File this one under the category of "Sequel That Didn't Need To Be Made," Saw II is a yawn-fest with a puzzle that only the most brain-dead audience member will not be able to figure out.
(Spoilers below, and I'll probably ruin the entire movie for you as well, since I hated it so much, which includes revealing the ending, so don't read if you want to see it. Don't say I didn't warn you!)
I am a fan of the original Saw, released in 2004 and directed by James Wan. It was a creepy little film that came out of nowhere (and filmed in 18 days, according to the Internet Movie Data Base) with an interesting (if overused) premise: two men awaken in a filthy industrial bathroom, with no memory of how they ended up in said bathroom, chained to the walls. To free themselves, lots of blood will have to be shed. What followed was some hilarious overacting by Cary Elwes and lots of blood and gore.
(Now, I warned you, I'll be spilling the beans, so stop reading if you want to be left with some surprises.)
Now we have Saw II, directed by newcomer Darren Lynn Bousman, bringing back the Jigsaw Killer from the first film (Tobin Bell), who is dying of cancer and has captured a bunch of people and locked them into a room. Poisonous gas is slowly being pumped into the room. The group is informed that they have only a couple of hours to live, and they have to figure out some puzzles in order to free themselves. Jigsaw tells everyone to follow his instructions explicitly, and of course no one does, because this is one of those films where everyone is incredibly stupid, like the guy who finds a note with a key. The note says "do not use this key to open the door" and the guy is all like "Freedom, sweet freedom!" and of course uses the key and a gun mounted on the other side of the door blasts a large hole through his head.