Robert Downey Jr steels, sorry, steals the show in this latest cinematic plundering of the Marvel comics universe. He serves up swagger and swank and arch one-liners in place of the earnestness of Spider-Man, or DC’s dour Batman. His charismatic performance holds Jon Favreau’s film together when it threatens to lose its way between the crash-bang set pieces.

Initially, Downey Jr is Tony Stark, a Scotch-swigging, philandering, billionaire arms manufacturer. Wounded by his own ordnance and captured in what is clearly Afghanistan — though Favreau keeps the politics deliberately vague — Stark escapes by building a suit of robotic armour around a generator implanted in his chest to keep his injured heart going. Back home he starts to question his trade, and builds a new suit with which to destroy his mis-sold armaments. He’s hard on the outside, soft inside, and we know his heart’s in the right place because we can see it glowing, like ET’s.

Fortunately, Downey Jr’s Stark doesn’t stop being flippant or flash. There are delightfully witty scenes of him trying out his armour like a new sports car, observed only by two dumb robots and a sarcastic electronic butler. Thank goodness for a hero whose flaws are arrogance and cockiness, because elsewhere, the human element is lacking. When Gwyneth Paltrow and Jeff Bridges appear as Stark’s assistant-cum-love interest and his mentor-cum-nemesis, the film slows noticeably.

While Bridges just happily chews cigars and the scenery, the growing intimacy between Stark and Paltrow’s fetishistically-attired but prim “Pepper” Potts feels agonisingly forced. Perhaps Favreau, who wrote Swingers and directed Elf, is happier with jokes and the ironic observation of boys and their toys. Sure enough, we’re soon back to Stark’s Iron Man, taking a mobile phone call in his helmet while playing aerial tag with a pair of F11 fighter planes. There’s a satisfyingly climactic CGI battle, but Downey Jr tops it in the witty denouement with pure acting skill.

Favreau’s film inevitably recalls other super-hero movies because all their storylines follow a broadly similar arc. A more telling problem is that it seems to treat itself as the first instalment in a franchise. Stark’s struggles here are basically internal: only at the very end does he contemplate what it means to be Iron Man. Other characters are sketched in and cued up for future glory. “Next time, baby,” says Stark’s bland chum Jim (Terrence Howard) as he eyes a spare metal suit.

I hope there is a sequel, though. Despite its flaws, the film delivers splendidly tense action sequences, a magnetic performance from Downey Jr, and Paltrow in tight dresses and high heels. Which, for superhero fans, is a near-perfect hat-trick.

Iron Man is released on 2 May