In a textile warehouse, worker Esther sees a silhouette of a figure when she turns the lights off, but sees nothing with the lights on. She warns owner Paul about the apparition, whom he ignores, and leaves. Paul is then chased and gruesomely killed by the figure.
Paul’s stepdaughter, Rebecca, lives away from her mother Sophie and brother Martin. Sophie has a documented mental illness and depression that has resurfaced, talking to an imaginary friend. One night, Martin sees Sophie talking to a figure and is horrified, gaining insomnia. Rebecca takes Martin to her apartment to protect him, against their mother’s wishes. That night, she awakens to find the figure, barely avoiding its attack just as the building’s neon sign turns on. The next morning, Rebecca finds “Diana” scrawled on the floor. She recalls Diana from her own childhood, the reason she left when her father seemingly abandoned them. Later, she breaks in and sees files about Sophie and Diana, and that she was killed when exposed to a bright light for an experiment
That night, Sophie has a movie night with Martin, but includes the figure, frightening him. She tells him a story about the figure, Diana, her friend when she was in an institution, who had an unusual condition that made her skin photosensitive. Martin sees Diana crouched above and turns on the light, escaping to Rebecca’s before Diana can attack him.
Rebecca confronts Sophie about Diana but she denies the accusations. Rebecca, her boyfriend Bret and Martin decide to stay overnight to protect her. Rebecca goes to her bedroom to reconcile; however, Sophie secretly passes her a note saying “I need help” before Diana tugs her back in. Rebecca realizes that she is being controlled and turns on all the lights in the house to keep her away.
Knowing their intentions, Diana baits Rebecca and Martin into the basement by shorting out the power. Bret is attacked but escapes and contacts the police. Rebecca realizes that using blacklight can allow them to see Diana and discovers scrawled writings along the wall, explaining how Diana won’t let anyone “take” Sophie and that she killed Rebecca’s father.
A pair of police officers arrive and free Rebecca and Martin but are killed by Diana. Rebecca sends Martin to Bret and goes back to rescue Sophie. Diana then attacks Rebecca violently, but Sophie confronts her with a gun, realizing she’s Diana’s tether to the world. She then commits suicide by shooting herself which makes Diana disappear. Distraught, Rebecca, Martin and Bret reconcile and vow to stay together.