This title uses a modified version of the Unreal 2.5 engine, and can be a little taxing on some systems. On the plus side, I'm running Windows 7 64-bit beta, and this game runs just fine on that OS.
In this title you return to the scene as Samuel "Sam" Fisher, expert at stealth and espionage, star agent for the NSA's "Third Echelon". The game begins where Chaos Theory ends, and you start off being flown to an arctic facility where you are to investigate "suspicious activities". In this mission you are working with a rookie agent named John Hodge. This mission ends in a less-than-optimal fashion, and is quickly followed up with a cut-scene involving sad news on a personal level for our man Sam. After this the game REALLY begins. The story takes you through a fairly high energy ride involving professional and personal losses, prison break, secret agendas, betrayals, nuclear chemistry, and story defining decisions.
Double Agent adds a few new game-play mechanics to the series, and it is HIGHLY recommended that you utilize the training exercises before beginning the story, even if you're a veteran SC player. Like Chaos Theory before it, Double Agent provides cooperative two-player missions which make references to the single player storyline, and are also referenced BY the single player story. The multiplayer VERSUS version of this game restricts the spy player more that previous titles, and was not as widely accepted as the versions in SCPT or SCCT.
All in all this is a sound addition to the Splinter Cell series, and I highly recommend it.