Star rating: ***
Director: Ang Lee
With: Tony Leung, Tang Wei, Joan Chen

For a director often noted for his cool sensitivity, Ang Lee has a flair for generating brush fires in the media. His Oscar-winning Brokeback Mountain shattered its share of taboos, and since Lust, Caution took the Golden Lion at Venice last year its graphic sex scenes have been the subject of endless "did they, didn't they?" discussions.

While one can sympathise with Lee's dismay at his painstakingly realised wartime thriller being given the Knocked Up treatment, he protests too much. Sex is to Lust, Caution what dance is to Singin' in the Rain. There is more to his film, far too much in fact, but nothing to sear a hole in the screen or the imagination like the physical drama between the two leads.

When the picture opens in Shanghai in 1942 there is little clue to the passion that will eventually break loose. Rodrigo Pieto, Brokeback Mountain's cinematographer, paints the city in shades of greys and black, the only flashes of colour coming from the blood red in the Japanese flag. Shanghai is occupied and its Chinese citizens preoccupied with the business of survival. In one home, however, a group of ladies is more concerned with the business of mah-jong and gossip. Mrs Yee is holding court, with her shy young friend, Wong Chia Chi (Tang Wei) among those in attendance. The only event for which the women draw breath is the arrival of Mr Yee (Tony Leung), man of the house, local dignitary and collaborator. The exchange of pleasantries that follows sets a pattern the rest of the film will follow. Although little is apparently going on, messages are being sent, clues set down and it will be a long time before the end game becomes clear.

Make that an agonisingly long time. Heeding the warning in the film's title, Lee resists any temptation to reveal too much of his story too soon. He insists on having everything, including a detailed picture of the politics of the time, in place before the tale proper begins, and his film suffers as a result. Indeed, by the time he leads us by the hand to the bedroom door, a whole hour and a half in, the exhausted and bored viewer may be more in the mood for nothing other than a nice cup of tea. But bear with him. When Lee lights the blue touchpaper, his movie takes some thrilling, beautifully executed, unforgettable turns.

Based on the story by Eileen Chang, the basic premise is sketched in swiftly enough in a series of flashbacks. Wong Chia Chi, a student, has been left alone after her father's departure for England. Joining her university's am-dram group, she finds both a band of friends and a cause. War is approaching and, as Japan's intentions towards China become clear, Wong enlists in the resistance. Her handlers, homing in on what they see as her only value, assign her a mission: she is to be the honey in the trap that snares the quisling Yee.

As the seduction gathers pace, the film becomes less about the act of sex itself than what it means. Yee and Wong become as intimate as it is possible for two people to be, yet theirs is a sexual dance of many veils, with each trying to conceal as much as possible from the other.

The scenes are certainly graphic, but the physical gymnastics are nothing compared to the mental workouts going on. This is sex as interrogation - brutal, raw, animalistic, sometimes tender, and always shocking in its intensity.

Tang Wei puts in an astonishing performance in what is her first film. Though the character is exploited at every turn, the Ang Lee of Sense and Sensibility, ever the feminist film-maker, never allows her essential dignity to waver. She is no cardboard cut-out Mata Hari; this is a complex woman who is under no illusions about what she is doing, and what it is doing to her. The scene where she spells this out to her handler is one of the film's best. Though the older man is meant to be the flint-eyed master to her novice, it is he who is left quaking with shock at her honesty.

Tony Leung succeeds in making a many-faceted monster out of Yee. Flitting from torture chamber to bed chamber, he reveals just enough of a conscience to keep the character interesting. These flashes of humanity, and the odd hint of sly humour, are never enough to lift the gloom that hangs heavy over the film.

There's a weightiness to Lust, Caution that suits the story but is ultimately the movie's undoing. Though never muddled, it is overcrowded with themes, history, characters and its own ambitions. With an ending that rivals Crouching Tiger, Hidden Dragon for excitement, Lee shows what a near-masterpiece his movie could have been had his focus been narrower. A clear case not of "lust, caution", but "information overload, beware".