia becomes paranoid when Emily visits, worried that Adrian is still a threat. However, she learns that Adrian has allegedly committed suicide. Following his death, Cecilia inherits a considerable fortune and estate from Adrian's brother, Tom, primarily to help with Sydney's college tuition and James' expenses. Yet, a nagging feeling of being watched lingers, and she becomes increasingly disturbed as strange occurrences happen around her. One night, while sleeping with Sydney, she experiences an unsettling event where the comforter is mysteriously pulled off, sending chills down her spine.
In her quest for normalcy, Cecilia attends a job interview for an architect position, only to find her portfolio missing, leading to a fainting spell due to being drugged again. Her doctor confirms that she was overmedicated with Diazepam, rekindling her fears. Shortly after, a distressing email from her account leads to a confrontation with Emily, further complicating their friendship. When Sydney is attacked by an unseen force, Cecilia's credibility comes into question, leading James to take Sydney away for her safety. During her investigation of the house, Cecilia uncovers hidden evidence, including her portfolio and photos of her taken while she slept.
Convinced that she is not alone, Cecilia devises a plan. She pours paint on the attic ladder, attempting to capture the intruder, only to confront a man clad in a dark suit. After a fierce struggle, she escapes but decides to go to Adrian's house, where she uncovers the high-tech invisible suit he had developed. In a tense encounter, Cecilia is attacked by the invisible version of Adrian, but her loyal dog intervenes, allowing her to escape yet again.
Later, while having a heartfelt conversation with Emily at a restaurant, tragedy strikes when a knife is suddenly drawn across Emily's throat, and Cecilia is unjustly implicated. Confined to a sanitarium after being declared unstable and discovering she’s pregnant with Adrian’s child, Cecilia faces threats from Tom, who manipulates her situation further. In a moment of desperation, she attempts to take her own life but is thwarted by the invisible man, leading to a climactic confrontation where she must show extraordinary resilience.
With a fire extinguisher in hand, Cecilia faces off against the invisible figure, ultimately revealing his identity as Tom. After a harrowing battle, she takes him down and unmasks him. Meanwhile, James discovers Adrian imprisoned in his home. As the story unfolds, Cecilia’s relentless pursuit of justice reaches a critical point when she agrees to meet Adrian under the guise of reconciliation. It is here that her true intentions come to light, as she orchestrates his demise, gloating over his lifeless body. As the night closes, Cecilia walks away with a mix of victory and trepidation, potentially holding onto Adrian's workings to carve out her future.
