After suffering a traumatic brain injury from a car accident, Mary Dunne is convinced that her husband, Grayson, has been replaced by an impostor. She receives phone calls from an unknown caller who claims to be the “real Grayson,” causing Mary to suspect that she’s living in a parallel universe with impostors who are gaslighting her and keeping her against her will. She’s also haunted by a gruesome presence that visits her frequently, making her question her own sanity. The “real Grayson” tries to convince Mary to move on with her life and accept her new reality, but Mary can’t let go of the feeling that she’s not safe. Tensions reach a breaking point when Mary finds out she is pregnant with the supposed impostor’s baby.