Rebecca, a young woman living independently, learns that her younger brother Martin is experiencing the same terrifying events she once endured: a malevolent spirit that only appears in the dark. Their mother, Sophie, has a history of mental illness and seems to communicate with this supernatural entity, which is tied to her past. The spirit, linked to Sophie’s childhood friend Diana, manifests itself when the lights go out, causing Rebecca to explore the connection between her mother’s troubled past and Diana’s menacing presence.

As Rebecca delves deeper, she finds clues that suggest Diana's influence over Sophie has dangerous ramifications for their family. The encounters with the spirit escalate, leading Rebecca to take matters into her own hands in an attempt to protect Martin and discover the truth behind Diana’s connection to their mother. Tensions rise as Rebecca and Martin fight for survival, navigating the harrowing ordeal that has brought their family into the grip of darkness, with every flicker of light becoming a matter of life and death.