The full story of Saw (2004) runs on two tracks, the bathroom in the present and a set of flashbacks that explain why these two men were chosen. Spoilers follow for the whole film.
In the bathroom, each man has a tape with instructions. Lawrence has until 6:00 to kill Adam, or his wife Alison and daughter Diana will be murdered. A flashback shows Lawrence questioned by detectives Tapp and Sing, who found his penlight at a Jigsaw crime scene. He was cleared, but he had seen the testimony of Amanda, a rare survivor of Jigsaw's traps. Tapp and Sing later found Jigsaw's warehouse, where Jigsaw escaped after badly wounding Tapp, and Sing died in a shotgun trap.
In the present, Alison and Diana are held captive in their apartment while the captor watches the bathroom on a hidden camera. Adam admits that Tapp paid him to spy on Lawrence, and Lawrence reveals that he had an affair, which is why he was chosen. Adam recognizes the captor from a photo as Zep Hindle, a hospital orderly.
When Lawrence does not kill Adam, Zep moves to murder the family. Alison fights back, and Tapp arrives and saves them. Tapp chases Zep into the sewers and is shot in the chest. Back in the bathroom, Lawrence hears only gunshots and believes his family is dead. He saws off his own foot and shoots Adam. Zep then enters to finish Lawrence, but Adam survived the shot and beats Zep to death with the toilet tank lid. Lawrence crawls out to find help.
Adam finds a tape showing that Zep was also a victim, forced to obey Jigsaw to get an antidote to a slow poison. Then the supposed corpse stands up and is revealed as John Kramer, the real Jigsaw. He tells Adam that the key to his chain went down the drain when Adam emptied the tub. Adam tries to shoot him, but Kramer shocks him through the chain, leaves and seals the door, abandoning Adam to die.
