The film takes place in two different times: the present and 11 years earlier. The two plotlines are told in parallel through flashbacks.
Eleven years ago, computer developer Alan Russell moves into a new house with his wife Marie, 10-year-old son Tim, and 13-year-old daughter Kaylie. Alan purchases an antique mirror to decorate his office. The mirror induces hallucinations in both adults; Marie is haunted by visions of her own body putrefying and decaying while she is still alive, while Alan is seduced by a ghostly woman named Marisol, who has mirrors in place of eyes.
Over time, the parents become detached and psychotic, with Alan isolating himself in his office and Marie becoming withdrawn and paranoid; during the same period, all of the plants in the house die and the family dog disappears after being locked in the office with the mirror. After Kaylie witnesses Alan interacting with Marisol and tells her mother, Marie becomes feral and, after a period of starving herself, attempts to murder the children. Alan overpowers her and chains her to their bedroom wall.
Alan remains isolated in his office for an indeterminate period of time; when the family runs out of food, the children attempt to seek help from their neighbors, who disbelieve their stories. Attempting to contact doctors or the authorities, Kaylie discovers that all of her phone calls are answered by the same man, who admonishes her to speak with her father.
One night, Alan unchains Marie, and both parents attack the children. Marie briefly comes to her senses, only to be shot dead by Alan. Alan corners the children in his office, but also experiences a moment of lucidity, during which he forces Tim to shoot him to death. The police arrive and take Tim into custody; before the siblings are separated, they promise to reunite as adults and destroy the mirror.
Eleven years later, Tim is discharged from a psychiatric hospital, having come to believe that there were no supernatural events involved in his parents' deaths. Kaylie, meanwhile, has spent most of her young adulthood researching the history of the mirror, obsessively documenting the lives and deaths of everyone who's ever owned it. Using her position as an employee of an auction house, Kaylie obtains access to the mirror and has it transported to the family home, where she places it in a room filled with surveillance cameras in an attempt to document its powers; using a "kill switch"—an anchor weighted to the ceiling and set to a timer—Kaylie intends to end the night with the mirror being destroyed, whether or not she herself survives.
Tim joins Kaylie at the house and attempts to convince his sister that she's rationalized their parents' deaths as being caused by an external force, in order to avoid facing the truth. The siblings argue for the duration of the evening until they find that cameras in the room have inexplicably moved in their absence; reviewing the video, they realize that the mirror induced them to rearrange the contents of the room without their knowledge. Tim accepts that the mirror has some diabolical power and attempts to escape the house with Kaylie, only for the pair to be repeatedly drawn back by the mirror's influence. Trying to call the police for help, they are only able to reach the same voice who spoke to them on the phone as children. Kaylie kills her fiance as she sees a ghostly figure having mirrors for eyes in place of him. The pair begin to hallucinate and experience visions of everyone killed by the mirror, who appear as ghostly figures with mirrors in place of their eyes.
As Kaylie is drawn to the mirror by an image of her mother beckoning to her, Tim experiences a hallucination of being trapped alone in the room with the mirror. He activates the kill switch, causing the anchor to descend and fatally impale Kaylie.
The police arrive and arrest a hysterical Tim, who insists that "the mirror did it." Tim is taken away in the back of a squad car as the ghosts of his parents and sister watch him from the house.