The three friends, Joe, Eric, and Hung, are avid fans of LARPing (Live Action Role-Playing) and decide to take their game to the next level. They set up camp in the woods and prepare to embark on a weekend-long adventure as medieval knights, wizards, and warriors.
Joe, who is the game master, orders a prop spellbook online to add to the authenticity of their game. Unbeknownst to them, the book is actually a real grimoire that summons demons when used improperly.
During the game, the group accidentally performs a ritual from the book and conjures up a succubus named Lilith. Lilith is a demon from hell who feeds on the life force of men. She quickly sets her sights on the friends and begins to stalk them through the woods.
Joe and Eric initially believe that Lilith is just a part of the game, but things quickly turn serious when they realize that she is real and dangerous. They enlist the help of a metalhead LARPer named Ronnie, who has real combat experience, to try to defeat Lilith and send her back to hell.
As the group battles Lilith and her demonic minions, they come to realize that the only way to defeat her is to use their real-life skills and weapons, not just their characters' abilities. In the end, they manage to defeat Lilith and save themselves from a gruesome fate.
The movie is a comedic horror film that combines the worlds of LARPing and demonic summoning. It stars Peter Dinklage, Steve Zahn, and Ryan Kwanten as the three friends, with Summer Glau playing the role of a LARPing fanatic who joins the group.