Please sit tight--this one's complicated. Lawrence Gordon (Cary Elweys) and Andy (Leigh Whannell) awaken to find thelselves chained at opposite ends of a filthy room with a bloody corpse between them. They are the latest prisoners of the 'jigsaw' killer, who plays sadistic "games" in which his victims end up killing themselves or others. We are led to believe that the jigsaw killer is Zep Hindle (Michael Emerson), an orderly at Lawrence's hospital. He is seen observing the chained Lawrence and Andy through video monitors, kidnapping Lawrence's wife Allison (Monica Potter) and daughter Diana (Makenzie Vega), and planning to murder them if Lawrence fails to kill Andy by 6 am. When Lawrence's time runs out, Zep approaches Allison. Allison has managed to loosen her bonds, and she fights Zep for his gun. When the gun fires, it draws the attention of former detective Tapp (Danny Glover), who has been keeping a close eye on Lawrence's home, thinking that Lawrence was the jigsaw killer. Lawrence, meanwhile, is able to hear the struggle back at his house through the cell phone provided by the killer. Frantic over the thought of losing his wife and daughter, he resorts to the previously-unthinkable choice, and goes to work on sawing off his left foot to escape his chains. Tapp pursues Zep out of the Gordons' house, and on a high-speed chase to what appears to be a defunct sewage plant. There, as Tapp and Zep struggle, Lawrence finishes the removal of his foot, retrieves the bullet and the pistol from the corpse on the floor, and shoots Andy. Zep manages to kill Tapp, and enters the room where Lawrence and Andy lie. Zep tells Lawrence that he's sorry, but that he's too late--according to the "rules," he needed to kill Andy by 6. Andy, however, isn't quite dead; Lawrence has only wounded him in the shoulder. Andy crushes Zep's skull with the toilet tank lid. Andy remains chained, however, so it's left to the one-footed and bleeding-to-death Lawrence to crawl for help. After Lawrence departs, Andy searches Zep's body for the key to the chains. Instead of a key, he finds another tape player, which reveals that Zep was simply another victim. He was poisoned, and he had to plan the girls' kidnapping to earn the antidote. The "corpse" on the floor rises, and we learn that the real jigsaw killer is John (Tobin Bell), the terminal cancer patient seen in Lawrence's first flashback. His motive? He wants people to stop taking "being alive" for granted. John turns off the room's light and slams the door shut, re-imprisoning Andy. Lawrence's fate is never shown, but his family is seen safe.
There's just one really annoying logical flaw in all of this--how did Zep know exactly where to go when Tapp was chasing him? John did a very good job of hiding his work, and certainly didn't treat Zep as an accomplice. Bottom line: it was just a trick to keep us believing in Zep's guilt for longer. Shame on you, Mssrs. Wan and Whannell, for pulling a dirty screenplay trick. I might have forgiven you for not letting Andy use Zep's gun to shoot the lock off of his chain, but this is just plain cheating.
I was looking forward to this for so long, too. Oh well, next week = Pixar + Star Wars + Brad Bird = Movie Heaven.