David F. Sandberg directs this horror, based on his own short, starring Teresa Palmer, Gabriel Bateman and Maria Bello.
After her husband is killed in a mysterious accident at work, Sophie (Bello) becomes mentally unstable and disturbed and spends much of her time communicating with an imaginary friend that lurks in the darkness.
When her behaviour begins to affect her son Martin (Bateman), his older sister Rebecca (Palmer), who remembers their awful home life all too well, offers to shelter him in her apartment to keep him out of harm's way.
With the reluctant help of her boyfriend Bret (Alexander DiPersia), Rebecca tries to uncover the mystery behind the entity her mother calls Diana as she researches her mother's past in a mental institution.
But it seems Diana has now made Rebecca a target of her attacks when the lights go out...