Welcome To New York is one of those films that needs to come with a warning beyond its 18 certificate.

This is partly due to lurid sex scenes that will have you squirming in your seat, and partly because the film's gaze upon depravity and moral bankruptcy is so unflinching. As with director Abel Ferrara's masterpiece Bad Lieutenant, watching this is akin to staring into the abyss.

A fiction derived from real-life scandal, it opens with a very lengthy sequence depicting a sex and drugs orgy, at the heart of which is a gargantuan, monstrously naked Gerard Depardieu. Buffeting us with boredom and revulsion, it is incredibly difficult to watch. But there's a point. And I would say "hang in there", because what follows is a compelling illustration of the corruption of power and the nihilism of addiction, with the sort of no-holds-barred performance that comes along very rarely.

Though Ferrera opens with the usual disclaimers, there's no doubt who his model is: Dominique Strauss-Kahn, the former head of the International Monetary Fund and French presidential hopeful, until allegations that he sexually assaulted a hotel maid in New York ushered an end to his public life.

The character played by Depardieu is named Devereaux, powerful head of a world bank who appears to have nothing on his mind except sex. In his Washington office, he brazenly tries to encourage a colleague to engage with a prostitute. He then shuffles on to a plane to New York, and the drug-fuelled orgy in a hotel suite. After the group leaves, Devereaux has a nap before starting over with two newly arrived escorts.

When he tells these women that "I'm not a spring chicken", he really isn't kidding. Depardieu takes "warts and all" to a new level. Marlon Brando used to carry his excessive weight with a certain grace - anarchic in his destruction of his own famous beauty, yet not without a certain dignity; Depardieu, no looker in the first place, is subversive in a different way, growling and wheezing and throwing his obesity in our faces.

This debauched preamble serves a purpose, in unveiling the animalistic quality of Devereaux's sex addiction. And the next morning, as a maid enters the suite to do her job, we find that the banker can't contain those appetites, whether sex is consensual or not. "Do you know who I am?" he asks her, with the chilling assumption that comes with privilege: that he can take what he likes with impunity.

This spontaneous and casual sexual assault propels the film into its main phase, in which the banker finds himself arrested and awaiting trial, and his equally powerful wife (Jacqueline Bisset) flies into New York to oversee his defence. Shockingly, she is concerned less with his adultery, or his crime, than with his sabotage of their political ambitions. The two enter a duel of reflection and recrimination.

Here Ferrara and Depardieu dare to give depth to their character. When Devereaux makes a nocturnal soliloquy on the loss of his idealism, just at the moment that he had the power to do good, this sociopath suddenly seems Shakespearean in the scale of his personal tragedy; like Richard III, though, he remains as much monster as man.

Like Harvey Keitel's performance in Bad Lieutenant, Depardieu's is as much soul-baring as it is flesh-baring, becoming increasingly subtle and disturbing as the dust settles. And as with Keitel, one knows not to call it brave, for this formidable actor wouldn't see it like that at all.