Adam, a photographer, awakens in a bathtub in a large dilapidated bathroom, and finds himself chained at the ankle to a pipe. Lawrence Gordon, an oncologist, is chained to a pipe across the room, and between them is a corpse holding a revolver and a microcassette recorder. Each man finds a tape in his pocket, and Adam is able to retrieve the recorder. Adam’s tape urges him to escape the bathroom, while 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 weren’t meant to be used for their chains, but for their feet, and identifies their captor as the Jigsaw Killer, whom Lawrence knows of because he was once a suspect.
Flashbacks show that while Lawrence was discussing the terminal brain cancer of a patient named John Kramer he was approached by Detectives David Tapp and Steven Sing, who found his penlight at the scene of a Jigsaw “game”. Lawrence’s alibi cleared him, but he agreed to view the testimony of the only known survivor of one of Jigsaw’s games, a heroin addict named Amanda Young, who escaped a “reverse bear trap”, and believes that Jigsaw helped her.
Meanwhile, Alison and Diana Gordon are being held captive in their home by a man who is watching Adam and Lawrence through a camera hidden behind the bathrooms two-way mirror. The house is simultaneously being watched by Tapp, who has since been discharged from the force. Flashbacks show that Tapp became obsessed with the Jigsaw case after Amanda’s testimony and eventually found Jigsaw’s warehouse using the videotape from Amanda’s game. He and Sing entered the warehouse, where they apprehended Jigsaw and saved a man from a drill trap, but Jigsaw escaped after slashing Tapp’s throat, and Sing was killed by a quadruple shotgun trap while pursuing him. Still convinced that Lawrence is Jigsaw, Tapp continued stalking him after his discharge.