Public Enemies (15) *** Dir: Michael Mann With: Johnny Depp, Christian Bale, Billy Crudup, Marion Cotillard Between the clatter of machine guns and the blasts of testosterone, you might get the impression Michael Mann's Public Enemies is a gangster movie. Sure it is, wise guys. But more than this it's a romance, an expression of love for a period in American history that spawned the last great outlaws.

Like any grand passion, Public Enemies has its glories and faults, the latter more towering than the former, but you can forgive this enthralling picture a lot. No-one is more fascinated by flawed men on both sides of the law than the director of Heat, and he asks that you are captivated by them too. If the offer proves resistible, it's because Mann proves too obvious in his ardour.

From the book by Bryan Burrough, Public Enemies is the true crime tale of men who shot first and never asked questions later, especially of themselves. Characters such as Pretty Boy Floyd and Baby Face Nelson appear on the margins, but this being a Michael Mann picture, it soon comes down to a case of mano-a-mano. It was De Niro and Pacino in Heat, Cruise and Foxx in Collateral. Here, Johnny Depp, playing bank robber John Dillinger, squares up to Christian Bale as FBI enforcer Melvin Purvis.

Mann comes out with all intentions blazing. No matter where you sit in the cinema it will probably feel too close, such is the grainy, in-your-face visual style. Everything swaggers in Public Enemies, especially the camera. The sound levels, similarly, are all over the joint.

We make Dillinger's acquaintance in 1933, four years into the Great Depression, as he is leaving prison by unconventional means. After a change of clothes in a nearby shack, the dirt poor lady of the house pleads to Dillinger: "Take me with you, mister." The man is a hoodlum but in her mind he's a prince of thieves who stole only from the rich. The Robin Hood of robbin' banks.

Where Depp's Dillinger is all Fedora set at a rakish tilt, Bale's G Man is a study in buttoned up decency. Like Dillinger, he lives by a code of his own, it's just that his is the law. And like Dillinger, he's surrounded by some rum fellows, chief among them FBI boss J Edgar Hoover (Billy Crudup). Hoover is portrayed as a shameless publicity hound, a man who, like the criminals he's chasing, is hungry for celebrity. Purvis just wants to get his man.

Initially, Mann is in two minds about Dillinger and Purvis. They are both made to seem like angels with dirty faces, complicated sorts who do what it takes even if they don't always like it. But as the story goes on the lead characters become as black and white as a Thirties newspaper.

You wait in vain for the screenplay, by Ronan Bennett, Michael Mann and Ann Biderman, to throw something unexpected into the mix. Yet Dillinger turns into a straight down the line anti-hero, an outlaw who, in the time honoured tradition of celebrity criminals, never did nothing to nobody who didn't deserve it, while Purvis grows more heroic and square jawed by the minute.

The trouble with working in such broad strokes is that characters start to act in ways that seem too pat to be believable. With Dillinger especially, even though his tendency towards savage violence is shown, the screenplay veers close to hero worship.

Playing Dillinger's girlfriend, La Vie en Rose star Marion Cotillard does battle with an American accent and loses. Like Bale and Depp, she's a bit too made over.

Wherever you look, Mann's picture oozes period style. Even the FBI telephone exchange looks like an art deco nightclub. In Depp and Bale, Mann has two of the finest movie stars around, actors who know how to act in a contained way while flooding the screen with their charisma. While Depp will get the plaudits, Bale's performance is the more quietly satisfying.

But it's the set pieces where Mann shows his hard, true class and makes you forgive his softly softly take on crime. He shoots a jailbreak as if it's a clumsy ballet, full of shuffles and coiled menace. Another scene set in a cinema, where Dillinger has just seen his picture flash up in a newsreel, is simply brilliant. The theatre audience is told to gaze right and left in case the public enemy number one is among them. As they do so, Depp stares straight ahead, looking fate in the eye.

Dillinger, as Mann's savvy picture makes clear, was running out of time. Crime was going semi-legit, the better to make more money. The law was changed to stop criminals skipping across state lines. In gangster movies, censorship codes began to frown on the glamorisation of tough guys. The credits were about to roll for the public enemies. They wouldn't be around to read the reviews.