In 1950s Iowa, Mary Weatherford sacrifices her sister Elizabeth in front of a large mirror, stabbing her to death on a bed. Decades later, Megan Gordon, a shy teenage goth, moves into the home from Los Angeles with her recently widowed mother, Susan. In her new bedroom, Megan finds the large mirror in the corner left behind by the previous owners. Emelin, the auctioneer in charge of the house clearance, finds a cache of journals that describe the mirror's apparent possession by a demonic force able to grant wishes.