Killer whales, lost limbs, bare-knuckle fighting, sex and love… In the hands of a lesser director De Rouille et D'os (Rust & Bone) could easily come over as a ridiculous mess – melodramatic, mawkish and nothing more. But this is the sixth feature from expressionist auteur Jacques Audiard and if there is one thing he knows how to do it's to create an appealing underworld of outcasts; conflicted characters they we can't help but root for.

Rust and Bone refers to the taste of blood in the mouth when, upon a blow to the face, the lips are crushed against the teeth. It is also the title of a collection of short stories by Canadian writer and amateur boxer Craig Davidson, published back in 2005. Audiard and scriptwriter Thomas Bidgain, who collaborated on the prison thriller A Prophet ("a jail movie with no women, no space, no light, no love" says Bidgain), have plucked their lead characters from two of those stories – in the book, they never meet – ramped up the sexual tension and laid a few more blows for dramatic effect. The result is one of the most unconventional and moving love stories of recent years.

A mesmerising Marion Cotillard plays Stephanie, a killer whale trainer at an aqua park in Antibes. She is a lost soul, desensitised, a self-confessed control freak who spends her nights alone at clubs trying to turn the heads of all the guys. She is in a dead-end relationship, has lost interest in sex and with it, we can sense, all passion for life. After an altercation one night at a club Stephanie meets bouncer Ali (Matthias Schoenaerts), a blank-faced brute drifting from one job and one-night stand to the next. His caustically honest and insensitive manner offends her at first. He makes a pass, she rejects him. But after disaster strikes and a freak accident at the park renders Stephanie distraught and hopeless, a friendship blossoms for those very reasons. He is the one person who won't patronise her or exclude her from everyday life.

On another level, the challenge of taming Ali excites her. She's aroused for the the first time in years. As Schoenaerts recently explained: "My character’s pretty animalistic. When I say animalistic, I mean he reacts very instinctively to things, he doesn’t react intellectually; he doesn’t think, he just acts and reacts. That’s where the honesty comes in, because his tenderness is not a choice, he doesn’t choose to be tender as he doesn’t choose to be violent; violence and tenderness both come naturally."

However Ali is not easy to like. He is solitary, selfish and insular. (We're not told why. It's refreshing when a director doesn't try to answer all our questions.) Accompanying him from Belgium is his five-year-old son who obviously needs constant attention. When we first meat them, Ali steals and scavenges to feed them both. But soon he passes responsibility over to his sister, with whom they have come to stay. His hands are always in the fridge and all over several women he beds, without a care for anyone else. His thoughtlessness even threatens the livelihood of his sister. Through a night security job he soon hears of bare-knuckle fights organised by gypsies and relishes the opportunity to not only earn money but indulge his thirst for action.

But it is through his relationship with Stephanie that Ali begins to redeem himself. He takes her to the beach to swim, he refuses to treat her any differently despite the recent tragedy ("do you even realise?" she questions, dumbfoundedly) and offers sexual favours to see if "everything is working down there". In a moment of delightful levity he says he's "OP", operational and at her service. By beginning to inhabit the more primal world of Ali she begins to live again.

The painter's analogy is a well-worn one in cinema but it is so apt in the case of Audiard. The surreal treatment of the accident by Audiard, together with cinematographer Stephane Fontain and editor Juliette Welfling, transforms a potentially gruesome moment into a something more transcendental. A wash of colours and celestial imagery. Audiard says that with this film he and Bidegain were trying to find an equilibrium between realism and stylisation. And that is apparent throughout the film, whether it's Stephanie's return to the aqua park, the potentially erotic undertones of her first few tastes of sex as a woman without legs, Ali's fights (where the audience, through the eyes of voyeur Stephanie, is thrilled by the slow-motion brutality before them) or the film's denouement on the frozen lake.

Stephanie, finally able to talk again with the help of prosthetics, visits an old friend at the aqua park

One of Davidson's stories that Bidegain adapted with Audiard is called Life in the Flesh. This became an overarching theme for the whole film. "Rust & Bone is about a world where people have just their body left to sell," he explains. "The characters are normal people, but their destiny is magnified by accidents." From a viewer's perspective, the resulting tension between brutality and harmony as these characters fight against the fate that befalls them is great cinema, particularly in the hands of Audiard, a master of patient, poetic exposition. He's often explored the lives of ordinary people with age-old demons, fighting for survival and some sense of fulfillment. In A Self-made Hero he made Mathieu Kassovitz's spineless post-Second World War draft dodger an accomplished impersonator in the Resistance; in Read My Lips Vincent Cassell's selfish criminal comes to the rescue of Emmanuelle Devos's deaf office secretary and becomes "a hero in a disaster of his own making"; in The Beat That My Heart Skipped Romain Duris' debt-collecting thug is a romantic who longs to escape to another life as a concert pianist; in A Prophet we witness Tahir Rahim's ascent from pawn to gangster, from nobody to made man, from sinner to saint.

Audiard sets about crafting another intricate scene

In the first of these examples, the French director's second film when it was released in 1996, the actor Albert Dupontel utters this basic truth about all of Audiard's leading men: "Losers can seem like winners, devils like angels and cowards like heroes." Ali is the latest flawed individual to be confronted with a life-changing situation and it is he who is the beating heart of this film, not Stephanie as many would expect. I did feel that Cotillard' s character faded far too quickly into the background towards the end but that shouldn't detract from either the quality of her performance or the brooding intensity of Schoenaerts who makes his character's volte face entirely believable.

Audiard is undoubtedly one of cinema's most singular and affecting filmmakers, a man whose work is capable of stimulating the parts that others just can't reach. Rust & Bone stands up to anything in his already formidable oeuvre but observes humanity in an often inhumane world from a slightly different angle. It questions what we have, who we love and asks what would happen if these were taken from us.

And the final word must go to Schoenaerts. Having announced his arrival with an impressive turn in Flemish thriller Loft and displayed a De Niro-like dedication to his craft in the Oscar-nominated Bullhead (he gained 30 kilogrammes, ate 1,000 cans of tuna and trained six days a week for a year and a half for his ferocious portrayal of a crooked cattle farmer addicted to steroids), the imperious Belgian has confirmed his status as a major talent.

Jacques Audiard's Rust and Bone It all begins in the North of France. Ali suddenly finds himself with a five year-old child on his hands. Sam is his son, but he hardly knows him. Homeless, penniless and friendless, Ali takes refuge with his sister in Antibes. There things improve immediately.

PS The killer whale debate

Briefly, it is worth noting that Animal Defenders International have called for a boycott of this film because it shows orcas in captivity. In a recent press release the organisation argues that prolonged isolation from natural habitats encourages unusual behaviour (including aggression) and significantly reduces life expectancy.

You would be a fool to argue that captivity is a good thing but I must agree with author Craig Davidson, who wrote a detailed response on his blog. This was a rash outburst from the ADI. If Audiard and his team shoot scenes at an aqua park that does not mean that they are supporting or promoting captivity and cruelty to animals. If anything, this is a cautionary tale showing mankind what happens if you try to capture and contain wildlife. Let's hope that Audiard makes the statement that the ADI wants and addresses the issue in the context of this film. A little common sense, please.