‘Iron Man 3’ loaded down by heavy metal

Starring Robert Downey Jr., Ben Kingsley, Gwyneth Paltrow

Rated PG-13 fpr sequences of intense sci-fi action and violence and brief suggestive content. At metro theaters. 2 hours, 10 minutes.

Bottom line: Heavy metal action weighs down light joy

In the galaxy of big-screen superheros — a rather glum lot — Robert Downey Jr.’s Iron Man is the snappy one.

He’s the sarcastic, motor-mouthed, preening, self-referential do-gooder, as opposed to all those self-serious crusaders. No matter how much of a scrap heap of metal-twisting mayhem the franchise piles on (and it’s a lot), Downey’s sheer charm — his unsentimental, offhand yammering — is the only real super power in Marvel’s “Iron Man” trilogy.

“Iron Man 3” follows not just “Iron Man 2” but the box-office busting “The Avengers,” in which Tony Stark, aka Iron Man, joined forces with other superheros. These global blockbusters are more produced than directed, but it’s nevertheless particularly fitting that Shane Black here inherits the helm from Jon Favreau, the director of the previous two.

Black’s film, more than any other “Iron Man,” is stuffed with this self-aware, winking style. This includes loads of references to “The Avengers,” an experience from which Stark has developed panic attacks and sleep-depriving nightmares.

He is pulled into a confrontation with a terrorist named Mandarin (a bearded Ben Kingsley), who, in hijacked broadcast transmissions, takes credit for public explosions that, in a movie such as this, chafe awkwardly in the wake of the Boston Marathon bombings. When reporters mob Stark for his response after an explosion puts his friend and bodyguard (Favreau, looking happily unburdened) in the hospital, Stark swears vengeance and brazenly supplies his home address for a fight.

When helicopter missiles collapse Stark’s Malibu estate into the sea, his companion Pepper Potts (Gwyneth Paltrow) is separated from him, and his damaged computer operator Jarvis (voiced in formal British by Paul Bettany in a manner not unlike the classic butler Jeeves) rockets Stark to Tennessee.

With a damaged suit and grasping at leads on the bombing, Stark has to rebuild himself, which he does with the help of a mop-headed, fatherless boy (an excellent Ty Simpkins). Tennessee isn’t an accidental landing spot, but a preprogrammed flight to a location where Stark begins to learn what’s behind the bombings.

Downey and Simpkins make a good team playing a kind of mock-Spielbergian pair, and it’s the best and most natural part of the movie.

The “Iron Man” films have always played in the world of the military industrial complex, one where the guys with the fancy weapons control the world more than politicians. Soldiers, and even terrorists, are merely pawns in a larger corporate battle.

But within “Iron Man 3” is a fight between screwball irony and blockbuster bombast. The script, by Black and Drew Pearce, contains the best dialogue of the series. But the wisecracking begins to feel suffocated under the weight of a whole lot of action, more Iron Man suit changes than Beyonce would even dare, and the lumbering machinations of a plot that closes in on a lengthy oil rig finale as if pulled by magnetic force.

This is the first “Iron Man” in 3-D, and the darkening effect is particularly disappointing for what’s been a bright-hued franchise. The action scenes, too, are cut too quickly so that your eye often feels like it’s racing to catch up. The heavy metal action could never sink the irrepressible Downey, but it weighs down the otherwise light joy of “Iron Man 3.”