In Darkness, Agnieszka Holland, 143 mins (15)

In this Holocaust true story neither the heroism nor the humanity follow the Hollywood format

A group of Polish Jews try to flee through the sewers (Jasmin Marla Dichan)

The more heroic a story, the more it demands to be told in a style that resists the obvious heroic mould. The Polish director Agnieszka Holland gets it about right with her true-life Holocaust drama In Darkness, about a group of Jews who survived by hiding in sewers for 14 months. The film's Oscar nomination might have been a warning sign: for its foreign language prize, the Academy has a penchant for real-life tales told as heartwarmingly as possible. But In Darkness is recounted with abundant grit and a sober view of human fallibility. The fugitives here bicker, mope, sometimes betray each other, and their Polish saviour begins as their exploiter. All this serves only to highlight the genuine, breathtaking heroism that gradually emerges.

The setting is Lvov, Poland, in 1943, where the Nazis are about to lay waste to the Jewish ghetto. Some residents plan an escape, burrowing into the sewers below – but no sooner do they break through the surface than they come face to face with Leopold Socha (Robert Wieckiewicz), a sewer worker and sometime thief. He's none too fond of the Germans, but doesn't much like Jews either; still, he agrees to guide them through the tunnels, at a price. It's only little by little, as horrors pile up, that he begins to realise the moral meaning of their dependence on him, that Socha starts to act out of real humanity.

The script by David F Shamoon – based on Robert Marshall's book In the Sewers of Lvov – concentrates on the flaws within and the tensions between people, eschewing the exaltation of moral nobility that often afflicts Holocaust cinema. The Jews and their reluctant "Moses", as one of them sceptically calls him, have no love for each other: "Never trust a Polack," someone says in Yiddish (one of the film's languages, alongside Polish, German and Ukrainian), while Socha tells his sidekick that they can always turn in the Jews if need be. One man abandons his wife and daughter to go underground with his mistress. The refugees bicker, prevaricate; the single overtly religious man among them, when heard uttering Hebrew prayers, is mocked by his fellows: "God isn't listening."

What drives these people is survival, which isn't always a flattering urge. But reminders of humanity come, with little trace of sentiment, in surprising ways: a 10-year-old girl offers a man her apple, a snatched moment of eroticism happens with running water as the only soundtrack. In an extraordinary, by all accounts true episode, one man (Benno Fürmann) infiltrates a concentration camp to find a woman left behind. A less capable storyteller would have had to constantly flash up the caption: "This Really Happened."

However, Holland's and Shamoon's commitment to telling the full story means that In Darkness sometimes feels diffuse. We're given the damp, the dark, the rats and a very vivid suggestion of stench. But as the film moves from crisis to crisis – nightmare close shaves, emotional moments of truth, a climactic flood – we miss a deeper sense of what would probably strike most of us as the ultimate horror of all this. I mean the sense of entombment, and what it must have done to the soul to live day after day with no light, or sense of time, or objects to occupy the mind.

The emphasis on event and atmosphere – and photographer Jolanta Dylewska terrifyingly evokes the abyssal claustrophobia – means that the film is less effective in depicting character. Only a few personalities emerge fully: notably Agnieszka Grochowska as a strong-willed woman named Klara Keller, and Fürmann's Mundek Margulies (partly because he makes sorties into daylight, where we can see him). It's Socha who is revealed most clearly, played superbly by Wieckiewicz as an ostensibly self-serving grouch. Socha doesn't realise he has much compassion until it jumps out and startles him; when he finds himself behaving generously to his charges, he's embarrassed: "I don't want them to think I'm a sucker who's doing this for nothing."

Agnieszka Holland has effectively been an American director for some time: she's made episodes of Treme and The Wire, and has directed some lukewarm dramas such as a handsome but mundane Washington Square. But here she gets back to her European roots – and not only because she's revisiting the underground setting of Andrzej Wadja's Polish Resistance classic Kanal (1957). One of her best films, Europa Europa (1990), was about a boy in Nazi Germany who survived by passing as a member of the Hitler Youth. In Darkness further explores the theme of survival by subterfuge, with the moral complexity that that entails. We learn that even in – especially in – extreme jeopardy, nobility can exist; but then, films have told us that before. More revealingly, In Darkness reminds us what can also prevail is bloody-minded, unpredictable human nature.

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