In Lust, Caution, director Ang Lee switches genres yet again to fashion a mesmerizing erotic thriller that risks much and expands the boundaries of cinema. Set mostly in Japanese-occupied Shanghai during World War II, the story (adapted from a short story by Eileen Chang) centers on a student actress who becomes the willing bait in a scheme to kill a Chinese collaborator. The film's first two-thirds unscroll in slowly smoldering fashion—too slow, some feel—finally exploding in graphic sex scenes between the woman and the collaborator. Their carnal dance—which Lee himself has called "the crux of the movie"—suggests that only by performing and impersonating do the two natural enemies tap into their truest selves.

Shy but radiant young Wang (Tang Wei) has been abandoned in Shanghai by her father, who has fled to London with her brother during the war. After joining a patriotic theatre troupe at the university, Wang discovers her calling as an actress. Spearheaded by its zealous leader, the troupe hatches a naïve plot to shoot collaborator Mr. Yee (Hong Kong idol Tony Leung Chiu-wai), using Wang, their star performer, as bait. Wang infiltrates Mr. Yee's family, playing mah-jong with his wife (Joan Chen in a wicked cameo). Eventually Wang becomes his mistress. But she's in way over her head with the dapper, feral Mr. Yee, and finds her patriotism subverted by desire.

First-timer Tang Wei, almost continuously on screen, is remarkable and sympathetic as a shy student reborn as a femme fatale. Leung nails the sinister intelligence chief Mr. Yee, who's often seen as a shadowy noir-type figure, gliding through Shanghai in his black car, as if it were a carapace. Without much dialogue, Leung delivers the sexiest villain in recent memory, going where few actors would venture. In their lovemaking scenes, the pair convey a wealth of conflicting emotions, blurring the boundary between impersonation and the true self, undermining the very notion of a stable identity. If the sex is simulated, you'd have fooled me. Lee has called what the actors deliver here "ultimate performance."

Lust, Caution will likely fail to find the crossover audience of Lee’s Brokeback Mountain, that other tale of forbidden love which became a cultural flashpoint. At 157 minutes, the subtitled film runs long, and the tango between Wang and Mr. Yee—while appealing to the voyeur in us all—is marked by a shocking cruelty. But for those willing to enter its dangerous world, the rewards of this film are great.