Convicted murderer Seth Baxter wakes up chained to a table beneath a pendulum blade. A videotape informs him that he can release himself by crushing his hands between two presses. He does so, but the blade still swings down and cuts him in half as someone watches through a hole in the wall.
In the meatpacking plant, FBI Agent Peter Strahm guns down Jeff Denlon and is locked in the sickroom. He discovers a hidden exit and a microcassette recorder, which urges him to stay in the sickroom, but he ignores it and is attacked in the tunnel by someone in a pig-mask. He wakes up with his head sealed in a box being filled with water, but survives by performing a tracheotomy with his pen. Outside, Detective Mark Hoffman delivers Corbett Denlon to the police and claims they are the only survivors, but is shocked when Strahm is brought out alive as well.
During a press conference, Hoffman is promoted to detective lieutenant and credited with closing the case. He finds a note in his office that reads “I know who you are” and learns of Lindsey Perez’s death while taking Strahm’s phone. At the hospital, Strahm tells Hoffman that Perez’s last words were his name, and questions how he escaped the plant. Strahm is also visited by his boss, Dan Erickson, who puts him on medical leave. Now suspicious of Hoffman and determined to piece together his involvement with Jigsaw, Strahm takes case files of past victims and begins researching them on his own.
In an underground room, five people awaken with collars connected to mounted blades locked around their necks, the keys to which are in glass boxes across the room. A videotape claims that all five of them are connected and implores them to “do the opposite” of their instincts throughout the game. Mallick, an arsonist, activates the timer and everyone reaches their keys except for Ashley, a former fire inspector, who is decapitated when the collars retract. In the next room, they learn that the overhead jars contain keys to three bomb shelters. Charles, an investigative journalist, strikes Mallick and begins smashing the jars one by one. Brit, a real estate vice president, and Luba, a city planner, each find a key and Charles takes Mallick’s, only to be struck by Luba and left to die when the bombs detonates. In the third room, five short cables must be connected to a full bathtub to unlock the door. Luba attacks Mallick to use his body to close the circuits, but Brit stabs her and they use hers instead. In the fourth room, they find a machine fitted with circular saws and a beaker requiring ten pints of blood to open the door. Mallick notes the five armholes and they realize that all five of them were meant to work together to survive the game. Brit also works out their connection: they were all involved with a building fire that killed eight people. With no other options, they saw their arms to provide the blood.