By Christy Lemire, The Associated Press

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, 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, and he juggles all the complicated, expensive toys deftly.

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.

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 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 antiwar argument in the multilayered script, 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. 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, 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, rocketpropelled, red-and-gold suit will surely cause the hearts of geeks and nongeeks alike to go pitterpatter.

But because the buildup 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”

Rating: PG-13 for some intense sequences of sci-fi action and violence, and brief suggestive content.

Running time: 2 hours, 6 minutes.