A full moon hangs in the night sky and lightning streaks across dark storm clouds. Sarah Tyler returns to her troubled family home in the isolated countryside, for a much put-off visit. As the storm rages on, Sarah, her family and friends shore up for the night, cut off from the outside world. But something comes out of the driving rain and darkness. Something that holds a dark secret so devastating that, in one night, it could wipe out the entire family. Trapped, Sarah and her brothers and friends must use their heads as well as their physical strength to survive not only the thing that is hunting them down one by one - but their own entrapment as the besieged group turns in on itself. Can they survive the horror stalking them? Or is their enemy already amongst them and has it always been? Can they survive for 13 hours?