You would be better off watching it to get the full effect but here is a brief plotline...
Jigsaw allows himself to be captured by a police detective and tells him he wants to play a game. He has kidnapped the policemans son and if the cop talks to Jigsaw for an unspecified time, he will have his son returned.
The policeman can see his son on a computer monitor in a house with many other people and they are told that a nerve gas is slowly killing them and the antidotes are all over the house but the key to the front door is locked in a safe in the centre of the room they are in.
In typical Saw fashion they all start to die in one way or another. One of the people in the house is Amanda from the first movie (the girl with the cage on her face who has to kill the guy to escape). You see her and the cops son escaping from one of the other people in the house and they end up in the basement. The basement is from the first movie with the two guys in the room.
Long story short.. Amanda is Jigsaws accomplice and the footage that the cop can see is on a tape and the boy is safe. The cop does try and kill Jigsaw at the end of the movie though.