Where has Robert Downey Jr. gone? There's no doubt he’s the star of Iron Man 3; he sprints through the picture like a neurotic panther. And yet he's curiously absent, detached in a Zenlike way from the whole affair. The nakedness that defines his best performances has become, paradoxically, a kind of mask, not unlike the sleek, airbrushed-looking one he wears as the superhero incarnation of cocky kajillionaire Tony Stark. Today, Downey could play Stark in his sleep. The jittery self-doubt, the look-at-me hubris, the Boy Scout cluelessness about women: He's become so proficient in his believability that you can hardly believe a minute of it. Maybe you don't need to believe much in Iron Man 3. This is the first in the franchise to be directed by Shane Black, and only the second picture the prolific action screenwriter has made. (The first was the marvelously nerve-jangling Kiss Kiss Bang Bang, also starring Downey.) On the plus side, Black has a puckish sense of humor, and shows a healthy resistance to the comic-booky self-seriousness of the Batman movies. The villains in Iron Man 3, for example, include the Mandarin, a pointy-bearded sage who’s half Osama bin Laden, half Ming the Merciless. He's played with bug-eyed hamminess by Ben Kingsley, and the movie is spooky, silly, or both whenever he's onscreen. But the big problems with Iron Man 3 are less specific to the movie itself than they are characteristic of the hypermalaise that's infected so many current mega-blockbusters-- too much plot, too much action, too many characters, too many pseudo-feelings. The mechanics of Iron Man 3 are complex and rambunctious, like Keystone Kops, bouncing off one another and ultimately canceling one another out.