HOLLYWOOD — Much of the allure of “Iron Man” comes from the fact that we are indeed talking about a man — a real man who has lived a life and made mistakes and experienced regret — not some scrawny, teenage boy who received his superhero powers through a bite from a radioactive spider.

No offense to Spidey, the other Marvel Comics hero who’s already provided billion-dollar summer blockbuster fodder. But there’s just something more relatable about Tony Stark, even though he’s a playboy industrialist of staggering wealth and arrogance.

And in the hands of Robert Downey Jr., he’s absolutely riveting. Downey may have seemed an unlikely casting choice at first, but it’s difficult to imagine any other actor in the role; he’s so quick-witted and he makes such inspired decisions with dialogue that, at times, might have seemed corny otherwise. Throughout his eclectic career — from “Less Than Zero” and “Chaplin” to “Kiss Kiss, Bang Bang” and “Zodiac” — he’s always been capable of both great charisma and vulnerability, and both are beautifully on display in this, the biggest movie of his life. “Iron Man” is a blast, too — the perfect start to the summer with its shiny mix of visual effects, elaborate set pieces and plenty of humor within its intelligent script.

This is also the biggest movie of director Jon Favreau’s life following “Made,” “Elf” and “Zathura,” and he juggles all the complicated, expensive toys deftly. The visual effects come courtesy of the venerable Industrial Light & Magic, with Matthew Libatique (“Requiem for a Dream,” “Inside Man”) providing the crisp cinematography

Stuff gets blown up real good, to the tune of AC/DC’s “Back in Black” and, appropriately, Black Sabbath’s “Iron Man,” but beneath the requisite spectacle is an issue-laden storyline with heart to go along with its brains. Tony’s weakened heart has always been his Achilles heel, but it’s also what gets him out of trouble and inspires his rebirth.

The first moments of “Iron Man” give us a telling glimpse of Tony: a close-up of his hand, cradling a tumbler of Scotch on the rocks, as he rides in the back seat of a Humvee that’s rumbling across the Afghanistan desert. He’s the brilliant and talented head of Stark Industries, the leading supplier of weapons to the U.S. military, and he banters comfortably with the soldiers who have been assigned to protect him during a trip to demonstrate his latest missile. They, in turn, are in awe of his high-flying ways.

(The Iron Man comic-book character was partly inspired by billionaire industrialist Howard Hughes, but the similarities to Downey’s own life are unmistakable: the well-documented highs and lows and, now, the shot at complete redemption. The subtext provides both knowing laughs and a sense of substance.)

But things go awry almost immediately. The Humvee is attacked by insurgents and Tony is abducted. While in captivity, with a battery attached to his heart to keep him alive, he’s ordered to reconstruct the missile. Instead, with the help of the doctor who saved him (a graceful Shaun Toub), he’s crafty enough to create a suit of armor and become a weapon himself to escape.

Tony returns home to his monstrosity of a mansion that’s carved into the face of a Malibu cliff but looks more like an old set from “The Jetsons.” He’s a changed man, and the changes he has welcomed to his life and company also bring enemies. His top executive Obadiah Stane (Jeff Bridges, deliciously villainous with a shaved head and devilish goatee) is appalled at Tony’s new purpose — to no longer make weapons. But Stane insists, “What we do keeps the world from falling into chaos.”

It’s an anti-war argument in the multilayered script from the writing teams of Mark Fergus and Hawk Ostby, and Art Marcum and Matt Holloway, but the delivery is hardly heavy-handed. The original “Iron Man” comic book that inspired the film took place in the 1960s during the Vietnam War, and Tony Stark was relevant, functioning as an important, fervently anti-communist cog within the military-industrial complex. Moving the film’s action to Afghanistan and the present day makes it just as relevant in its own way.

In his tricked-out underground workshop, his own personal bat cave, Tony creates his Iron Man uber-suit, even though he’s not quite sure what to do with it once he’s finished: the right thing, perhaps, for the first time in his life?

His right-hand woman, Pepper Potts, stuck by him and kept his life organized when he was a shallow pig, but seems to like the more enlightened Tony better. (In another unexpected bit of casting, Gwyneth Paltrow brings understated smarts and class to the role.) Meanwhile, his best friend, Rhodey, an Air Force colonel played by an underused Terrence Howard, just seems confused by this person he no longer thinks he knows.

Tony undergoes plenty of trials and errors on the road to becoming Iron Man, which are both amusing and thrilling. But the moment he finally climbs inside that streamlined, rocket-propelled, red-and-gold suit — with its perfectly intertwined pieces that lock together like the most comfy, high-tech pair of ski boots — will surely cause the hearts of geeks and non-geeks alike to go pitter-patter.

But because the build up is so successfully engaging, the ending feels like a letdown. It’s just plain silly watching versatile, Oscar-nominated actors behave like a couple of middle-aged Transformers.

That’s merely one bump in an otherwise satisfying ride, though. And there’s plenty of opportunity for improvement: The last line clearly sets up a sequel. But you knew that was ironclad from the beginning.

“Iron Man,” a Paramount Pictures release, is rated PG-13 for some intense sequences of sci-fi action and violence, and brief suggestive content. Running time: 126 minutes. Three and a half stars out of four.