You all know Sam Fisher. Muscle-bound guy, voice like a moose with a hangover and one of the leading men in video games thanks to his acrobatic sneaking, versatile gadgets, and witty knifepoint interrogation banter? Well, he's at it again in Splinter Cell: Double Agent. The excellent, albeit very familiar, stealth action gameplay is spiced up by the fact that Sam now has to serve two masters, juggling opposing objectives in order to maintain their trust - and stay alive.
Trust is the crux of the gameplay in Splinter Cell: Double Agent, and the main factor that keeps this game feeling like a fresh entry in the series. Fisher has to remain a trusted member of both the National Security Agency and the JBA - the domestic terrorist organization that he's infiltrated on "the most dangerous mission of his career." This is measured by the Trust Meter, a sliding scale with the NSA and JBA on opposite ends.
Almost all of your actions, from triggering a laser tripwire to killing a mercenary, affect the meter, which in turns affects your situation. Kill an inmate who threatens your terrorist buddy, and that buddy will give you a gun to use while you two escape. But the NSA won't be happy about the killing, so your old buddy Lambert won't be able to get you all the weapons you'll want for your next mission.
Decide instead to let the inmate live, and your terrorist amigo won't trust you with the gun, leaving you to get creative on the armed guards. Makes things trickier, but you'll get hooked up by Lambert for your next mission. And so on. You have to play both sides, too - let your trust slider get far to one side, and it may very well be game over.