The film opens with two people wearing a device on their head that will crush their skulls if they do not add a proper amount of weight (using their own severed body parts) to a weight scale. The girl (Tanedra Howard) severs her arm off halfway up her bicep. She then throws her arm onto the scale and survives, but watches the other victim die.
William Easton Peter Outerbridge, an insurance businessman, is in his office when he denies a man a loan to get a medical procedure, which later results in the man's death. William is captured and wakes up in a room with a mask on. Moments later, it is revealed that another man is in the room. With each breath taken, cement-like objects are moved closer and closer to their stomachs. William yells to other man and tells him to hold his breath. The other man, revealed to be a smoker in the tape, is unable to free himself from the trap and his chest is crushed. William later walks through and removes one of the bombs from his arm. William continues on and sees a light that says "grab these" and when he refuses, he is forced to grab the two handles. Billy comes out and informs him that that letting go of one the handles will result in one person being hanged while the other will be saved. After much deliberation, William lets the woman live, and watches on as the man gives him a greasy look before dying. William walks into the next room (the steam room) and is told that he has to disable the steam, which will result in him getting scalded, to let the other person advance. William is burned by a jet of extremely hot steam, but lets the woman (who is believed to be having an affair with William) advance. She climbs a ladder and is greeted by William, but is told that the key to the device that will spear her head is inside Williams chest. She picks up a buzz saw and attempts to kill William, resulting in a struggle where she is eventually killed.
William proceeds to the next room (the Carousel room) he is told he has to pick two people to live. After much arguing William allows two women live. William proceeds to the next room, and arrives with but a second to spare.
Hoffman Costas Mandlyor goes along with Agent Perez (who is revealed to be still alive) and Erickson as they are deciphering the voice from Seth's tape. Hoffman is identified as the voice, and to protect him identity Hoffman slashes Erickson's neck and stabs the light panel to confuse the others. He then throws his coffee into the computer technician's face, grabbing and using her as a human shield while Perez shoots at him. Hoffman charges Perez and stabs her in the stomach. Perez, in her final words, utters "everyone knows." Hoffman douses the bodies in gasoline and torches the room.
Also, through flashbacks, Jigsaw and Amanda arrive at Jill's clinic, telling her his methods work. Amanda goes along with Jigsaw and then there is another flashback of Amanda pushing John in a wheelchair to Hoffman, who is messing up the Rack Trap from Saw III. Amanda pushes John over to Jill Tuck where he gives her the key to the box from Saw V.
William walks out of the final room and to his left are an old woman and a boy, and to his right is Pamela Jenkins, who is revealed to be his partner. The mother and son to his left are the family of the man who William denied the loan for in the beginning of the film. The woman and son have a switch in their room that has the words "Live and Die." The woman attempts to flick the switch to kill William but cannot force herself to do so. The son, in a moment of anger, walks up to switch and says "I can do it," flicking the switch to kill William. A small bladed pendulum swings down and stabs William in the back, with hydrofluoric acid being injected into his back. Pamela is forced to watch William melt. The fate of the other survivors is not revealed.
The letter in saw 3 is revealed, with it stating that Amanda got Cecil to get the drugs from the chemist, and that John and Hoffman knew. The letter also notes that Amanda was ordered to kill Lynn. Later, Hoffman is captured by Jill as part of Jigsaw's final request (one of his six requests in the envelopes). He is placed into the reverse bear trap and is given sixty seconds to save himself, but he is completely strapped down with seemingly no way to get out. Hoffman leans down and uses the front the bear trap to break the bones in his hands, allowing himself to get his hands out of the shackles. He then frees himself and is able to walk around. Hoffman picks up a screwdriver and attempts to pry the bear trap off, but is unsuccessful. With only seconds remaining Hoffman runs up to the door that Jill left through and smashes the glass. There, he utilizes the horizontal bars in a small window to free himself by placing the jaws of the bear trap between the bars, preventing the trap from fully opening. Hoffman is then able to remove his mouth from the bear trap. The movie ends with Hoffman screaming looking upwards.
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