Billie, a twenty something burnout, is tasked with house-sitting for her aunt and uncle, but after inviting some friends over, becomes convinced her teenage cousin Grace has stolen a notebook containing her latest screenplay. While hunting for the book in "Grace's Room", the gang uncovers a bloody trove of occult paraphernalia. Unbeknownst to Billie and her friends, said cult has been watching them from the nearby woods, and now must kill them for discovering too much. A gruesome assault on the house ensues as the gang struggles to survive the night.

The seemingly random killings of an assassin puzzle her former lover, a wealthy Greek crime boss whose organization is jeopardized by his love for her, and the detective following her rising body count