In Iron Man, director Jon Favreau’s superlative, breathlessly paced adaptation of the Stan Lee comic, Downey’s narcissistic musings seem perfectly suited to Tony Stark, the billionaire playboy who presides over the world’s foremost weapons manufacturer. He is defiantly cavalier, casually irresponsible and brilliant to boot. He hardly seems introspective enough to suffer a crisis of conscience, but after witnessing the devastation his high-tech artillery has wrought on the impoverished villages of Afghanistan, he opts out of the arms race, after a fashion.

Rather than supply Middle Eastern warlords with weapons of mass destruction, Stark reinvents himself as a superhero, endowed not with otherworldly powers but with limitless ingenuity and resources. Shielded by a sleek suit of titanium armor equipped with all the standard accoutrements – propulsive jets and missile launchers, all powered by an artificial heart – he returns to Afghanistan to clean up the mess his corporate empire helped create, then turns his attention to his rogue business partner, Obadiah Stane (Jeff Bridges, minus his trademark mane).

If Stark seems more accessible than your average superhero – he’s neither a genetic anomaly, like Spider-man, nor an emotionally scarred vigilante, like Batman – Stane, as his backstabbing colleague, is a classic villain, all bluster and brimstone in his bid to facilitate a new era of global military proliferation. Bridges, with his sinister salt-and-pepper goatee and a diabolical glint in his eye, burns with rage once Stane’s journey to the dark side is complete, setting up a climactic brawl in which Downey’s hero seems at first overmatched.

Conceived in 1963 as a dynamic new warrior in the fight against Communism – Starks was modeled on the eccentric mogul Howard Hughes – Iron Man has been ushered gracefully into the present as America’s last best hope in the war on terror. It is a risky proposition, thrusting Lee’s unlikely hero into the thick of a polarizing conflict, but it works. As one of the finest comic-book adaptations ever, Iron Man is smart and sophisticated, but firmly rooted in the kind of goofy mythology that has always informed the backstories of Marvel’s most popular creations, from Spidey to the Hulk. In other words, it’s serious fun.

While Iron Man has never ranked among the most iconic of American superheroes, Downey’s brilliantly nuanced performance as a hedonist in the grip of a midlife crisis might change that. As Stark, he is quick-witted, charismatic and effortlessly engaging – more Bond than Batman, but with an appealingly devious sense of humor. Even as he disappears inside his shiny armor, flexing his metallic muscles against a wondrous (and wholly unobtrusive) background of CGI, his presence elevates Iron Man into the realm of unforgettable fantasy.