A security guard is running through a subway station, he eventually enters a room he cannot escape from and starts begging his reflection in a mirror for his life. Suddenly, his reflection cuts its throat with a mirror shard, killing the "real" security guard.
Ben Carson (Kiefer Sutherland), a suspended police detective, begins his first day as a night security guard at the Mayflower, a luxury department store that was gutted by a fire and shuttered five years prior. The building still contains numerous mirrors from the store.
On Ben's first night of patrol he finds a mirror that appears to be covered with handprints, but only on the reflected side of the glass. He sees an open door in the reflection while it is actually closed. Over time, Ben begins to see more intense visions, which he initially shrugs off as hallucinations. He soon finds the wallet of Gary Lewis, the previous night guard (who died at the beginning of the film). Inside is a note that says "Esseker". After viewing Garyâs crime photos Ben is convinced that the mirrors make people do things to themselves that they are not actually doing.
Meanwhile, Ben's sister, Angie (Amy Smart) is killed by her reflection as it grips its jaw and slowly pull its mouth apart, causing her to bleed profusely. Ben is distraught when he finds her body. In anger, he attempts to destroy the mirrors at the Mayflower, but they are impervious to damage. He demands to know what the mirrors want, and cracks appear on one of the mirrors, spelling out the word "ESSEKER".
Ben enters the flooded basement of the Mayflower and finds a small sign stating "Psychiatric Studies" and "St. Matthew's Hospital" underneath. He moves to the site of the leak and begins pulling at the tiles and brick of the wall and finds a room with a chair surrounded by mirrors beyond it, a Psychomanteum. Realizing that the Mayflower was built on the site of an earlier hospital, Ben asks his police friend Larry (Jason Flemyng) to help him locate the patient-employee manifest for the hospital. Larry finds the name Anna Esseker, a patient of the psychiatric hospital. She was twelve years old at the time and died in a mass suicide.
Ben looks through Anna's file, and finds an Authorization and Consent form that negated her Death Certificate, stating that she had been discharged from the hospital two days before the suicide and is led to believe that Anna is still alive. Meanwhile, Ben's wife Amy (Paula Patton) discovers her son Mikey's (Cameron Boyce) reflection acting differently from the real Mikey. In a panic she calls Ben, who immediately returns home. Together they cover every reflective surface in the house with green paint.
Ben locates Anna Esseker's childhood home, and discovers that as a child she was violent and uncontrollable, and diagnosed with severe schizophrenia. She was taken in by a doctor from St. Matthewâs Hospital, who believed that she was suffering from a rare personality disorder. His treatment was to confine Anna to a chair surrounded by mirrors, believing this would cure her disorder by forcing her to confront her own reflection. Ben is told by her brother that when she returned, apparently cured, strange things started to happen with the mirrors in their home. As a result, her family sent her to a convent, Saint Augustine's Monastery, where mirrors are forbidden.
Ben visits the convent, and finds Anna (Mary Beth Peil), who explains that she was actually possessed by a demon, which was drawn from her and became trapped in the mirrors. She explains that it collects the souls of those it kills and if she were to return it would make it possible for the demon to be brought back into the mortal world. She refuses to go back.
Meanwhile, Amy discovers that Mikey is missing at home and a thin reflective layer of water is completely covering the floor. After putting her daughter in a safe closet, she finds Michael using a chef knife to scrape the paint from the mirrors. Amy tries to stop him but he escapes, obviously possessed.
Having threatened her at gunpoint, Ben returns with Anna to the Mayflower and straps her into the chair in the Psychomanteum. Back at Ben's house Mikey is suddenly pulled through the water on the floor by his reflection and begins to drown. At the Psychomanteum the lights begin to flicker and the building begins to shake as the demons in the mirrors are released. They repossess Anna and all the mirrors in the Mayflower explode. Simultaneously, Mikey is released from the demon's grip and Amy is able to pull him to safety. Ben is then attacked by the repossessed Anna. He manages to kill her by igniting a nearby gas line, setting off a huge explosion. The old building collapses, killing the demon, and trapping Ben under the ceiling as he rushes toward the exit.
Ben pulls himself out of the rubble and stumbles out of the building. Policemen and firemen are everywhere in the street, and a body is seen being taken in a bag by paramedics, but nobody notices Ben. He looks at the older security guard's name tag, and sees it is written backwards, realizing everything is in reverse (like in a mirror). He comes upon a mirrored surface in the city and fails to see his own reflection as he reaches out to touch it. He realizes that he is now trapped in the mirror world: in the living world his hand appears as a handprint on the glass surface.

Question: What explanation does Anna give to Ben for her erratic behavior?
Answer: She was possessed by a demon. 
Alternative answer: That she was possessed with a demon and he could come back to the mortal life.