Adam Faulkner (Leigh Whannell), a photographer, awakens in a bathtub full of water, at one end of a grimy and disused bathroom. At the other end is Dr. Lawrence Gordon (Cary Elwes); both men are chained by the ankle to the plumbing pipes. A corpse holding a revolver and a microcassette recorder lies on the floor between them. Lawrence and Adam each discover tapes in their pockets labeled "Play Me". From these tapes, Adam is told that he must escape the bathroom, while Lawrence is told that he must kill Adam before six o'clock, or else his wife and daughter will be killed and he will be left to die where he sits. They find hacksaws, though neither is sharp enough to cut through the chains. Adam breaks his and throws it away in frustration. Lawrence realizes that the saws are not meant for the chains, but instead for their feet. Lawrence determines that their captor is the Jigsaw Killer. The name is a misnomer, says Lawrence, as he never directly murders anyone and his intentions are for his victims to survive with a better appreciation of life. In a series of flashbacks his previous victims are presented, including his only known survivor up to that point, a highly traumatized heroin addict named Amanda Young (Shawnee Smith). She believes that her experience in the test, in which she risked having her jaws ripped apart, has made her a better person. While Adam and Lawrence search for alternative escape routes, Zep Hindle (Michael Emerson) breaks into the Gordon house and captures Lawrence's wife, Alison (Monica Potter), and daughter, Diana (Makenzie Vega). Zep is an orderly at Lawrence's hospital who was present when he was talking with some students about John Kramer's terminal brain cancer. While psychologically toying with Alison and Diana, Zep monitors Adam and Lawrence through video surveillance. Simultaneously, the house is being observed by Detective David Tapp (Danny Glover). Tapp became obsessed with the Jigsaw case after hearing Amanda's testimony. He and his partner, Detective Steven Sing (Ken Leung), illegally broke into a warehouse that turned out to be one of Jigsaw's lairs. In the lair, they saved a man from being killed by drills aimed at his neck. Before they could secure Jigsaw's arrest, however, Sing was killed by a booby trap, and Jigsaw escaped after slashing Tapp's throat. Tapp was dismissed from the police force and is now stalking Lawrence, convinced that he is the Jigsaw Killer. In the bathroom, Dr. Gordon finds a cell phone that can only receive calls. He and Adam try to stage Adam's death, but an electric shock through the latter's ankle chain foils this plan. Following these events, Adam and Lawrence recall their abductions; they were both ambushed and knocked unconscious by a stranger wearing a gruesome pig mask. Lawrence receives a call from Alison, who warns him that Adam knows more than he is telling. Adam explains that he had been paid by Tapp to trail and photograph Lawrence and shows him a pile of pictures that he found with the hacksaws. Lawrence begins to berate Adam, but grows defensive when Adam shows Lawrence evidence that he has been cheating on his wife. The two begin arguing, but are distracted when Adam notices a picture of Zep in Lawrence's house. They deduce that Zep is their abductor. Just as this realization is made, however, Adam points out that it is six o'clock, the deadline. Alison frees herself from Zep, and a struggle ensues. Gunshots are fired, attracting the attention of Tapp, who distracts Zep long enough for Alison and her daughter to flee. Zep shoots Tapp in the chest and races to the sewers, intent on killing Lawrence, who is only aware of the sounds of gunfire and screaming. Desperate to help his family and unable to reach the phone after throwing it aside, he saws off his foot and shoots Adam with the corpse's revolver, using a bullet that had been enclosed with his tape. Zep enters the bathroom to kill Lawrence, but Adam springs up (having suffered a non-fatal wound) and beats Zep to death with a toilet tank cover. Lawrence slowly crawls away, promising to get help and return for Adam. Adam searches Zep's body for a key, but finds another microcassette player instead. Adam learns that Zep was only a pawn in Jigsaw's game, following rules laid down for him in order to get the antidote for a slow poison he had been given. The "corpse" in the middle of the floor slowly rises to its feet, peeling some bloody makeup from its face and revealing itself as John Kramer, the Jigsaw Killer. Adam reaches for Zep's dropped pistol, but Jigsaw stuns him with an electrical shock from a hidden remote control. He tells Adam that the key for his chain was in the bath, which Adam accidentally pulled the plug out of with his foot at the beginning of the film. Jigsaw then flicks off the lights and shouts "Game over!" before slamming the door and sealing Adam in the bathroom.