Robert Downey Jr in Iron Man 2

Robert Downey Jr gives a stunning performance in the Iron Man sequel, a movie with a message about modern America.

Tony Stark, the creation of Marvel Comics in 1963 and the subject of 600 issues of Iron Man magazine, is a curious superhero. In his private life, if that is what it can be called, Tony (being impersonated for the second time on screen by Robert Downey Jr), is no retiring, bespectacled Clark Kent or shy schoolboy Peter Parker. Rather, he's a handsome, eccentric, technological genius, clearly based on Howard Hughes, who has inherited from his father (appropriately named Howard Stark) a vast business specialising in, among other things, state-of-the-art military weapons and turned it into a concern worth billions.

The secret identity that he attained, in the manner of the Scarlet Pimpernel, Superman, Batman, Spiderman et al, by devising the impregnable flying suit that made him Iron Man, was blown before the end of the first cinematic blockbuster, which appeared two years ago. So how does Tony progress and develop? Well, it could be said that he's been shaped to please the right-wing critics of Avatar.

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The big political juggling act of Iron Man 2 is how to get the extensive support that the producers need from the Department of Defence and the top brass at the Pentagon and yet retain Tony's position as a maverick genius. This is initially done in two ways. Tony more or less adopts the motto of the Strategic Air Command that in the 1960s Bertrand Russell thought so ironically amusing: "Peace is our profession". The cleverest spokesman for the arms business since Bernard Shaw's Undershaft in Major Barbara, he presents his company in a showbiz-style exposition in New York with a chorus of dancing girls joining lethal weapons on stage, all in the interest of world peace.

Then, when he's called to Washington to appear before a Senate committee presided over by the deviously smarmy Senator Stern (Garry Shandling), he refuses to hand over the Iron Man equipment to the government on the grounds that it's better developed by private enterprise. In this, he has the covert support of a close friend, the handsome black soldier Lt Col Rhodes (Don Cheadle). The Washington hearing is modelled on an actual event just after the second world war when Howard Hughes faced down his political critics, an incident also celebrated in Scorsese's equally Hughes-aggrandising The Aviator.
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Meanwhile, having established Tony as an impeccable combination of patriotism and capitalism, the movie sets the scene for a replay of the cold war by introducing a wild Russian mirror-image of the American superhero. Not unlike the bizarre commie villains Sylvester Stallone confronted as Rocky and Rambo, Mickey Rourke's Ivan Vanko is a disfigured, tattooed giant from the same cracked mould that produced the shambling loser Rourke played in The Wrestler. He acts like Rasputin, has the technical skills of a professor at the USSR Academy of Sciences and has cold war issues to settle.

Hanging over Tony Stark, as with most American heroes, is the shadow of a father who, he thinks, didn't love or appreciate him. Ivan's dad was a brilliant Soviet scientist who defected to the west only to be unjustly accused of espionage by Stark Sr. Returned home, he was incarcerated in the gulag and died of drink in Putin's squalid Moscow. Ivan, who has the means to create his own Iron Man, embarks on a revenge trip that begins spectacularly when he confronts Tony on the track of the Monaco grand prix.

This first titanic battle ends in apparent victory for Tony, who rightly observes, in the best line from Justin Theroux's variable script: "You look like you've got friends in low places." This is proved only half true when Ivan is abducted by Stark's deadly enemy, Justin Hammer, the unacceptable face of capitalism, played with considerable verve by Sam Rockwell as Batman in a three-piece designer suit. Hammer is using the military-industrial complex to undermine Stark and is naturally unaware of the Russian's secret agenda.

As a political fable, it's oddly revealing about current American thinking and it is interwoven with a variety of other topical themes. Among these is female empowerment, as the enlightened Tony appoints his long-time secretary Virginia "Pepper" Potts (Gwyneth Paltrow looking uncannily like a less passive-aggressive version of Mia Farrow) as the CEO of his corporation.

The special effects and the big action sequences are well enough handled. But when Downey compliments Cheadle at the end by saying: "You kicked ass back there by the way", he's acknowledging the conventionality of the whole affair. If the movie has a certain distinction and a suggestion of depth, then this derives almost entirely from the presence and performance of Downey as Stark. One of the most gifted, versatile and daring actors of his generation, he was quite brilliant as Charlie Chaplin in Richard Attenborough's biopic nearly 20 years ago. He's been unforgettable in little scenes here and there, as when dangerously provoking Mike Tyson in James Toback's semi-improvised Black and White (1999), and he was superb recently as the Australian method actor who loses himself within the character of a black GI he's playing in Tropic Thunder.

I was impressed though dissatisfied by his Sherlock Holmes in Guy Ritchie's picture last Christmas. His strange, sometimes fractured diction makes him not always easy to understand. But he exudes intelligence, internal conflict and a witty doubt about the world and its absurdities. To the role of Stark he brings a sense of his own troubled life and upbringing and his much publicised struggles with drink and drugs. He is a man who, having decided that conversing with his inner child is no proper occupation for an adult, has invited demons in to engage with his soul. Downey holds our attention as few other American stars do today.