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. Yet he's absent, detached in a Zenlike way from the whole affair. The nakedness that defines his best performances — in any role, up to and including the first Iron Man (2008) — 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.

In the first Iron Man, Downey's performance wasn't so far from the one he'd given years before in Chaplin. Both characters shouldered enormous egos surrounded by shells of fragility — you had to love the whole egg. But today, Downey could play Stark in his sleep. The jittery self-doubt, the look-at-me hubris, the Boy Scout cluelessness about women: He can dial up whatever he needs. 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 he 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 on-screen.

But the big problems with Iron Man 3 are less specific to the movie itself than they are characteristic of the hypermalaise that has 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.

The movie opens with a flashback to 1999, before Tony Stark suffered the near-fatal injury that led to his becoming a superhero. He's a playboy having a fling with a nerdy-gorgeous scientist (played by a beguiling but underused Rebecca Hall) who has discovered a breakthrough nanotechnology that, in the wrong hands, can turn human beings into flaming weapons. The wrong hands, it turns out, belong to Guy Pearce's sleazy-debonair Aldrich Killian, who, in the movie's present, plans to use this fancy science to kidnap the president.

Along the way Killian tries to seduce Stark's now–live-in girlfriend, Pepper Potts (Gwyneth Paltrow). He may have a shot: It's Christmastime, and Stark's gift to her is a stuffed rabbit the size of a water tower. For this and other reasons, Stark is banished to the bad-boyfriend doghouse, though he claims to have a good excuse for his erratic behavior: He's a sleepless mess, self-tortured to the point of exhaustion, and he can concentrate on nothing beyond adding to his collection of increasingly sophisticated Iron Man suits — he's become Flash Gordon's answer to Pierre Cardin.

Stark will need those suits to fend off a fusillade of special effects, including but not limited to bad guys whose flesh glows fiery orange, the usual explosions of increasing intensity, and the destruction of Stark's futuro-groovy seaside home and suit-building atelier. The gratuitous grandness of these effects shouldn't be underestimated: They're big, all right, and the finale includes a soaring phalanx of Iron Men that's at least momentarily awe-inspiring.

Yet all of this adds up to so little. (To the extent that directors of photography even matter in superhero movies anymore, Iron Man 3 was shot by John Toll, and it looks generically extravagant.) There's no real drama here, or if there is, Black gives us no time to let it sink in before moving on to the next cataclysmic signpost. The picture's climax, especially, strives for emotional grandeur: Black and Downey even toy with the idea of a tragic ending.

But any despair we might feel is quickly and cheerfully reversed. Nothing in Iron Man 3 is all that dark or serious, despite Downey's autopilot efforts to prove how dark and serious his character really is. Stark broods, he wisecracks, he offers throwaway advice to a winsome kid (played by Ty Simpkins) whose father has split: “Dads leave; there's no need to be a pussy about it.” Downey's firecracker dialogue sometimes feels improvised — maybe it is — and it's often bitterly funny. To respond to his vulnerability is to thrill to his sharpness as well; keeping up with him is much of the pleasure of watching him.

But Downey may have taken Tony Stark as far as he can. He's as fine an actor as we've got, always alive and receptive without being too precious about it. Which is why it's strange to watch a Downey performance and to feel nothing. He has a great deal of swagger and perhaps more physical agility than ever. But perhaps even he is exhausted by the too-muchness of Iron Man 3 and its ilk. Or maybe it's time for those of us who adore him to face facts: It's too hard to love a man who dresses like a toy.

IRON MAN 3 | Directed by Shane Black | Written by Drew Pearce and Black | Walt Disney Studios Motion Pictures | Citywide

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