Cecilia Kass finds herself ensnared in a violent and controlling relationship with the affluent optics engineer, Adrian Griffin, who is the founder and CEO of the human-computer augmentation company Cobalt. In a desperate attempt to escape, Cecilia, with the assistance of her younger sister Emily, manages to drug Adrian with diazepam and flee from his heavily secured home.
Seeking refuge, she turns to her childhood friend, James Lanier, a detective with the San Francisco Police, and his teenage daughter, Sydney. Just two weeks after her escape, shocking news strikes — Adrian appears to have committed suicide and bequeaths her $5 million. His brother, Tom, takes care of the legalities involved. However, strange occurrences begin to plague Cecilia, prompting her to believe that there is more at play within the house. Although James reassures her that her fears stem from trauma and paranoia, Cecilia's sense of impending doom escalates when she discovers evidences that suggest Adrian is not truly dead.
During a job interview, she finds her portfolio mysteriously altered, leading to a fainting episode, which later reveals alarming levels of diazepam in her system. When Cecilia uncovers the very bloodied diazepam bottle she had used on Adrian in her own bathroom, her fears intensify. Determined to uncover the truth, she meets with Tom again but is met with disbelief when she voices her suspicions that Adrian may have faked his death and might be utilizing his expertise to stalk her invisibly.
As tensions mount, an unseen assailant hits Sydney, leading to confusion and fear. Alone in Adrian’s house, Cecilia embarks on various methods to capture this invisible threat. In a thrilling search, she finds Adrian's phone in the attic, receiving a chilling text that simply reads "surprise." Testing her theory further, she dumps paint down the attic trap door, successfully revealing an invisible figure in her home. A fierce struggle ensues, but Cecilia manages to escape.
In her quest for answers, she infiltrates Adrian's lab and discovers an additional invisible bodysuit—evidence that validates her fears. Moments later, however, chaos erupts when the invisible presence assaults her again. In a desperate bid for help, she meets Emily at a restaurant, only to witness a horrific betrayal as Emily is brutally murdered with a knife placed in Cecilia’s hand, framing her as the murderer.
While awaiting trial, Cecilia finds herself confined to a psychiatric hospital where she learns she is pregnant. Tom proposes a sinister deal: he will drop the charges if she agrees to return to Adrian and raise their child, revealing that Adrian had maliciously tampered with her birth control. Horrified by his implications, she refuses and cleverly steals a fountain pen.
Undeterred, Cecilia plans to confront the invisible assailant by luring him out. In a chilling showdown, she stabs the figure, causing the invisibility suit to falter. When the security arrives, the figure escapes, only to threaten Cecilia’s loved ones instead. In a frantic race to protect them, she rushes to James's home and finds that the figure is attacking him and Sydney. In a climactic moment, she shoots the figure, only to discover that it was Tom concealed within the suit.
Following these events, police storm Adrian’s home, revealing him alive and tied up, claiming his brother held him captive. However, Cecilia remains skeptical, believing Adrian used Tom as a pawn in his manipulative game.
In a tense yet strategic meeting, Cecilia aims to extract a confession from Adrian, wearing a wire while [James](hugo-cruz) listens from afar. She feigns reconciliation, demanding he admit to Emily’s murder. Adrian, however, deflects blame onto Tom while expressing that his time away transformed his perspective on life. But upon uttering "surprise" again, Cecilia slips away and moments later, security footage captures Adrian's apparent suicide, leaving her to call 911 amidst apparent distress. Yet, in a subtle twist, she reveals her true emotions as she taunts Adrian with a lingering "surprise" before departing, knowing fully well the implications of his demise.
When [James](hugo-cruz) arrives, Cecilia reassures him that Adrian took his own life while subtly concealing evidence of her own deceptions, walking away into the night.
