Armed with nothing more than twigs, their imaginations and a simple set of rules, a group of 12-year-olds engaged in a lively game of Capture the Flag in the neighborhood woods start dangerously blurring the lines between make-believe and reality. Paint-filled balloons = Grenades. Trees = Control towers. Sticks = Sub-machine guns. The youthful innocence of the game gradually takes on a different tone as the quest for victory pushes the boundaries of friendship. The would-be warriors get a searing glimpse of humanity's dark side as their combat scenario takes them beyond the rules of the game and into an adventure where fantasy combat clashes with the real world.

Kim and her boyfriend Matt have just broken up, but they travel together to a cottage nearby a lake to spend the weekend with their friends Eve, Dash and his wife Amber and Baker and his new girlfriend Lily. The atmosphere becomes heavy with the situation between Kim and Matt, and one of them suggests them to play "Dead Mary". Matt, Eve and Dash summon the evil witch repeating her name three times in front of a mirror with a candle. Along the night, Matt hears weird noises, is killed and returns to life, while part of the group is possessed by the fiend. Without knowing who is possessed, the rest of the group fights to survive.