In Splinter Cell: Chaos Theory you need to follow the main objectives without being detected by the enemy. You'l also have a few bonus objectives each level which you don't need to complete, but they give you a nice reward for after you beat the game.
In this game you play as Sam Fisher, and you are walked through alot of the parts through a com-unit.
Throughout Splinter Cell: Chaos theory your objective is to stay in the shadows and stealthily make your way through the levels.
If you succesfully finish all the bonus objectives you can unlock "costumes" for Sam Fisher to wear when you replay levels in the game, and cheats such as unlimited ammunition and health.
This game also has up to 4-player Co-Opertive mode to play nine extra levels with your friends. Plus a versus mode where the mercaneries need to protect the computer terminals against the spies which have to hack them without being found by the merceneries.