Yes, I've seen it. Basically:
Tony Stark/Iron Man challenges a terrorist called The Mandarin to a fight when Stark's old pal Happy Hogan falls into a coma as the result of an explosion Stark thinks The Mandarin caused. In the meanwhile, a scientist Stark blew off thirteen years ago is back with a project called Extremis, which turns its subjects into something akin to nuclear-powered human weapons with rapid regeneration ability and super strength. After Mandarin blows Stark's house up, Stark finds himself crash-landing in the middle of nowhere, where he's left with a damaged and de-powered suit, and discovers that the explosion he thought the Mandarin caused was actually an Extremis soldier who malfunctioned, so he sets off to take down both The Mandarin and the scientist behind Extremis.
That's the nutshell version.