Notes

The film starts with a chase between police and a criminal named Edgar Munsen (Josiah Black). After crashing his car against police cruisers, Edgar runs to a rooftop and is surrounded by police as he holds a trigger in his hand. He asks for Detective Halloran (Callum Keith Rennie), who shows up as the other cops have their guns drawn on Edgar. He tells Halloran that he has to decide who will die, or else Edgar himself will die. Halloran takes out his gun just as Edgar pulls the trigger. The cops blow off Edgar's hand and destroy the trigger, but someone has shot Edgar in the chest against Halloran's orders. A weakened Edgar tells Halloran that the game has begun.

The scene then moves to a room where five people - Anna (Laura Vandervoort), Ryan (Paul Braunstein), Mitch (Mandela Van Peebles), Carly (Brittany Allen), and an unnamed fifth person - are chained to a wall with buckets on their heads. A recording plays from John Kramer/Jigsaw (Tobin Bell), explaining that in order to survive, blood must be given. The chains then start to pull the group toward the wall where buzz saws are sticking out. As they get closer, Anna cuts her arm and is freed. She tells the others to cut themselves to free themselves. Ryan and Mitch follow suit while Carly gets cut from behind. The fifth person is unconscious and realizes too late what to do, and he is seemingly killed.

A woman is jogging through the park when she stops next to a crowd of people gathered by the bridge. She turns around and screams when she sees a body hung from the bridge by the neck with a bucket over the victim's head. The body is brought to the morgue where forensic pathologists Logan Nelson (Matt Passmore) and Eleanor Bonneville (Hannah Emily Anderson) inspect it, just as Halloran and his partner Hunt (Cle Bennett) arrive. Hunt offers condolences to Logan, whose wife passed away recently. The head on the body has been severed in half, presumably by the buzz saw. On the neck is a chunk of flesh cut off in the shape of a jigsaw puzzle. A small audio file is pulled out of the neck, with Kramer stating that now there are only four left.

The four victims are now in a barn with the chains still on their necks. They start getting pulled toward another potential death until Mitch grabs a tape next to the infamous Billy puppet. The chains stop, and another audio recording from Kramer plays. He mentions that someone among them has been injected with a poison, and there are three needles before them - one with the antidote, one with a saline solution, and another with hydrochloric acid. Kramer personally calls out one of them as being a purse snatcher. It turns out to be Carly, who stole a purse from a woman that later died after she suffered an asthma attack and couldn't reach the inhaler in her purse. Carly notices one of the needles has the number 3.53 on it, which is how much she stole from the woman and how much Kramer deems her own life is worth. The chains start to pull the four up toward the ceiling to hang them until Carly takes a needle. Ryan, who grabbed all three, yells at Carly to take one, but she never does. Ryan sticks all three needles in Carly's neck, which releases them from the chains but also causes Carly to suffer a painful, bloody, burning death.

Detective Halloran later finds the body of a woman whose face has been melted off by acid outside a building. Logan and Eleanor inspect the body and see another jigsaw-shaped flesh chunk missing.

The remaining three victims figure that they are there for something wrong that they have done and must confess. Ryan admits to doing a number of bad things like cheating on his wife (and his second). Mitch sold a motorcycle to a young man who later died in an accident. Anna only recalls being with her husband when their baby died, supposedly as a result of him falling asleep and rolling over on top of the baby, smothering him. As they progress toward their next task, Ryan tries to walk out a door with a "no exit" sign, only to have his foot fall through the floorboard and get caught in razor-sharp wires that tighten around his leg. The next tape from Kramer instructs them on how they can get out of this. Anna and Mitch try to reach a lever to help Ryan, only for them to be trapped in a silo that starts to fill with grain. The grain stops pouring, and is followed by sharp falling objects (a rake, knives, needles, buzz saws). Ryan hesitates but reaches for the lever, saving Anna and Mitch but causing the wires to sever his leg.

Edgar has been in an induced coma at the hospital following the gunshot. He then disappears when nobody notices. With the belief that the Jigsaw Killer is still out there, Halloran orders Kramer's grave to be dug up. The police open the coffin to find Edgar's body in there.

Logan meets with Eleanor in a bar. He mentions how he doesn't trust Halloran and that he may have purposely shot Edgar during the initial rooftop standoff. Eleanor later invites Logan to her studio, which is full of recreations of Jigsaw's traps. She admits to having a strong fascination with his work, but also worries that this may implicate her in the recent murders. Hunt, who followed them, takes photos and plans to bring them to Halloran, but Logan tries to persuade Hunt that Halloran is the true culprit.

Back at the barn, Mitch is put through the next test. He is hung up by his foot with wires and is slowly lowered toward a mechanism that would shred him. The next tape has Kramer stating that the young man he sold the motorcycle to was his nephew, and that Mitch willingly took the man's $600 knowing the brakes were faulty, and Kramer's nephew was soon struck by a truck. As Mitch is lowered, Anna tries to stop the mechanism by stopping a motorcycle engine that is powering it. Although she stops it long enough for Mitch to pull the lever that frees him, it turns back on and Mitch is completely shredded. Anna tries to make her escape through an open door, only to get caught.

Anna wakes up in a basement, chained to the wall, as is Ryan, still alive with something on his leg to stop the bleeding. A man in a pig mask is standing there. He removes his mask to reveal himself as John Kramer. He addresses both people as having done horrible things. Ryan was reckless in his youth and caused his best friend to get into a fatal car accident when he tried to stop his drunken antics. Anna, who turns out to have been a neighbor of Kramer's, actually suffocated her baby and planted his body next to her husband to frame him. Her husband later hung himself in prison out of guilt. Kramer places a shotgun with one shell in the barrel, telling them their "key to survival" is now in the middle of the room. After Kramer leaves, Anna rushes to get the shotgun to kill Ryan. She grabs it and pulls the trigger, but it backfires and blows her brains out. Ryan sees the shell having contained a destroyed key to their chains. He weeps knowing he is doomed.

Logan and Eleanor head over to the barn, only for Halloran to find them. Eleanor gets away, but Logan is caught by an unseen assailant.

Logan and Halloran both wake up in a basement with devices around their necks that are rigged with laser cutters. They are standing in the middle of the room with two buttons, with a final tape giving them the choice to press a button that would kill or save the other. Halloran goes first and presses Logan's button. As the lasers start to activate, Logan admits his sin, which is that he was the doctor that mixed up Kramer's X-rays with another patient, meaning his cancer could have been detected earlier. The lasers appear to kill Logan, and Halloran thinks he is free, until he is pulled to the back of the wall, with the lasers activating on his device. Halloran pleads for mercy, admitting that he let innocent people die while real criminals walked free. Logan then rises, alive. He removes the device and walks over to pull off a sheet revealing the long-decomposed corpses of Anna and Ryan. Logan tells Halloran that he was the fifth person in the initial trap, and that it was ten years ago when it happened (making this entire film a half-prequel). Kramer let him live, knowing Logan made a simple mistake, and he took him in as an apprentice and to continue his legacy after his demise. The bodies that were found identical to Carly and Mitch were other criminals that Halloran let go. In fact, one of the criminals he let go was Edgar, who killed Logan's wife. He also let Eleanor live to establish an alibi. With Halloran's confession recorded, Logan starts to leave just as the lasers cut Halloran's head down the middle like a tomato. Logan says "I speak for the dead" before shutting the door.