Cecilia Kass is trapped in a violent and controlling relationship with wealthy optics engineer and businessman Adrian Griffin. One night, Cecilia drugs Adrian with her diazepam and escapes his highly secured house with the assistance of her younger sister, Emily.

Cecilia hides out in the home of her childhood friend, Detective James Lanier and his teenage daughter, Sydney. Two weeks after Cecilia's escape, Adrian seemingly commits suicide and leaves her $5 million. His lawyer brother, Tom, handles the arrangements. Cecilia suspects another presence in the house after several strange events, but James assures her she is just traumatized and paranoid. 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 bloodied diazepam with which she drugged Adrian during her escape.

Cecilia, accompanied by James, meets 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 while comforting Cecilia; assuming that Cecilia did it and is becoming unbalanced, she and James leave. Alone at the house, Cecilia tries various tactics to catch the figure she believes hit Sydney. She finds Adrian's phone in the attic, on which she immediately receives a text saying "surprise". Cecilia dumps paint down the attic trap door and it coats a previously invisible figure. A violent struggle ensues but she escapes. She goes to Adrian's home to investigate his lab, where she finds an additional invisible bodysuit, confirming her suspicions. Right after she hides the suit in their former bedroom's closet, the invisible figure attacks again, so she flees and contacts Emily. The pair meet at a restaurant, where 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 psychiatric 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 suicide. He reveals Adrian tampered with her birth control to impregnate her. Cecilia refuses the offer and steals a fountain pen from him. That night she fakes attempting suicide to lure out the invisible figure. When the 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's house and finds the figure attacking him and Sydney. She shoots the figure to death but finds Tom in the suit. Police storm Adrian's house and find him alive, tied up and claiming that Tom held him prisoner. They conclude that Tom killed Emily as well, but Cecilia believes that Adrian set his brother up as the scapegoat for his crimes.