Some creepy dude with a metal mask melted into his face hands out tickets to a free screening at a movie theater that's been closed for many years. The people who end up going to the show find out that either the movie or the theater is cursed when a prostitute who was out on the town with her pimp scratches her face while trying on a silvery devil mask that's part of a display in the lobby of the theater and turns in to a demon, just like in the movie they're watching! Mayhem ensues.
Lesson learned? Don't go to movies at closed theaters advertised by mute disfigured freaks.
Features the greatest example of a Deus Ex Machina in a movie that I can think of.