The short film dealt with a woman who noticed a dark silhouette in the shadows whenever she turned off the light, but when the light is turned back on the figure vanishes. The movie opens with the same basic concept as we see a woman (Lotta Losten who played the part in the original short) working in the gloomy hallways of a textile warehouse, when the motion activated lights turn off she spots the silhouette of a woman with monstrously long fingers, but upon waving her arms and the lights coming back on the strange woman is gone. This goes on for a bit as this mysterious being hounds the poor woman, who vainly tries warning her boss Paul (Billy Burke), but he’s to wrapped up in his own problems to take her seriously.
Needless to say it doesn’t end well for him.We are then introduced to Paul's stepdaughter Rebecca (Teresa Palmer) who has commitment issues, refusing to acknowledge that her lover Bret (Alexander DiPersia), who she has been exclusively seeing and sleeping with for eight months, is her boyfriend and just someone she has sex with on a regular basis. She is alerted by her half-brother Martin’s (Gabriel Bateman) school that the kid has been falling asleep in class, and that her mother has not been answering the phone. Turns out Rebecca's mother Sophie (Maria Bello) suffers from depression, she even spent time in a mental institution as a child, and the recent death of her husband Paul has apparently sent her down the rabbit hole again. Rebecca takes Bret to her place, an apartment above a tattoo parlor, and that night she to encounters a strange figure that only becomes visible in the dark.
The nice thing about Lights Out is that it doesn’t waste time with our lead characters trying to convince other people that there is a ghost haunting them, having been attacked in her room Rebecca immediately believes Martin that this ghost, who he calls Diana, has been living with him and his mom for some time now. That the mysterious Diana is the reason for Rebecca’s father disappearing, and her running off to live on her own, just adds credence to what the kid tells her. The ghost also having carved her name into Rebecca’s wooden floor, causing a flash back where she remember the nasty entity ruining a picture she drew as a child, kind of seals the deal.