Two men are chained to opposite corners of a room with a corpse between them, his brain hanging out in a pool of blood. While trying to find a way out of this pickle, one of the men recalls a murderer who doesn't actually murder people: He simply puts people in situations which make them kill themselves or each other. His name according to the news is "Jigsaw." As the movie goes on the men find clues to why they are here and who is behind the mayhem.
The film makes you believe the murderer is a skinny white dude, but he is also one of Jigsaw's victims told to set up the men in the room or he will die. At the end we find that the real Jigsaw had front row seats to his "game" when the dead man in the middle of the room stands up and wipes off the blood and brain he had been lying in from the beginning.
Conclusion: The killer is lying in the room from beginning to end. (I considered myself fooled.)
— Thanks to Michael Rusk!
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