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Lust, Caution won the Golden Lion at the Venice Festival last September because it’s a film of pure nerve. Only a director as revered as Ang Lee could have shot a story about China as explicit and raw as this. The sex is brutal. The history is taboo. Tony Leung is a topflight informer in Shanghai in 1938, a rich and sophisticated Quisling who does night shifts for the Japanese.

His impeccable manners and wealthy background give Leung’s character Mr Yee unique access into the secret lives of his social and intellectual peers. He specialises in hunting down Chinese resistance fighters, and when time permits he attaches their genitals to the national grid. Not with any great glee. His flair for the job is matched only by his polished indifference to human emotion. Indeed that’s exactly what Mr Yee’s marriage is like to his chattering, empty-headed wife.

Tony Leung has played many inscrutable parts in his time, but the deep menace he generates simply by being well-dressed and polite is precisely why this film is such a gripping watch. The clever distraction is a beauty called Wang Jiazhi, and Tang Wei plays the young assassin quite brilliantly. How brilliantly will decide whether she lives or dies. She is a humble actress studying to be the next Jean Simmons. Her decision to cosy up to Mr Yee to bring him within murdering distance of a Chinese loyalist in a cupboard is the sort of plan that is doomed before anyone has actually thought of it.

The sex scenes between Leung and Wei have already sparked an international scandal – every detail is true. I felt helpless when Tony flung the groaning Wei face-down on a mattress to have his beastly way.

The critics are in seventh heaven. This is Ang Lee well and truly unplugged. The gentle Taiwanese director (although there are 32 websites sponsored by the Chinese Government that remind us that their Oscar-winning hero is indisputedly related to Mao) is rapidly becoming a cause célèbre. Lust, Caution is Lee’s erotic masterpiece. This is China’s X-rated riposte to Nagisa Oshima’s Ai No Corrido.

The hardcore yoga makes Broke-back Mountain’s gay shenanigans look like interval drinks. The intensity of the sex is far more honest and revealing than the secrets each lover tries to hide. That’s the beating heart of the film. The real shock is how much Wei seems to enjoy the graphic sadism. Leung is absolutely ferocious in the sack. He rips Wei’s clothes off as if they were so much wrapping paper. Moments later he leaps back into his Sunday best.

It’s no mystery that the Chinese censors skinned seven minutes out of the film. Until they invent a rating system – which has been on the Communist Party’s to-do list since 1926 – the local fans will have to scribble their own fantasies around Wei’s startled looks and Leung’s Roger Moore eyebrows. At more than two and a half hours the film could easily shed half a dozen further slices of overdressed drama. But every layer tells a guilty story.

18, 158mins

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