Modern Warfare 2 doesn't really continue the story of Call of Duty 4. Taking place nearly 5 years after the events of the first game, aside from a intro cutscene showing events that happened during the first game, including the end of the first game, the stealth mission and a couple others, there is no real connection between the titles. You again, play as a member of the S.A.S. and the U.S. Rangers. You meet up with some familiar faces including 'Soap' McTavish and Cpt. Price.
The story is dark, in the sense that its content material is much more mature than that of Call of Duty 4. I don't know if I should discuss this, but if you don't want spoilers, skip this next part. Later on in the game, you fight a battle in a war torn Washington D.C. *SPOILER END!*
The A.I. in this game is pretty dumb and there are no really difficult sequences in this game, such as the Ferris Wheel in Call of Duty 4. You get some variety of mission selection and some new toys to play with. For example, you get to fight on a Hummer machine gun and a Chopper Machine Gun. You also get access to a Predator missile and a heartbeat sensor.
Multiplayer is good (when it works). The problem with playing multiplayer on the Xbox 360, is that all of the n00b's who play Halo 3, have purchased this game as well because their parents won't let them have any other console. The multiplayer game has added an entire new level to it, including death streaks, adjustable kill streaks and medallions.
Where the game falls is in its story. By the time you're into whats going on, its over. Their are also too many filler missions. But you already know if your going to buy this game already so enjoy it you already have it and if you don't, well that's just too bad for you isn't it?