The Shane Black-directed sequel will pick up soon after the events of The Avengers, which have left Stark more aware than ever of his comparative weakness.
"It's almost like post-traumatic stress [disorder]," Feige told MTV. "He has retreated after the events of The Avengers into his workshop, where he's building advanced versions of his suits.
"He has [designed] a version of the suit that can latch onto him in individual pieces, anytime, anywhere. He starts to realise [that] he doesn't want to be out of the suit.
"He's seen a lot of things in Avengers and has encountered a lot of powerful people, much more powerful than he is."
The sequel will see Stark seeking vengeance on those who have "destroyed his private world", including Ben Kingsley's villainous Mandarin who Feige described as "relentless... a non-stop threat".
"Much of the movie is Tony in the middle of the country without his tools and a fairly broken suit to help him," Feige continued. "But that's his superpower: his power is his brain. It's fun to put Tony Stark in a corner with nothing and see how he can get out of it."
Feige has previously stated that, despite the dark tone suggested by the synopsis and teaser trailer, Iron Man 3 "is not a serious movie".
Iron Man 3 will be released in the UK on April 26, and in the US on May 3.
Watch the teaser trailer below: