The film, a fictionalized account based on historical events, stars Thure Lindhardt and Mads Mikkelsen as two Danish resistance movement fighters nicknamed Flammen and Citronen, during the Nazi occupation of Denmark in World War II.

Set after the Nazi invasion of Denmark, the film focuses on the Holger Danske resistance group's Bent Faurschou Hviid (known as Flammen) and Jørgen Haagen Schmith (known as Citronen).

Aksel Winther, Bent and Jørgen's handler, asks them to kill Elisabeth Lorentzen, Horst Gilbert, and Hermann Seibold–members of the Abwehr, German military intelligence.

[5] Even in the wake of major companies' refusal to support the project, Madsen and Andersen continued to research archives in England, Germany and Sweden.

[3] Originally, Madsen planned to create a docudrama-style film[4] but when he discovered Ketty was a Russian spy and had a love affair with Flammen he changed his mind.

[6] Mads Mikkelsen was chosen to play Citronen by the director before his international career was launched because, as Madsen said, "he has this grand masculinity, and he approaches his roles like an animal".

[2] Wally Hammond, for Time Out London, said its cinematography varies "between atmospheric, noir-esque period evocation and modern widescreen stylings, with excellent use of low-key lighting, silhouettes and location".

[6] Manohla Dargis of The New York Times affirmed, "You can see the Melville touch in the impenetrable shadows that spill across Mr. Madsen's carefully composed mise-en-scène and in the fedoras and trench coats worn by his two heroes.

[17] Turan even said it is "more nihilistic than idealistic",[20] while Tirdad Derakhshani of The Philadelphia Inquirer asserted that it "balances the whizzing bullets and political intrigue with an elegiac tone and an existential edge just this side of nihilism".

[26] Writing for The Washington Post, Michael O'Sullivan said the film also has "a surprisingly contemporary subtext, as when Hoffmann, in an abortive showdown with Flame, calls his would-be assassin a well-intentioned, but ultimately misguided, terrorist.

[20] Derakhshani stated it "rehearses virtually every element of the classic genre piece: violence, sex and romance, gunplay, spies, betrayals, a femme fatale, and a murderous Gestapo officer".

[41] Critical response to the film was mixed but mostly positive in Denmark,[42][43][44] including praise for Madsen's direction, the performances of Lindhardt, Mikkelsen,[44] Stengade and Mygind;[42] opinions were divided about the portrayal of the main characters.

[47][48] Berlingske's Ebbe Iversen commented that it can be both good and bad to have morally ambiguous characters as it can be seen as "an artistic force" to have "authentic protagonists psychology", but it can also be frustrating to have their motives portrayed as "enigmatic".

"[49] Henrik Queitsch of Ekstra Bladet praised the action sequences for its details, even saying it is the best Danish war film ever and said it is also "a booming interesting history and a multifaceted psychological portrait – not only of Flame and Citron, but also of the many people they come across in their path".

[52] DR's Per Juul Carlsen declared on Filmland [da] that although the film is visually beautiful, he is not sure it should be this way: "Had it not been better and more correct to tell the story really ugly and blurry in the rain instead of sunshine ... with realism instead of polished exquisiteness[?]".

[56] SFGate's Walter stated, "[t]hough the material might lend itself to heavy-handedness, director Ole Christian Madsen is steady, and he gets fine performances from the two leads and [Stine] Stengade.

Beyond that is a drop-dead gorgeous period noir, rife with paranoia, femmes fatales, and good men inexorably sinking into the bloody mire and opaque texture of life (and death) during wartime.

Beautifully choreographed and filmed in deep shadows that cut the violence into shadowy, brutal fragments, Flame and Citron's jobs are at once thrilling and disconcerting".

[61] Nick de Semlyen from Empire considered it an average film, saying "It's familiar ground for anyone who's seen Black Book or Sophie Scholl, but director Ole Christian Madsen steers a skilful course, keeping things grim but not to an off-putting extent, bringing a David Lynch-esque vibe to Flame's hotel dalliances with a shady lady and pulling off an incredible death scene for one of the leads.

[64] Burr mostly praised it but said that "Madsen eventually loses his way", stating that there was "not enough" information about Citron and that the film "feels packed with events and frustratingly unfocused".

[24] Jenkins said the best scenes in Flame and Citron are the ones who share Army of Shadows's "chaos", and he criticised what he described as an aspiration "to be a noble national epic, rather than the rougher, more universal tale of two desperate men fighting for a cause".

"[65] Although he praised the "beautifully choreographed and shot" action sequences, Murray criticized Flame & Citron for "lean[ing] toward the handsome and thoughtful when it could stand to be a lot dirtier and more visceral", citing Black Book as a "superb counter-example".

"[65] Ella Taylor argued in favor of Flame & Citron, saying it "is the film that the horribly overrated Black Book could have been, had Paul Verhoeven not indulged in the puerile reversals of sensitive Nazis and treacherous partisans.

"[21] Fuchs also compared the moral dilemmas the characters have to deal with to Black Book and Steven Spielberg's Munich but said Flame & Citron does not have "the splendid surrealish excess" and "the weird conflation of maternal bodies and motherlands" of the two films, respectively.

[61] St. Louis Post-Dispatch's Joe Williams said that in contrast to "lavish thrillers" Black Book, Inglourious Basterds and Avatar of Hollywood-ish "heroic defiance", Flame and Citron is "lean and psychological, rooted in the either-or of wartime choices".

[57] Nick Roddick of the London Evening Standard commented that "it could succumb to the Sod's Law of foreign-language cinema: make a arthouse movie too commercial and you risk losing both audiences.

But Flame [& Citron] 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.

[76][77][78] The involvement of the Danish people with the Nazis and the assassinations committed by the resistance movement had been a taboo subject since World War II, with scholarly literature on the topic only being produced from the 1980s and 1990s.

Birkelund rejected the idea of Jacobsen giving orders, but Madsen said the meetings were created by him to show that the resistance movements of that time had approaches different from those of their present-day counterparts.

[48] While Madsen's film has been highly criticized by historians, Max Manus gained the status of "real past" and received only a few criticisms that were dismissed by resistance veterans, politicians and even the king Harald V.[48] Bjerg et al. argued that the difference in reception may be attributed to the films' content, saying, "In stark contrast to Flame & Citron, [Max Manus] doesn't challenge the notions of the right and the wrong side and the unambiguous good cause.