Some ultranationalist political party wishing to restore Russia back to its former "glory" during the Cold War teams up with some militant, Middle-Eastern group and help them take over a country as a diversion for a whole load of nukes they're preparing to use against the Western world. First you control the president of the Middle-Eastern country as you're executed by the revolutionaries, then you do your military training as a special forces soldier named Soap, then Soap and his friends raid a Russian ship and discover their nukes.
Then you jump back and forth between Soap and his team trying to get their hands on the leader of the Russian ultranationalists and a three different Americans kicking ass, blowing stuff up, taking over TV stations, getting nuked and helping ground forces from air planes. Finally Soap and his friends get the the Russian ultranationalist leader's son, who turns out not to be the talkative type -- commiting suicide. Then you flashback fifteen years and play Soap's boss on a mission trying to assassinate the Russian ultranationalist leader. Then the Russian ultranationalist leader somehow finds out his son is dead, and fires some nukes, and then you have to fight yourself through the Russians base, disable the nukes, and then drive out of the place while it explodes behind you.
The Russian bad guy is naturally quite pissed off at this point and chases after you in a car. After a short car chase something explodes and you end up on your back with all your mates unconscious. The Russian bad guy approaches you to finish you up, but as he does, your commander throws you his gun, you kill the Russian main bad guy and peace is once again restored.
Then you fight yourself through this air plane or something, and parachute out of it for really no explicable reason.
In the sequel, some new Russian bad guy has taken over the ultranationalist party, and Soap has been assigned as the commander of a new special task elite force.
Also, The Russian antagonist in COD4 kept an underling in check, this underling ,aka Vladmir Makarov, is the main villain in MW2 (most likely, the way the trailers portray him). Since Soap killed Zakhaev (The COD4 antagonist), Soap is the catalyst to whatever Makarov will do in MW2.
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