Informant Michael Marks awakens in a room with a spike-filled mask locked around his neck. A videotape informs him that in order to unlock the device, he must cut into his eye to obtain the key. He sets off the timer and finds the scalpel, but cannot bring himself to retrieve the key and is killed after sixty seconds when the mask closes.
At the scene of Marks’ game, Detective Allison Kerry finds a message for her former partner, Eric Matthews, and calls him in. Despite not wanting to be involved in the case, Eric reluctantly joins Kerry and Sergeant Daniel Rigg in leading a SWAT team to the factory which produced the lock from Marks’ trap. There they find and apprehend John Kramer, the Jigsaw Killer, who is weak from cancer. He indicates several computer monitors showing eight people trapped in a house; including his only known survivor Amanda Young, and Eric’s estranged son Daniel. The other victims are called Xavier, Gus, Jonas, Laura, Addison and Obi. A nerve agent filling the house will kill them all within two hours, but Kramer assures Eric that if he follows the rules of his own game, by simply talking with John, he will see Daniel again. At Kerry’s urging, Eric agrees in order to buy time for the tech team to arrive and trace the video signal.
The victims are informed by a microcassette recorder that antidotes are hidden throughout the house; one is in the room’s safe, and a cryptic clue is provided. Xavier ignores a warning note and uses a key on the door, which fires a bullet through the peephole and Gus’ eye, killing him. They search the house for more antidotes after the room lets them out, with no success. Obi, who is revealed to have helped with the abductions, is forced into a crematory oven to obtain two antidotes inside, but inadvertently activates the trap and is burned to death before the others can save him, destroying the antidotes. In another room, Xavier throws Amanda into a syringe-filled pit intended for himself, and though she is able to retrieve the key, he is unable to unlock the door containing an antidote and he leaves out of frustration. Throughout the game, the victims discuss connections between them and determine that each has been jailed before; with the sole exception of Daniel.