After spending an evening with her family at which a large mirror breaks for no apparent reason London radiologist Gina McVey (Lena Headly) begins to sense she has a doppelganger. When she sees herself drive past on the street, she follows the doppelganger to an apartment, the outcome of which the viewer doesn't see. Immediately afterward she wrecks her car and while recuperating at her boyfriend's apartment comes to believe that he too has been replaced by an evil doppelganger. From her dreams it becomes clear that the doppelgangers gain access to the world through broken mirrors. Unknown to Gina, first her sister-in-law and then her father are replaced. When she finds the corpse of her boyfriend which has been left behind by his doppelganger, she returns to the apartment where it all started - her apartment though she has forgotten this - and finds her own corpse there. The memory of what she is, which had been erased by the accident, returns to her and she takes her place in a doppelganger London being taken over person by person.