Three Knocks Behind the Glass

An old mirror hangs in Rebecca Holloway's bedroom, inherited with the house and never questioned until now. One night the glass stops reflecting her movements correctly, and a second later she sees a woman standing on the other side who looks exactly like her, down to the scar on her wrist. The woman raises a hand and knocks, once, twice, three times, and asks in Rebecca's own voice to be let out.
As the knocking returns night after night, Rebecca turns to her partner Cole Donovan, who insists it's stress and sleep deprivation, and to Nathan Graves, an antiques restorer who recognizes the mirror's make and warns her that some glass was never built to only show a reflection. What began as a strange trick of the eye becomes a countdown, because the woman behind the glass is growing more insistent, and every object in the house that holds a reflection, windows, spoons, still water, is starting to answer back.