Saw I | Saw II | Saw III | Saw IV | Saw V | Saw VI | Saw 3 D | Jigsaw | Spiral | Saw X
The film starts off from the ending of Saw II, with Eric Matthews trapped in the Bathroom after being abducted by Amanda Young. After hesitating to attempt it with a hacksaw, he eventually manages to free himself by breaking his chained foot with the toilet lid, allowing him to slip the foot through the shackle.
The scene immediately cuts to a much later time, where Allison Kerry and Daniel Rigg find the scene of a Jigsaw game in a elementary school classroom, with a forensic named Mark Hoffman intervening in the investigation. Base on the prominentevidence they have, they figure that the game's victim, Troy, was restrained by a number of chained barbells pierced through the skin of various parts of his body, and had to rip them off in order to free himself and escape the room before a nail bomb exploded. However, Kerry deduces that the trap couldn't have been escapable due to the door having been welded shut for the SWAT team to open by force, so she thinks John Kramer isn't responsible for setting up this game.
When she returns to her apartment at night, Kerry is abducted by a pig-masked figure and finds herself in a trap of her own upon waking up. She has one minute to get a key inside filled with acid, which will dissolve it once the time runs out, in order to free herself from the device attached to her chest. She manages to get the key in time, but finds out that the trap was rigged much like Troy's, and she gets a brief sight of the one who did just before the trap rips her ribcage apart.
The film then cuts to a surgeon, Dr. Lynn Denlon, finishing a work shift, with two flashbacks to her at an affair with a man (named Chris) and her trying to treat a child, respectively. She's abducted by Amanda, who takes her to her and the now-bedridden John's current hideout inside a meatpacking plant. John tells Lynn that she must keep him alive until a man, mentioned by Amanda, makes it through a series of tests. At the same time, Amanda locks a collar connected to John's heart monitor around Lynn's neck; should she let John die or move out of range, the collar will detonate and kill her. Lynn is thoroughly confused, believing that John wants to be treated even though there isn't anything to treat his condition.
Somewhere else, the aforementioned man, Jeff, wakes up inside a hanging crate, with a tape telling him that he has to go through several tests in order to find "the man responsible for the loss of his child". Just after Jeff breaks out of the cage, there's a flashback that explains he had been in grief since the death of his son Dylan three years ago, keeping Dylan's original bedroom and properties under security, as shown when he takes one of the toys that his daughter Corbett was holding. The flashback ends with a pig-masked person abducting Jeff. In the present, Jeff gets up and, while walking through the hallway, finds a box containing a message telling him to open a door and a cut piece from a photo featuring him.
John suddenly gets a cerebral seizure that Lynn stops before it risks his death. Lynn tells Amanda that he needs to undergo brain surgery in order to survive, to which Amanda replies that she must perform it herself at the plant.
By then, Jeff finds a woman, named Danica Scott, chained in the nude at a freezing room. A tape explains that Danica was the only witness present in Dylan's death, who refused to testify about what happened at the incident; this led to the drunk driver responsible for killing Dylan receiving a rather light sentence at court. In the game itself, Danica is progressively sprayed in cold water that will quickly freeze and encase her body in ice; Jeff can either let her die or save her by getting a key behind several metal bars. Danica convinces Jeff to save her, but due to the late timer he's unable to stop her from being frozen to death, even though he's able to get the key after suffering a mild injury. After exiting the room, Jeff finds another box, this time with the message "One bullet will end it all." and a piece from a photo of Dylan.
Lynn tries to convince Amanda to allow John to be brought to the hospital to no avail. Therefore, she decides to tell her that, among other tools, she would need anesthetics and a drill to perform the surgery, so Amanda leaves to get such supplies.
The film cuts to a montage of flashbacks when Amanda picks the Reverse Bear Trap midway through. First, John is seen making a copy of Billy the Puppet, with a photo of Amanda being briefly seen. The second flashback sees John setting up Amanda's game from the first film, and the third shows John meeting Amanda for the first time after the game. After the montage, Amanda closes a box and brings the supplies to Lynn.
As he continues walking, Jeff finds a copy of Billy lying on the floor. In the next room he enters, he encounters Halden, the judge who presided at the drunk driver's court trial and have his sentence. Much like the previous one, the tape for this test tells Jeff that he can either let Halden die by being submerged in liquefied pig intestines, or save him by getting the key to his shackle; to do so, he has to incinerate Dylan's possessions this time. After a considerably long period of hesitation, Jeff turns on the incinerator and saves Halden just before he completely drowns.
Lynn begins the brain surgery on John as Amanda watches in fear. During the surgery, John hallucinates and begins to think about a woman he loves, whom Amanda mistakes for Lynn as he professes a love speech. Several flashbacks show John spending time with said woman, who's named Jill. Another series of flashbacks begins as Amanda returns to the room with the box she previously picked up before and begins to cut herself. These flashbacks include a "ceremony" being held by John to convert Amanda to his ideals, Amanda kidnapping Adam, and her setting up the Bathroom game with John. In the latter, it's revealed that Amanda had deliberately placed the key to Adam's shackle in a way that it would get into the drain no matter what happened.
Back to the present, Amanda covers the wound she just made on herself and goes back to see how Lynn is going with the surgery; it had already ended, and John had to sleep until he recovered. Amanda immediately gets angry with Lynn and grabs her by the throat when she tells her that he couldn't notice her during the surgery. Upon loosening her grip on Lynn, Amanda leaves once again as another flashback begins: shortly after the ending of the first film, Amanda, inside a lair with John, feels wracked with guilt about leaving Adam to die, so she returns to the Bathroom to give him a mercy kill.
Meanwhile, Jeff walks through another hallway with Halden following him. He finds another box with the message "One step closer to your revenge." and a third photo piece featuring Corbett; he's able to put the three photos together into one larger part, which shows that it's a family photo. The mother, however, only has a brief part of her shown through Corbett's piece. In the room of Jeff's third test, he and Halden find Timothy Young, a former medical student and the drunk driver responsible for Dylan's death, strapped to a device. The tape on Timothy's neck instructs that the device will slowly twist his limbs before passing on to his neck; should Jeff decide to save him, he has to get a key connected to the trigger of a shotgun that will shoot anyone in the line of fire. Due to Jeff's sheer hatred of Timothy this time around, Halden has to convince him to get the key, but the latter is killed by the shotgun when he walks in front of it. Timothy isn't fortunate either, since Jeff is unable to find a keyhole that would deactivate the trap.
Amanda, who had watched the test through camera, goes to tell John and Lynn that Jeff had finished the test, but not before finding a letter on a desk that gets her worried. John orders Amanda to unlock the collar from Lynn, but she refuses to do so, holding Lynn at gunpoint instead. As John tries to calm her down, Amanda tells him that she does not believe in his philosophy, admitting that she was the one who had manipulated the first two traps in the film. A flashback also reveals that following the opening scene, Eric tried to fight back against Amanda until the latter left him to die for good.
Jeff finds a fourth box with the message "Last chance." and a pistol that he picks up. Amanda ultimately shoots Lynn in the back just a she sees and runs up to Jeff, who in turn shoots Amanda in the throat. As Amanda falls to the floor, John tells her that Lynn and Jeff are married, and he hid that fact from her to test her, as he had already known what she did before. This is accompanied by a montage of flashbacks featuring Amanda, Jeff and Lynn, ending with Amanda succumbing to the gunshot.
Jeff attempts to shoot John too, but finds out that the pistol had already run out of ammunition. John tells Jeff that he promises to call an ambulance for Lynn should he pass a final test: Jeff can either kill John or allow him to live if he "forgives" him. Vengefully, Jeff sarcastically tells John that he forgives him before slashing his throat with a buzzsaw. As Lynn yells Jeff's name, a brief flashback montage featuring events from the previous films happens before the collar detonates and destroys Lynn's head, followed by the door in the room automatically closing. A final tapenote informs Jeff that he had failed the test, revealing that Corbett is also locked somewhere else in the plant, and her place was only known by John. Jeff lets out a scream at John's corpse as the film ends.