Tom Hardy—along with some skilled technical craftsmen who skillfully insert two images of him into single frames—manages an impressive acting tour de force playing twins in Brian Helgeland’s “Legend.” If the result doesn’t equal Jeremy Irons’ feat in “Dead Ringers,” that’s because the brothers in David Cronenberg’s film were far more subtle creations than the real-life gangsters Hardy portrays, who are both more easily differentiated—one wears glasses and the other doesn’t—and, each in his own way, far more theatrically extravagant.

Hardy portrays Reggie and Ron Kray, a couple of Cockney crooks who became celebrities in the London of the swinging sixties and remain notorious in the British pantheon of criminals. The brothers’ interlocked careers were already treated, fairly well, in Peter Medak’s “The Krays” (1990), where they were played by siblings Gary and Martin Kemp. That picture—which put a good deal of emphasis on the baleful influence of their mother, whom Billie Whitelaw invested with a compelling degree of manipulative power—was a full biography, going from their youth to their end. Though “Legend” is longer, writer-director Brian Helgeland opts to cover less territory. He begins with the brothers already in business, though Reggie is just coming out of prison, and concentrates on their expansion from the East End to the tonier areas of the city, in the process taking out their main competition and striking a partnership with the American mob, represented by an emissary from Meyer Lansky played, perhaps inevitably, by Chazz Palminteri.

The Krays’ mother Violet receives only minimal attention here—she appears in just a couple of scenes in the person of Jane Wood, who’s so unctuously doting that you might regret her presence is so fleeting. The chief feminine presence instead is that of Frances Shea (Emily Browning), the sister of one of the gang’s drivers (Colin Morgan), with whom Reggie is immediately smitten and eventually weds despite the opposition of her disapproving mother (Tara Fitzgerald). She actually narrates the picture with a degree of omniscience that will be explained only in the last act, and the crux of the plot becomes the deterioration of the marriage because of her husband’s insistent protection of his brother Ron, an obvious psychotic who’ll recklessly kill a rival in front of witnesses, endanger the gang’s finances in an absurd scheme to build a utopian city in Nigeria, and order a hit on the duo’s fiscal manager Leslie Payne (David Thewlis), who’s been instrumental in building their empire but whom he’s never liked. The other major figure reappearing throughout is Inspector “Nipper” Read (Christopher Eccleston), a smoldering cop obsessed with nabbing the brothers.

Helgeland, to tell the truth, doesn’t do a particularly good job of explaining either the Krays’ psychological dependence on one another or the specifics of their business operations. He’s far more successful at recreating the seedily glamorous milieu in which they operated, and the endemic political corruption that they both used and were threatened by (two of the most garish performances come from John Sessions as Lord Boothby, a lord who shared Ron’s homosexual tastes, and Kevin McNally as a worried Prime Minister Harold Wilson). And he draws colorful if broad turns from many in the supporting cast—including Wood, Morgan, Fitzgerald, Thewlis, Palminteri, Taron Egerton as Ron’s favorite boy toy Teddy, and Shane Attwool as the most flamboyant member of the rival Richardson gang. (Paul Bettany appears uncredited as Charlie Richardson.)

Unfortunately, Eccleston does little with the part of Read, who’s presented as a singularly ineffectual fellow, and Browning is an engagingly gamin-like but emotionally rather pallid presence as Frances; even her final act of defiance is oddly unmoving. That leaves most of the heavy lifting to Hardy, who’s certainly up to the task in technical terms. He makes Reggie a fellow of great surface charm and inner darkness, and puts on a tremendous show as the almost ghoulishly unbalanced Ron. He manages the two most energetic action scenes in which both participate—a showdown in a diner with the Richardson mob, in which Ron wields a couple of hammers, and a brutal brawl between the brothers in their once-posh nightclub—with considerable relish (though his stunt doubles and the technical crew obviously contribute much to them). But his performance—or performances, if you prefer—still amount to a stunt that one can admire without being fully convinced by it. You can enjoy the turn the sheer exuberance at work, though.

“Legend” has a lavish look, with Dick Pope’s cinematography complementing the rich period décor provided by Tom Conroy’s production design, the art direction by Gareth Cousins and Marco Restivo, Crispian Sallis’ set decoration and Caroline Harris’ costumes. Carter Burwell’s score, however, must contend for prominence with the plethora of pop tunes chosen by music supervisor Liz Gallacher.

British audiences will respond to “Legend” the way American viewers would to a stylish one on any of our iconic home-grown gangsters. American audiences, on the other hand, may find the Krays a less than fascinating duo. While Helgeland’s film as a whole is little more than a workmanlike job, many on both side of the Atlantic will respond to Hardy’s energetic double turn as proof of his versatility.