Cecilia Kass is trapped in a violent, controlling relationship with wealthy optics engineer and businessman Adrian Griffin. One night, Cecilia drugs Adrian with diazepam and escapes his highly secured home with the bungling assistance of her sister, Emily.
Cecilia hides out with acquaintance Detective James Lanier and his teenage daughter, Sydney. Two weeks after Cecilia’s escape, Adrian seemingly commits suicide and leaves her $5 million in his trust. His lawyer brother, Tom, handles the arrangements. Cecilia suspects another presence in the house after several strange events and calls James, but he assures her she is just traumatized. During a job interview, she finds her work portfolio’s contents removed, then faints. The doctor says high levels of diazepam were found in her system. In her bathroom, she finds the same bottle of diazepam with which she drugged Adrian, and dropped, during her escape.
Cecilia, accompanied by James, arranges a meeting with Tom. She believes that Adrian faked his death and used his optics expertise to become invisible in order to torment her, but this idea is rebuffed. Later, Sydney is hit by an unseen force, and she and James assume Cecilia did it. Alone at the house, Cecilia tries various tactics to catch the figure. After finding Adrian’s old phone and dumping paint on the figure, a violent struggle ensues before she escapes. She goes to Adrian’s home to investigate his lab. There, she finds an optical bodysuit, confirming her suspicions. Right after hiding the suit in the home, the invisible figure attacks again, so she flees and contacts Emily. The pair meet in a restaurant, but the invisible figure slits Emily’s throat and places the knife in Cecilia’s hand, framing her for murder.
While awaiting trial, Cecilia is remanded to a mental hospital, where she learns she is pregnant. Tom offers to get her charges dropped if she agrees to “return to him” and raise the child, implying that Tom helped stage his brother’s fake suicide. He reveals Adrian tampered with her birth control to impregnate her. Cecilia refuses the offer and steals a pen from his briefcase. That night, she attempts to commit suicide. When the invisible figure tries to stop her, she stabs him repeatedly with the pen, causing the suit to malfunction. The security team arrives, but the figure incapacitates them before fleeing the hospital, with Cecilia in pursuit. To protect her unborn child, the figure instead threatens to attack those she loves.
Cecilia races to James’ house and finds the figure attacking him and Sydney. She shoots the figure to death, but when unmasked, she finds Tom in the suit. Police storm Adrian’s house and find him alive, tied up and claiming Tom held him prisoner. Cecilia insists the brothers shared the suit, with Adrian sending Tom to the house knowing he would be killed.
To get Adrian’s confession, she meets him at his house while James listens on a wire. She agrees to mend their relationship, but only if he confesses to being the figure. Adrian insists that Tom did kidnap him and claims the experience changed his outlook on life and how he treated her, but when she cries, Adrian alludes to his abuse with similar phrasing to that of the figure. Satisfied, Cecilia departs to use the restroom. Moments later, the security camera captures Adrian seemingly slitting his own throat. Cecilia returns and, apparently distraught, calls the police; out of the camera’s sight, however, she taunts a dying Adrian, revealing that she had used the second spare bodysuit to kill him.
When James arrives, she confirms what the camera captured. He notices her carrying the suit, but allows her to leave. Relieved to be free of Adrian, Cecilia leaves the house, taking the suit with her.