Last summer, Transformers proved that if you make a movie about giant robots smashing each other and aim it squarely at 11-year-olds, millions of adults will come along for the ride. Given that precedent, it’s a pleasant surprise to see Iron Man, a comic-book-derived movie about giant robots smashing each other that’s not dumb. While it appeals to the inner child with action scenes that defy the laws of physics, Jon Favreau’s film also offers humor that doesn’t involve bodily fluids, and character development that doesn’t center on a winsome adolescent’s coming of age. Could it be . . . a superhero movie for grown-ups?

As weapons developer Tony Stark, Robert Downey Jr. is definitely no kid — he looks weathered — but it would be hard to call him mature. An engineering genius who inherited war-profiteering billions, Stark spends just enough of his time tinkering in his basement lab and the rest of it cavorting like Hugh Hefner. He offers no apologies for his profession: “I like the weapon you only have to fire once,” he crows, before showing his U.S. Army clients a device called “Jericho” that lays waste to what appears to be an entire Afghan mountain range. But he’s not so smug after being captured by a multinational terrorist gang who want him to make them a Jericho of their very own.

Re-enacting the origin story from Stan Lee’s 1963 “Iron Man” comics — where the setting was Vietnam — Stark busts out by instead creating a big metal suit that enables him to hit things really hard, throw flames and, yes, fly. And a new superhero is born — sort of. Besides crafting an improved suit that won’t crash and burn, the liberated Stark has to grapple with what he saw in Afghanistan — bad guys stockpiling his weapons. When he tells the press he wants out of a “system that’s comfortable with zero accountability,” his company’s stocks plummet, and Stark’s affable mentor and father-figure, Obadiah Stane (Jeff Bridges), gets a bit perturbed. He also gets way too interested in Stark’s pet project.

Despite the crisis-of-conscience theme, all of this could have been pretty standard fare. But Favreau had the sense to cast Downey, who does for this potential franchise what Johnny Depp did for Pirates of the Caribbean. Both of them are actor’s actors who love to exploit the inherent silliness of their material, yet know when to give it dramatic weight. Downey’s standard mode is warm, disarming, a bit fey on occasion — if you left him alone with a wooden post, he’d probably flirt with it. (When no one’s around, Stark banters with the robots in his basement.) Played by a more solemn actor, the golden-boy character could have been insufferable. But Downey makes us believe Stark charms everyone around him, including his straight-arrow Army liaison (Terrence Howard) and his long-suffering Gal Friday, Pepper Potts (Gwyneth Paltrow).

Paltrow rises to the challenge of playing a superhero’s eye candy/potential love interest by not acting like an ingenue. Pepper has worked for Stark long enough to be unfazed by his antics — she’s like a sexier Moneypenny — and the two actors ease into a subtle screwball chemistry. In one scene, they find themselves getting a bit too close and then talk about it, as adults in a professional relationship might do, rather than simply fleeing in mortification, like middle schoolers and bashful movie superheroes the world over. The strong script — reportedly supplemented by improvisation from Downey — keeps the wit coming and the movie buoyant, even when it heads into a very predictable climax laden with heavy-duty CGI.

Stark’s superhero garb is lacking in personality, to say the least — the slit-eyed suit looks like a cross between the Futurist robot in Metropolis and Boba Fett. But its simplicity works. And, more important, the movie never lets us forget there’s a screwy, disreputable, ultimately decent guy inside.