The movie starts with a man running away, escaping from something. He then runs into a locker room, where all of the lockers open to show his reflection. To his horror, the room's largest mirror begins to crack as he approaches it; he desperately apologizes to his reflection therein for running away in hopes of forgiveness, and begins fervently cleaning the mirrors. A piece of the mirror then falls off. Trying to redeem himself, he picks up the shard with the intention of putting it back on the mirror. However, his reflection slowly slashes his own throat, and the man dies, experiencing the effects of his throat being slashed despite not doing it himself.
Meanwhile, sometime later, Ben Carson (Kiefer Sutherland), a former undercover detective who was suspended after shooting another officer, is forced to take a night time security job at a department store that was gutted by a fire. The department store used to be a psychiatric hospital that experimented in treating schizophrenia. In 1952, a mass killing took place and the hospital was closed. It was later reopened as a luxury department store, the Mayflower. Carson comments on the pristine mirrors throughout the store, and the guard comments that the man Carson is replacing was obsessed with cleaning them.
Carson's round begins normally, though on his first night he sees a door open in a mirror's reflection while it is actually closed. After investigating, he finds nothing. Following nights expose Carson to more intense visions, which he initially shrugs off as hallucinations (due to a strong drug he started taking as a result of his alcohol dependency). He notices mysterious handprints on the mirror. Curious, he touches them, causing the mirror to break and injure his hand. Carson further hallucinates being set on fire, as well as seeing victims in various parts of the store who were burned to death. Afterwards, he finds the wallet of Gary Lewis (the man who dies at the beginning of the film), the night watchman he is replacing. The only piece of information is a note that says "Esseker." Ben then receives a package from Gary Lewis that was sent several days before his death. The package contains newspaper clippings about the fire and other crimes. The man convicted of burning the Mayflower was also convicted of killing his wife and children.
Carson tells his younger sister (whose apartment he is staying at) about what he sees, believing that it is not him looking into the mirrors, but rather, someone is looking at him. However, his sister is unconvinced. Later, he decides to see the body of Gary Lewis at the morgue (where his wife works), and she reluctantly agrees to let him see Gary Lewis and the photos of his death. He notices in the photo of Gary Lewis' body, that in the reflection of the mirror, the glass shard is bloody, but the one in reality is not. This convinces him that the mirrors are actually making people do things to themselves that they aren't actually doing.
Meanwhile, Angela (Amy Smart), Ben Carson's sister, is getting in the bath and begins to relax, when her reflection grips its jaw and begins to slowly tear it off, killing her. After arriving on the scene the next morning to see his sister's body, he returns to the store and attempts to destroy the mirrors, but they prove impervious and even regenerate from several bullets he fires at them. He demands to know what the mirrors want, and ESSEKER is written on the mirror. Ben investigates and finds the name Anna Esseker, a patient of the psychiatric institute. She supposedly died in the mass killing, but Ben discovers that she was actually transferred out two days prior to the event. After Angela's death, he realizes that the mirrors will eventually kill his family if he does not bring Anna Esseker to them. He goes to his wife's home and attempts to remove or paint the surface of every mirror in the house, but his estranged wife Amy (Paula Patton) believes that he is undergoing a breakdown. He tries to show Amy what happens when he tries to break the mirrors by shooting them, but nothing happens and she does not believe him. He looks in the window of his house and sees his children looking at him. Ben then leaves in frustration and embarrassment. However, Amy starts to believe Ben when she discovers her son Michael talking to his reflection. After calling him away, Michaels reflection remains, smiling at her. Amy calls Ben in a panic, who immediately returns home, and together they cover every reflective surface in the house.
Shortly after, Ben discovers Anna Esseker's home, but she is not there. The owner of the house says that when Anna was here strange things would happen with the mirrors, so they tried to get her help. The doctor's treatment was to lock Anna in a room of mirrors. Whatever was in her, left her and "entered" the mirrors. After that, Esseker was sent to a convent, where mirrors are not permitted. After finding Anna, she explains to Ben that she was possessed by a demon, and while in the hospital she was confined to a chair in a room which was surrounded by mirrors, as the doctors' believed this treatment would cure her schizophrenia by forcing her to confront her own reflection. In reality, the demon within her was drawn from her and became trapped in the mirrors.
Ben begs Anna to come back to the department store so that the demon will leave his family alone, though Anna refuses. Meanwhile, Ben's family is attacked by the mirrors. Because he feels the demon in the mirrors is a friend, Michael cleans the paint from all reflective surfaces with a butcher knife and turns all the faucets on, covering the floor with a thin, reflective layer of water. Amy is almost drowned by Michael's reflection in the bathtub, but she saves herself by pulling the drain plug. Meanwhile, a reflection of Amy almost slashes her daughter's throat but the real Amy manages to save her. Amy calls Ben for help, and he abducts Anna at gunpoint. Anna returns to the mirror room and tells Ben to strap her tight, then leave immediately. Amy finds Michael playing in the water. Michael is pulled through the surface of the water by his own reflection and is trapped on the other side, underwater.
As Anna opens her eyes and becomes repossessed, the mirrors explode. Ben returns to the mirror room and discovers that the demon is now crawling on the ceiling and walls and has superhuman strength. Ben shoots at it but fails to wound the demon. He then impales it on a broken steam pipe that ignites a nearby gas line. He tries to escape the crumbling building but the demon attacks again and he is forced to fight it off. The building gives away and various rubble comes down on the demon, killing it, and supposedly Ben as well.
At the family's home, Amy claws at the surface of the water in an attempt to rescue her drowning son. Suddenly, Michael is released from the other side of the reflection and Amy is able to pull him to safety and revive him with CPR.
Ben finally pulls himself out of the rubble and stumbles his way out of the building. Police and firemen are everywhere in the street, and a body is seen taken in a bag by paramedics, but nobody stops Ben as he leaves. He realizes that something is different because the name on a badge is written in reverse (like a mirror), so is the word "Police" on a car; his hand wound from earlier in the movie is on the opposite hand. Ben then realizes that he was crushed to death under the rubble and is now trapped in the mirror world. Fleeing the scene, he happens upon a mirror in town, and he can not see his reflection. He places his hand on it. The scene then changes to the real world and the audience sees Ben's hand print on the mirror. The camera pans and credits roll.
