NOTE: This spoiler sent in by Luke.
The movie starts with Seth in the pendulum trap. The aim of the trap is for Seth to push his hands into a device that rips them off, or have his body torn apart by a huge swinging axe. Seth get's his hands ripped off, but that still doesn't stop the machine as it rips him in half. We see someone watching this through a peephole in the wall.
Saw V appears on screen
We now cut to the end of Saw IV, where Agent Strahm shoots Jeff and enters the sick room. After investigating and finding Jigsaw's dead corpse, the door is shut and locked on Strahm. Strahm now exits the sick room through a secret door, but get's attacked by someone in a pig mask.
Strahm wakes to find his head in the head box trap, the box quickly starts to fill with water but Strahm sticks an empty pen into his neck and is able to breathe.
The police (Led by Fisk) make it to the Gideon warehouse (where all the mayhem is happening). Hoffman emerges carrying Corbett; he claims that he saved her. Strahm is carried out on a stretcher, still alive but badly injured.
The next day the police hold a service for all the people who have died chasing Jigsaw. Tapp, Rigg, Eric and Kerry's photos are shown. The Chief Of Police officially says that the Jigsaw murders are over.
We cut to Jill; She is given a box, which Jigsaw saved, for her before he died. She looks inside and is disgusted. She storms out of the room carrying the box.
Strahm is shown mourning next to an empty hospital bed; it turns out Perez died in the night. Strahm has a bandaged neck. Hoffman walks into the room; Strahm starts to act strange and says that Perez's final word was Hoffman. Hoffman says he has no idea what Strahm is talking about.
Hoffman heads back to his office and find a note for him saying "I know who you are". Hoffman get's worried that the secret: he is the new Jigsaw, may get out.
We cut to five people waking up in a sewer (Ashley, Charles, Brit, Mallick and Luba); they are in the necktie trap. In this trap they must push against a rope to get a key at the end of the room to release themselves, if they do not do this in 3 minutes the rope will yank them back into some sharp metal and they will get their heads chopped off. Mallck gets anxious and runs ahead (starting the timer). In the end, the whole group apart from Ashley get their keys. Ashley in eventually yanked back and gets her head chopped off in bloody fashion. The group walk through a door to another room.
Strahm starts to go mad and heads over to FBI headquarters to find some files on Hoffman. He finds a file saying that Hoffman's sister was murdered, and her killer was killed in a Jigsaw trap (The killer is Seth). Strahm comes to the conclusion that Hoffman killed Seth and made it look like a Jigsaw trap. Strahm leaves with the files.
We now cut to a flashback of Hoffman looking at the pendulum trap, as he was the person looking through the peephole at the start of the film. A couple of weeks after this Hoffman is kidnapped by the real Jigsaw. Jigsaw pretty much blackmails (even though he says otherwise) Hoffman into working for him.
We now view shots of:
-Hoffman Setting up the Saw II house trap
-Hoffman talking to Jigsaw before Saw III
-Hoffman and Jigsaw kidnapping Paul from Saw I
By the end of this Hoffman is a full apprentice of Jigsaw.
We now cut back to present day as Charles, Mallick, Luba and Brit make it to the next trap. In this trap, the group have to smash jars and find keys to enter bomb shelters before a bomb goes off in the sewer. Charles turns badass as he smashes Mallick and tries to go for a key himself, in his madness he doesn't actually see Brit and Luba pick up keys. A hurt Mallick attempts to pick up a key but Charles steps in his hand and takes it. Just as Charles is about to enter his shelter, Luba hits him with a pole and gives his key to Mallick. The trio enter their shelters and leave Charles to explode.
Strahm is now getting more sure that Hoffman is really the new Jigsaw, as he investigates the razor wire trap crime scene from Saw I.
Luba, Mallick and Brit make it to their next test. In this test, they have to find a way for a cord of electricity to make it into a bath of water; the cord is not long enough to reach the bath, so the group think they need to use a person's body. All of a sudden Luba tries to use Mallick's body, but Brit stabs her in the neck and claims she never trusted her. Mallick and Brit use Luba's body for the electricity to reach the bath, it works and the door opens to their final test.
Hoffman plants Strahm's phone at the house where Brit and Mallick's game is being held, in a hope to frame him.
Strahm is now fully sure that Hoffman is the new Jigsaw, as he tracks Hoffman to a house.
Brit and Mallick make it to their final test. In this test they have to stick their hands in a saw, and bleed into a beaker. When the beaker fills with their blood, they will be free to go. Before they start the test they realize that there are five saws, they then find out that everyone could have made it through every test seeing that...
-The keys fitted in every lock in the first trap
-The bomb shelters had room for two in the second trap
-The electricity could have just flowed through everyone a small bit in the third trap
If everyone had made it through every test, the final one would've been much easier.
Never the less, Brit and Mallick begin sawing their own arms and try to fill the beaker with their blood.
While this is happening, Strahm finds the glass box trap from Saw IV in the house he tracked Hoffman to. He also finds a tape attached to it, telling him to enter it if he wants to live, Strahm just takes this as another Jigsaw game.
Just as Brit and Mallick fill the beaker with their blood and finish the test, a police officer named Erickson makes to the scene. Both pass out from blood loss as Erickson calls for backup. After, Erickson finds Strahm's phone at the scene (which Hoffman planted earlier) and comes to the conclusion that Strahm is the new Jigsaw.
Hoffman walks into the room with the glass box, but doesn't see Strahm anywhere. Strahm then comes from behind and shoves Hoffman into the box. Hoffman signals for Strahm to play another tape, which is lying on the floor.
*Hello Zep starts to play*
The tape tells Strahm that he made the wrong choice, and he is going to die and not live as promised. The walls begins to close in on the room as the glass box lowers into the floor (with Hoffman safe from the walls). Strahm attempts to get out, but is brutally squished.