Mammuth, Benoît Delépine, Gustave Kervern, 92 mins (PG)

A meat-processing worker mounts his bike in search of his pension; but the real story is that, although Gérard Depardieu has become porky, his career is still in good shape

(Axiom Films )

Gérard Depardieu is one of modern cinema's foremost "sacred monsters" – but, for years, his career has been more monstrous than sacred.

In the early Nineties, he could still be relied on to bulk out prestige adaptations of Balzac and Zola. But, perhaps for fear of becoming an establishment totem, Depardieu has spent much of his time since then taking the international dollar as a jobbing jolly Frenchman, from his crowd-pleasing turn as Obélix the Gaul to Hollywood hack jobs like 102 Dalmatians (as Jean-Pierre Le Pelt, if you please).

Amid this concerted campaign to devalue his brand, Depardieu will occasionally chalk up a performance that proves he can still be magnificent when he deigns to stretch himself. In two weeks' time, you can see his affable and rather touching turn in François Ozon's Potiche, as an ageing communist romancing Catherine Deneuve. His superb performance in The Singer (2006) was all but an overt self-portrait – a hard-bitten crooner on the town-hall circuit, retaining his dignity despite accusations of being ringard, or tacky. "You're only tacky if you've lasted," says his character – Depardieu's own profession of faith as a survivor.

Now here's another self-portrait of sorts. Mammuth is a monument to the magnificence of Depardieu's ruin – and his occasional on-screen resurrection. The star plays Serge Pilardosse, a greasy-locked pork-processing worker who hasn't achieved much, but who can count on the support of his sharp-tongued but loving wife (Yolande Moreau). Serge doesn't have any friends – when he retires, his co-workers turn up for a hilariously joyless farewell drink, chomping crisps through the boss's by-rote speech. He also has few interests, although he takes pride in his knowledge of pork products – occasioning a prickly face-off with a supermarket charcuterie assistant (a nicely sullen turn by one of the film's two writer-directors, Gustave Kervern).

It emerges that Serge has never gathered the paperwork required to draw a pension, so he sets off on his long neglected Mammuth motorcycle (below) to find his documents. That involves meetings with some loathsome people – petty officials, a sneering nightclub bouncer – and some agreeably eccentric ones, notably a musical gravedigger. The film is a testament to strength of character, and a condemnation of the indignity of labour – themes beloved of directors Benoît Delépine and Kervern (aka "de Kervern"), French TV satirists who have also made a handful of brittle anarchist comedies, most recently the riotous screw-the-system story Louise-Michel.

Once he's on the road, Serge's romantic back story emerges – as a youth, he lost the love of his life in a bike crash. She eerily materialises now and then, played by the rarely sighted Isabelle Adjani, here resembling a well-preserved Gallic cousin to Morticia Addams.

Like all Delépine-Kervern films, this one rolls along episodically in a way that suggests the duo are making it up as they go along. Mammuth feels as if it's headed somewhere until Serge meets his niece – played by real-life outsider artist Miss Ming, a specialist in creepy doll-part bricolage. Her impassive, child-like delivery puts the film on a slower, sweeter footing, imparting a neo-hippie vagueness to the proceedings. With its high-contrast pastel-toned colour photography, Mammuth harks back to late-Sixties road quest movies, even culminating in the unnerving sight of Depardieu in a billowing kaftan.

The mock-beatific stuff is neither here nor there: the film really scores when it's scabrous. There's a nice sequence in which Serge meets a vampish con artist (silky-voiced Anna Mouglalis), and an outrageous, unsettling sight gag about what happens when two flabby male cousins get together after decades apart. It's not something that bears looking at, but Depardieu's expression, and his weary "Oh la la ..." are priceless.

As for filling the screen, when Serge strips off to bathe in a river, it's like witnessing some ancient hippo god in its primeval splendour. The film-makers also delight in extreme close-ups in which their star's bag-o'-spuds physiognomy sprawls across the screen, his bifurcated nose pointing east and west simultaneously.

Depardieu was once one of European cinema's great romantics, France's definitive bit of on-screen rough. These days, he likes to send himself up with a total lack of vanity, but rarely this brazenly. Mammuth is his announcement to the world that he doesn't give a toss what we think of him; it's one thing to be ringard, here he's a walking disgrace, and proud of it. His Serge is at first a shambling, clueless lunk who's screwed up his life – and perhaps Depardieu's picture of how he himself might have ended up if he hadn't got hooked on acting. But the character's final triumph is also the actor's. Mammuth is too soft-centred to be Delépine and Kervern at their best (for that, seek out their demented Aaltra, a road movie with wheelchairs). But the film is a raucously anti-narcissistic triumph for its star, who shows that there's still a magnificent cheval de guerre lying dormant inside the ragged old pantomime horse.

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