A chance encounter on the snowy slopes of a castle moat throws together three lonely teenagers: Emily, Simon, and the highly enigmatic Marcus, who is the catalyst for all that follows.

At first the castle's ruins are merely the backdrop to a day in the snow. But when the three break into the forbidden interior of the keep, they find the castle taking on a different and deeper meaning. Despite the freezing conditions, they spend the night there, to experience the power of occupation. But when the re-enactment gets out of control and a very real siege ensues, their playful dare soon turns into a frenzy of nightmarish action.