NEW YORK -- From the opening black-and-white footage of Nazis invading Copenhagen, "Flame and Citron" draws you into its doom-laden atmosphere and keeps ratcheting up the tension. This searing, stylish account of World War II heroism from Denmark's Ole Christian Madsen avoids period realism, conveying the story of two heroes of the Danish resistance as a noir thriller, complete with shadowy alleys, double-crosses galore and the requisite femme fatale.

Beneath its stylized surface, "Flame" is also a provocative film of ideas, exploring the notion of heroism. Although based on true events, it unspools like a fever dream, circling back to the hero's opening voiceover, which at the end takes on new poignancy.

This icy portrait of two assassins shooting Nazis point-blank offers no Hollywood-style uplift to mollify mainstream viewers. But "Flame" should pull in a niche group of World War II connoisseurs and will delight art-house and fest audiences with its innovative mix of drama and history filtered through genre. The film opens in New York on July 31, then in L.A. and a other markets August 14.

It's 1944 and Copenhagen is occupied by Nazi forces. Two resistance fighters with the noms de guerre of Flame (Thure Lindhardt) and Citron (Mads Mikkelsen) work undercover for the Holger Danske Group, assassinating Danish turncoats. They also itch to off the German invaders, in particular the silver-tongued Hoffman, head of the Gestapo in Denmark. Flame and Citron take orders from the well-fixed Aksel Winther (Peter Mygind), who in turn receives orders from London.

Known for his red hair and fearlessness, Flame, barely 20 and the younger of the duo, has become notorious throughout Copenhagen and carries an inflating price on his head. He acquires Ketty (Stine Stengade), a femme fatale in a Veronica Lake blonde wig, who works for the underground as a courier.

Following the loss of two comrades in the cell, it becomes apparent there's an informer in their midst. Now all loyalties appear murky. To the film's credit, it captures the confusion among the renegades -- call it the fog of resistance -- keeping the viewer as off-balance as the fighters.

Winther, it turns out, may be using his position as a front to protect financial interests tied to the Germans. Meanwhile, Ketty, though seemingly in love with Flame, may be a double -- or triple -- agent. Disillusioned with their self-seeking superiors, Flame and Citron become resistance "outlaws," pursuing their own vendetta to its inevitable bloody denouement.

It helps that the vendetta is carried out by two gorgeous, charismatic actors. Lindhardt, with his orange shock and milky skin, makes a riveting screen presence. Despite the film of cold sweat over his grizzled face and unflattering glasses, Mikkelsen's sculpted, exotic beauty pierces through.

The Occupation literally makes Citron sick to his stomach, leaving him with no choice but to fight it. Though more wed to battle than family life, he also loves, in his fashion, his wife and child. In a wrenching scene, he clumsily comes on to his wife, his need for her palpable, but she knows that at heart he's a rootless wanderer.

Flame, from a privileged background, developed his hatred of Fascists after witnessing anti-Semitism in Germany. Through Flame the filmmaker looks deep into the character of a hero, suggesting that he loses some of his humanity to the Cause and risks resembling the enemy he fights.

In a telling face-off, Flame visits his hotelier father, owner of the mountain retreat favored by Nazi bigwigs, who simply wants to get by. The film indirectly challenges viewers to ask what they might do in a like situation.

For "Flame's" bravura style, credit goes to below-the-line contributors. Cinematographer Jorgen Johansson favors overhead shots of figures in fedoras and stormtrooper uniforms fanning out or closing in like pawns directed by a higher force. Production designer Jette Lehmann has contrived a palette of gunmetal greys and livid whites daubed with red velvet, especially striking in the barny backrooms of cafes.

In its tough-mindedness "Flame" owes much to Jean-Pierre Melville's "Army of Shadows." Avoiding the docu-style string of anecdotes of many fact-based films, it offers the shapeliness and irony of classic drama. For beneath his stony exterior, it's Flame's romantic soul that will prove his worst enemy. This masterful film is at once a portrait of wartime heroism and a poignant journey into a boy's secret heart.

Opens: July 31 (IFC Films)
Production company: Nimbus Films
Cast: Thure Lindhardt, Mads Mikkelsen, Stine Stengade, Peter Mygind, Mille Hoffmeyer Lehfeldt, Christian Berkel, Hanns Zischler, Claus Riis Ostergaard, Lars Mikkelsen, Flemming Enevold, Jesper Christensen
Director: Ole Christian Madsen
Screenwriters: Lars K. Andersen, Ole Christian Madsen
Producer: Lars Bredo Rahbek
Executive producers: Bo Ehrhardt, Birgitte Hald, Morten Kaufmann, Jorgen Ramskov
Director of photography: Jorgen Johansson
Production designer: Jette Lehmann
Music: Karsten Fundal
Costume designer: Manon Rasmussen
Editor: Soren B. Ebbe
No rating, 130 minutes