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.
Jigsaw abducts a doctor in order to keep himself alive while he watches his new apprentice put an unlucky citizen named Jeff through a brutal test.
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.
Agent Strahm is dead, and FBI agent Erickson draws nearer to Hoffman. Meanwhile, a pair of insurance executives find themselves in another game set by Jigsaw.
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.