John Kramer, the "Jigsaw Killer," traps victims in situations that test their will to live through physical or psychological torture. According to Jigsaw, if they survive, they will be "rehabilitated."
Adam, a photographer, awakens in a bathtub in a large dilapidated bathroom, chained at the ankle to a pipe. Lawrence Gordon, an oncologist, is chained to a pipe across the room. Between them is a corpse holding a revolver and a microcassette recorder. Both men find a tape in their pockets, and Adam is able to retrieve the recorder. Adam's tape urges him to escape the bathroom. Lawrence's tape tells him to kill Adam by six o'clock, or his wife and daughter will be killed and he will be left to die. Adam finds a bag containing two hacksaws inside a toilet, which they try to use to cut through their chains, but Adam's saw breaks and he throws it at the mirror in frustration. Lawrence realizes that the hacksaws are meant to be used on their feet, not their chains. He identifies their captor as the Jigsaw (Saw character), whom Lawrence knows of because he was once a suspect.
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.
Saw II, Eric Matthews (Saw) breaks his foot with a toilet lid to escape his shackle. Six months later, the aftermath of a Jigsaw "game" is discovered by a SWAT team. The victim, Troy, was meant to rip chains from his body in order to escape from a bomb. Detective Allison Kerry arrives at the scene, and points out that the room's exit was welded shut, breaking Jigsaw (Saw character)'s modus operandi of giving his victims a chance to survive. While reviewing the videotape, she is abducted and awakens in a harness hooked into her ribs. Even though she retrieves the key from a beaker of acid and unlocks the harness, she is unable to remove it and is killed when time runs out.
A wax-coated microcassette is found in Jigsaw (Saw character)'s stomach during his autopsy, and reveals to Detective Mark Hoffman that, despite his death, the games will continue and he will still be tested. Elsewhere, two men – one with his eyes sewn shut, the other with his mouth sewn shut – awaken in a mausoleum, chained at the neck to a winch. The blinded man panics and activates the winch before attacking the muted man, who kills him in self-defense and uses a key from his collar to free himself.
Convicted murderer Seth Baxter wakes up chained to a table beneath a pendulum blade. A videotape informs him that he can release himself by crushing his hands between two presses. He does so, but the blade still swings down and cuts him in half as someone watches through a hole in the wall.
Two Predatory lending wake up in a room with a caged-in scale, wearing head harnesses with screws aimed at their temples, and the one who sets more flesh weight on their scale will survive. The overweight Eddie cuts fat from his torso, but Simone chops off her arm, and Eddie is killed when the timer expires. As he reviews the game's footage, Detective Lieutenant Mark Hoffman is called over by FBI Agent Dan Erickson, who found Agent Peter Strahm's fingerprints at the scene. Also present is Lindsey Perez, whose survival was covered up by Erickson for her protection. They learn from Dr. Adam Heffner, the coroner who performed the autopsies for all of the Jigsaw victims thus far, that the knife used to cut the jigsaw piece from Eddie's flesh was only used once before, on Seth Baxter. The agents intend to find the Baxter audio tape to prove Strahm's involvement.
Bodies are turning up around the city, each having met a uniquely gruesome demise. As the investigation proceeds, evidence points to one suspect: John Kramer, the man known as Jigsaw, who has been dead for over 10 years.