When shy seventeen-year-old Riley Borden's abusive stepfather is brutally murdered before her eyes, she bravely tells the authorities exactly who did it: a monster. Not surprisingly the evidence points to her and she's shipped off to a rural Northern California mental institution, leaving her ten-year-old sister in mortal danger at the hands of her deranged aunt. Bedeviled by inmates, emotionally vanquished, Riley becomes convinced that her life is over... but when she awakens to a presence that manifests as the very creature that got her here, with it comes a serendipitous escape and a desperate journey to save her sister—and along the way she learns that the primeval being that calls her soul home may hold a secret more ominous than anything she ever feared.