Ang Lee's "Lust, Caution" is heavy on both counts, to the detriment of what could have been an extraordinary film. Set primarily in Japanese-occupied Shanghai during the Sino-Japanese War that coincided with much of World War II, it's a highly watchable film, even an admirable one. Yet it is strangely inert and passionless, with a running time that is far too long for a movie so devoid of compelling emotion.

The story is like something from a classic Hollywood noir: spy falls in love with the person he's spying on. Except the spy is a woman, and I'm not sure it's love. She is Wang Jiazhi (Wei Tang), a wallflower college student who has gotten involved with a resistance group dedicated to ending the Japanese occupation. A Chinese leader, Mr. Yee (Tony Leung Chiu Wai), is believed to be a traitor, claiming to have China's interests at heart while selling out to the Japanese. The students' plan is to infiltrate his inner circle, learn his secrets, and assassinate him.

Wang is the bait, posing as a Mrs. Mak and joining the regular mahjongg games hosted by Mr. Yee's wife (Joan Chen). This makes her privy to the officers' wives gossip and gives her a few brief encounters with Mr. Yee, who occasionally pops in to see what his wife and her flock of friends are up to before heading off to a dark meeting somewhere.

Mr. Yee and Wang eventually become lovers -- though, again, "love" has little to do with it. Reserved and inscrutable in conversation, Yee proves to be a forceful, violent sex partner; Wang, meanwhile, has no experience at all other than the perfunctory encounters with a fellow resistance fighter that served as her dress rehearsal for Yee.

She is Yee's mistress, his kept woman, and their sex scenes are fairly explicit. (Worthy of an NC-17 rating? Eh.) There is a progression in their rendezvous. He becomes somewhat more tender, and she develops an attachment to him that jeopardizes the mission (which, you'll recall, was to kill him). What there is not, however, is any indication that their feelings go beyond physical attraction. If she is infatuated with him because he is her first lover, and if that clouds her judgment, Lee isn't interested in exploring that.

That's not the only facet of the story that is underdeveloped. The leader of Wang's resistance group is Kuang (Lee-Hom Wang), typically idealistic and zealous for the cause but also harboring a crush on Wang. There comes a time when violence is called for, and Kuang answers the call -- shockingly and grotesquely. Where did that come from? Who knew he had it in him? What does it mean? The subject is never addressed again.

The film is based on a short story by Chinese writer Eileen Chang -- a short story, mind you, which has been adapted to its 158-minute movie length by regular Lee collaborators James Schamus and Hui-Ling Wang ("Crouching Tiger, Hidden Dragon"). It is too slow and glum to be an espionage thriller, save for two or three energetic sequences. It lacks the passion and emotion necessary to be a tragic romance. It is not interested in the details of the Sino-Japanese War, so it's not a historical epic.

So what is it? It's a feast for the senses, for one thing, with gorgeous, lush cinematography by Rodrigo Prieto ("Brokeback Mountain") and a haunting musical score by Alexandre Desplat ("The Queen"). Joan Chen is a subtle strength as Mrs. Yee, always on the verge of being dangerous and catty, the kind of character you wish had more scenes in the movie. Does she know her husband is cheating on her with her mahjongg buddy? Does she care? We don't know.

I keep thinking about the things I wish the film had done instead of the things it did do. Maybe that's because the former list is so much longer than the latter.

Grade: C+

Rated NC-17, a scene of graphic violence, a few scenes of very graphic sexuality and nudity

2 hrs., 38 min.; Chinese with subtitles