Iron Man (Robert Downey Jr.) battles a new super-villain at the Monaco Grand Prix in Iron Man 2. (Double Negative/Marvel Entertainment/Paramount Pictures)

Two years ago this month, Robert Downey Jr.’s Iron Man soared his way into our comic-book-loving hearts. Released in the same superhero-cluttered year as The Dark Knight, Iron Man was the flippant answer to that movie’s tagline, “Why so serious?” Sure, the film had a dramatic arc – irresponsible weapons-peddling industrialist sees the error of his ways, decides to fight for good as iron-clad warrior – but it also had a witty script, some super-powered slapstick and the roguish charm of Downey Jr.

Iron Man 2, the long-awaited sequel, is as overloaded as a sumo wrestler’s plate at an all-you-can-eat buffet.

So, if it ain’t broke, don’t fix it, right? Not according to Marvel Studios. There, the credo is: If it ain’t broke, give it all the fixings. Iron Man 2, the long-awaited sequel, is as overloaded as a sumo wrestler’s plate at an all-you-can-eat buffet. The supporting cast is heaped high with A-listers: Mickey Rourke, Scarlett Johansson, Don Cheadle, Sam Rockwell and Samuel L. Jackson. The screenplay, by Tropic Thunder co-writer Justin Theroux, has more quips than an Oscar Wilde comedy. We get a double serving of villains this time around, and two Iron Men as well (another reason to call it Iron Man 2, perhaps).

In theory (or at least to Hollywood’s way of thinking) this should make it twice as good as the original. Instead, it ends up falling short. Not all that short, really, but we’re still left dissatisfied. Where most of us came to the original Iron Man with low expectations and were pleasantly surprised, the sequel has the opposite problem: it looked so unbelievably juicy on paper that it was almost doomed to disappoint us.

It does make a strong start. In its early scenes, Iron Man 2 quickly recaptures the levity of the first film, while also introducing a more sombre tone (ironically, with shades of The Dark Knight) and further developing Downey Jr.’s cocky but troubled Tony Stark.

It’s been two years since Stark introduced the world to his alter ego and, with typical impulsiveness, announced that he was the man in the iron mask. Now we see that the big reveal was partly to stoke his ego. Since then, Iron Man has successfully beaten the global bad guys and brought peace in our time – something a swaggering Stark trumpets from the stage of the Stark Expo, a year-long extravaganza the hard-partying billionaire has staged in suburban New York to celebrate his vision of the future.

But not keeping his identity secret has come with a price. The U.S. government, in the person of a cranky senator (Garry Shandling), is pressuring Stark to hand over the Iron Man technology to the military. An unscrupulous industry rival, Justin Hammer (Rockwell), will do anything to beat him. And in Russia, a hulking, stringy-haired physicist named Ivan Vanko (Rourke) — who has a mouthful of metal teeth and an axe to grind — is busy building a suit of his own to take down our hubristic hero.

Natasha Romanoff (Scarlett Johansson) attracts the attention of Tony Stark (Downey Jr.) in Iron Man 2. (Industrial Light and Magic/Marvel Entertainment/Paramount Pictures)

Looking partly like his battered Randy the Ram from The Wrestler, partly like a tattooed gangster out of David Cronenberg’s Eastern Promises, Rourke gives off a cold, freakish vibe. (He’s certainly unlike any physicist we’ve ever met.) When he dons his Whiplash armour, which features long, crackling electrical appendages that can slice through steel like butter, he could be some sci-fi nightmare version of a Russian peasant brandishing a pair of knouts. His confrontation with Stark, smack dab in the middle of the Monaco Grand Prix – shot with eerie brilliance by cinematographer Matthew Libatique – is genuinely frightening as comic-book showdowns go.

You get the impression that, given more dialogue and more screen time, Rourke might’ve turned Vanko into a villain to match Heath Ledger’s insane Joker. But that’s the problem with Iron Man 2. Director Jon Favreau has given himself so many characters and subplots to juggle that many of them bounce to the side.

There’s the half-realized father-and-son theme, in which Vanko’s love for his wronged Soviet dissident father is meant to contrast with Stark’s bitter feelings for his own late dad. There’s the promised but undelivered Betty-and-Veronica rivalry between Stark’s faithful secretary, prim Pepper Potts (Gwyneth Paltrow), and Scarlett Johansson’s sultry Natasha Romanoff, a SHIELD agent planted at Stark Industries who catches the boss’s ever-roving eye. Then there’s the fractured friendship between Stark and Lt.-Col. Rhodes (Cheadle), who betrays his old buddy for what he believes are Stark’s best interests.

Finally, there’s that pesky stuff with Jackson as the eye-patch-sporting Nick Fury, chief agent of SHIELD, who is there to recruit Iron Man for the Avengers initiative – a tiresome set-up for the future Marvel superhero ensemble film. Jackson brings his own smooth authority to the character, but I kind of miss the Fury of the old Marvel comics, a cigar-chomping, wisecracking, ex-army tough guy.

Ivan Vanko, a.k.a. Whiplash (Mickey Rourke), has a score to settle in Iron Man 2. (Marvel Entertainment/Paramount Pictures)

The other actors are as fine as we’d expect them to be, but seldom more than that. The likable Cheadle does his best with his underwritten buddy role. Paltrow pushes Pepper into high-strung territory, but at least she makes more of an impression than Johansson. The curvy actress’s one big action scene, in her cat-suited femme-fatale persona as Black Widow, is so digitally enhanced that it makes Buffy the Vampire Slayer look like kitchen-sink realism. Only Rockwell really gets to run with his part. He’s hilarious as the smarmy Hammer, a wheeler-dealer who sells weapons like they were used cars.

Rockwell’s funny shtick and the presence of the legendary Shandling — as well as Mad Men’s John Slattery as Stark’s father in some wry ’60s-era footage — betray the director’s comic roots. Favreau even gives himself a small goofball role as Happy Hogan, an all-purpose sidekick to Stark, Natasha and anyone else who happens to be around. However, he doesn’t bring back the endearing slapstick touch he brought to the original picture.

Nor does Downey Jr. charm us quite like he did the first time out. Granted, his Stark spends much of the movie grappling with a life-threatening health crisis (another subplot), which causes him to go off the rails. Still, even when he’s back to fighting form, he lacks a certain zest.

Like its predecessor, Iron Man 2 flaunts a retro-rock soundtrack and AC/DC’s Highway to Hell is heard blasting over the closing credits. Let’s hope that’s not an unwitting portent that the Iron Man franchise is heading down the same path.

Iron Man 2 opens May 7.

Martin Morrow writes about the arts for CBC News.