About a month after Emma Callan (Elisabeth Moss) and her family move into their seemingly picture-perfect Victorian home, Emma starts to have ghastly visions of a girl who appears to be her twin but is pure evil. Since no one else has seen this doppelganger, Emma is faced with two terrifying prospects: Either she's going insane or she's actually being haunted by a malevolent spirit determined to destroy anyone who dares delve into the dark mysteries of The Attic.