A photographer named Adam awakens in a dilapidated bathtub, with his ankle chained to a pipe. Across the room is oncologist Dr. Lawrence Gordon, also chained. Between them is the body of an apparent suicide victim holding a revolver and a microcassette recorder. Both men find a tape in their pockets, and Adam retrieves the recorder. Adam's tape urges him to survive, while Gordon's tape orders him to kill Adam by 6 o'clock, or his wife Alison and daughter Diana will be killed. Adam finds a bag containing two hacksaws inside the toilet. Both men try to saw through their chains, but Adam's saw breaks. Gordon realizes that the saws are not intended for the chains, but to cut off their feet, and identifies their captor as the Jigsaw Killer, a serial killer testing his victims' will to survive through lethal traps referred to as "games," whom Gordon knows about because he was once a suspect.

Five months prior, Gordon, while discussing the terminal brain cancer of patient John Kramer, was interrogated by Detectives David Tapp and Steven Sing, who found his penlight at the scene of one of Jigsaw's games. Gordon's alibi cleared him, but he agreed to view the testimony of heroin addict Amanda Young, the only known survivor of one of Jigsaw's traps, who had been forced to kill and disembowel a man to obtain a key to free herself from a reverse beartrap strapped to her face. Tapp and Sing later found Jigsaw's warehouse using the videotape from Amanda's game. There, they apprehended Jigsaw and saved a man from a trap, but Jigsaw injured Tapp and escaped. Sing pursued Jigsaw down a hallway, but in doing so accidentally triggered a shotgun trap which killed him.