Iron Man, 12A

Whatever the calendar and the weather might suggest, the summer blockbuster season is officially upon us, as heralded by Iron Man. The film has been keenly anticipated, not just because it's the first in a remarkably promising batch of big-budget popcorn-spillers (Indiana Jones and the Dark Knight among them), but because all the signs indicated that it might well be the best of the bunch. It's directed by the capable Jon Favreau (Elf, Zathura), with a screenplay by the co-writers of Children of Men, and its heavyweight cast boasts more Oscar nominees than the average period drama – all of which put it some way ahead of, say, The Fantastic Four. Anticipation was also stoked by a trailer so encouraging that it inspired a perfect headline from the spoof news website, The Onion: "Wildly Popular 'Iron Man' Trailer To Be Adapted Into Full-Length Film: Fans are worried that the feature film adaptation of the beloved trailer won't live up to the original 90-second story's vision."

In fact, the original vision belongs to Stan Lee and Jack Kirby, who co-created Iron Man, among many other Marvel superheroes, in the early 1960s. But Iron Man's alter ego, Tony Stark, skates through life more happily than the nerdy Peter Parker or those paranoid X-Men. As played by a neatly bearded Robert Downey Jr, Stark is a computer genius and playboy tycoon who has amassed billions of dollars by designing military hardware. When he's due to accept an award in Las Vegas in recognition of his achievements, he's gambling big bucks in the casino, surrounded by a bevy of models. And when an attractive journalist asks him how he feels about being labelled the "merchant of death", it only takes him a couple of nonchalant comebacks to get the reporter into his sportscar and back to his Malibu cliff-top mansion. Downey is neither young enough nor imposing enough to be the most obvious casting as a superhero, but, like Johnny Depp in Pirates of the Caribbean, he injects a mainstream movie with offbeat indie insouciance, tossing out offhand one-liners with devilish timing. Tony Stark must be the only superhero alter ego who would have been worthy of a film even if he'd never put on a costume and sworn to fight evil.

But, of course, that's exactly what he does, after he gets a taste of his own bad medicine. While he's in an Afghan desert, demonstrating his latest weapon of mass destruction, he is ambushed by rebels armed to the teeth with Stark's own guns and bombs – a bold bit of irony for a superhero film. The rebels drag him to their cave and coerce him into building a missile, via a torture montage which could have younger viewers wailing. But, instead, Stark welds himself a suit of strength-enhancing mechanical armour, and makes his escape.

Back in the USA after months in captivity, the repentant armourer declares that he's getting out of the weapons business, and then he retires to his laboratory so he can get on with refining his super-suit. This volte-face is a jolt to his sweetly devoted Girl Friday, Virginia "Pepper" Potts (an under-used Gwyneth Paltrow), and to his oldest friend, James Rhodes (Terrence Howard), an air force colonel who acts as Stark's military liaison. But it's particularly galling to Stark's business partner, Obadiah Stane. With a name like that, and with Jeff Bridges barely recognisable as a bald, grey-bearded giant, Stane could well turn out to be less than heroic.

Iron Man is enjoyable, snappily scripted, unpatronising entertainment, but it can't dispel a niggling sense that it's not quite as good as you want it to be. Although it's 10 times more consistent than Transformers, Pirates of the Caribbean 3, and the rest of last summer's blockbusters, it still has too much tongue-in-cheek comic-book excess for audiences who want a political espionage thriller, and too much grim commentary on global strife for audiences in the market for "pows", "blams", and "ker-splats".

It's also vague on what Stark is planning to do with his new identity. His withdrawal from the arms trade is presented as a Damascene conversion, but his first mission in his snazzy eco-skeleton is to zoom back to the Middle East and dish out some "shock and awe" to a gaggle of Taliban-alikes, which doesn't seem all that different from what the military was doing with his weaponry.

Essentially, Iron Man is a film about research and development. Like Batman Begins, it obsesses over how its hero invented his gadgets, so the central chunk has Stark tinkering in his workshop with his robotic assistants. Imagine if the Q scene in a James Bond film lasted half an hour, with Q telling 007 about the prototypes of each underwater car. It's a fun sequence, but if a superhero movie can allot its second act to the calibration of the hero's jet boots, that probably means that the plot could do with some calibration, too.

Mind you, the action is terrific when it comes along. Iron Man's gleaming red and gold get-up is much more flattering than anything in Spider-Man or Superman's wardrobe, and the CGI is incorporated so seamlessly that it's impossible to tell where Downey ends and his digital stunt double begins. All the fight-and-flight set pieces are choreographed with clarity and pizzazz, too; it's just a pity that there aren't more of them. You're left thinking that the film isn't so far from the original 90-second trailer, after all. It's there to whet our appetites for the sequel, like a scene-setting pilot episode of a TV series. The proper adventures of Iron Man, it seems, won't get going until the summer blockbuster season of 2010.

Jonathan Romney is away