Another May, another big comic-book movie designed to start a franchise. If the tactic worked for “Spider-Man,” who’s essentially the Marvel equivalent of Superman, can it send the high-flying “Iron Man”—who’s sort of its Batman figure in metal instead of spandex—into the boxoffice stratosphere too? The answer is, probably, but not quite as high. Not because Jon Favreau’s movie isn’t a perfectly good variant of “Batman Begins,” with which it shares its basic plot. But because the character of Iron-Man just isn’t all that interesting. (In contrast to his alter-ego Tony Stark, that is.)

I-M, to shorten things a bit, has a long history with the publisher; the character first appeared in 1963 in Tales of Suspense, and then graduated to his own title in 1968, running pretty continuously since then, though with a couple of reboots. Despite the longevity, however, he’s never been a top-tier costumed hero. And though he’s been given A-list treatment here, with a major star in the lead, an impressive supporting cast and state-of-the-art effects, “Iron Man” can’t escape being a little—dare I say it?—tinny.

The movie’s major strength is Robert Downey, Jr., who brings his customary quirky, don’t-give-a-damn persona to Stark, the billionaire arms maker whose traumatic near-death experience leads him to renounce the idea of profit through unprincipled weapons sales and use his scientific know-how to combat evildoers encased in a state-of-the-art metallic suit that allows him to fly and gives him massive firepower. (He quite literally has a change of heart, given his injuries.) The original scenario traced the transformation to his capture by Vietcong and his construction of the first, primitive outfit to the need to escape them. The script holds to the same basic explanation, but updates it to present-day Afghanistan—though to minimize the potential for jingoistic exploitation (not entirely successfully), it identifies the bad-guys not as Islamic fundamentalists but as generic nasty warlords without any noticeable religious affiliation.

The remainder of the picture is largely taken up with Stark’s fabrication and testing of a far more advanced and stylish combat suit, his mopping up operation against the Afghan nasties who’d captured him and are now using his firm’s weapons in their brutal operations against the locals, and—most importantly—a struggle over the future of his company with his long-time partner Obadiah Stane (Jeff Bridges), a plot turn that allows for corporate skullduggery and amoral capitalism to be added to the mix. Along for the ride are his loyal, long-suffering and obviously infatuated secretary Virginia “Pepper” Potts (Gwyneth Paltrow) and James “Rhodey” Rhodes (Terrence Howard), the straightlaced Pentagon liaison he loves to twit. The obligatory final confrontation is between Stark’s Iron Man persona and a another supersuited type, bigger and more powerful than he.

This is all pretty much standard-issue origins stuff, raised somewhat above the merest mediocrity by Downey’s laid-back charisma and some witty banter (some of which he must have improvised). Downey’s great fun in the role, even though his performance is basically a vaudeville routine that never invests Stark with any real emotional depth. Howard and Paltrow are fine, too, if very much relegated to second-fiddle status, and while Bridges is overindulgent (and what’s with the baldpate Daddy Warbucks look?), presumably Stane is supposed to be larger than life. There are also a few jovial moments involving a robot assistant in Stark’s lab that he perhaps unwisely assigns the task of fire-prevention duty. (On the other hand, cameos by CNBC’s “Mad Money” guy and the sadly inevitable Stan Lee would have been better excised. Lee has certainly replaced Larry King as the most overused celluloid whore on the planet.)

But the movie is undercut by big action sequences that frankly disappoint. The initial assault on the convoy carrying Stark through Afghanistan is a solid piece of ordinary military combat, but the one in which the billionaire escapes in his makeshift suit is pretty feeble as far as these set-pieces go, as is his return trip, where he saves some brutalized villagers from the warlord’s troops. Nor are the flying sequences, in the longest of which he’s pitted against some fighter jets, especially impressive, particularly when they’re compared, for instance, to the far more elegant ones Bryan Singer contrived for “Superman Returns” a couple of years back.

But they’re still superior to the mano-a-mano combat between I-M and his bigger brother in the final reel. That’s obviously intended to be the picture’s piece de resistance, but it’s much too reminiscent of the clattering chaos of “Transformers.” (Actually, it resembles nothing more than a more opulent version of the clash of monster machines that closes every “Power Rangers” episode ever made.) The sequence is decently fashioned by John Nelson’s special effects team, but it’s rather messy and blurred, and like the other fly-and-fight moments, it never really enthralls.

Given all that, it’s probably a blessing that these action sequences in “Iron Man” are relatively few and far between; much more of the film is devoted to Downey’s human shtick, and to tell you the truth, he makes it much more enjoyable than the parts in which he’s supposedly encased in his titanium costume.

It remains to point out that the movie is a handsome production, with an elaborate production design by J. Michael Riva and sets by Ernie Avila and Noelle King, classy art direction supervised by David Klassen, and impressive widescreen cinematography by Matthew Libatique, which uses L.A. landmarks like the Disney Music Hall. And you have to respect the work of Favreau, who, along with editor Dan Lebetal, maintains a sprightly pace—though even here one senses that Downey is really the determining factor.

“Iron Man” reflects the middle-grade level of the original comic character. It doesn’t belong with the bombs of the comic-movie genre: it’s no “Catwoman” or “Daredevil.” But it doesn’t reach the heights of the best of them: it’s no “Spider-Man 2.” Thanks to Downey, it’s a decent popcorn flick, but nothing more. It doesn’t crash, but it doesn’t soar, either.