Detective Eric Matthews, who was involved in the apprehension of the Jigsaw killer (John Kramer) in the first film, finds himself trapped in one of Jigsaw's twisted games. Matthews awakens to find himself in a sealed room with no apparent exit, and is informed by Jigsaw that his son, Daniel, has been kidnapped and is in a safe spot in one of Jigsaw's lairs; the detective has two hours to free his son before a deadly gas is released into the Hideout, killing everyone inside. Jigsaw subsequently reveals that eight people are trapped in the building, all of whom have a connection to Matthews, and they must all escape from a nerve-gas-filled house of horrors before they succumb to the lethal fumes.
Meanwhile, Matthews' partner, Detective Allison Kerry, investigates an abandoned house where she discovers the remnants of several other Jigsaw traps. As the game progresses, Jigsaw's methods are revealed through a series of flashbacks regarding the grisly origins of several of his victims, all of whom are forced to confront their own demons. One participant is Amanda Young, a survivor of Jigsaw's previous games who has been rehabilitated and made a protégée of Jigsaw. She helps the trapped people and provides insight into Jigsaw's mind, while Matthews' son, Daniel, also becomes an unwitting pawn in Jigsaw's twisted game.
As time runs out, Matthews becomes increasingly desperate to rescue his son and uncover Jigsaw's true identity. The games and traps become more difficult and deadly as the participants struggle to survive, and with each passing moment, the odds of escaping Jigsaw's deadly grasp grow slimmer. Ultimately, Matthews is lured into a final encounter with Jigsaw, where the shocking truth about the killer's motivations and identity is finally revealed in a plot twist that leaves viewers gasping.
