The film is gliding along, well into its second hour of stately intrigue, as a young woman in Japanese-occupied China woos a Chinese collaborator, hoping to get close enough to kill him. Then the man (Hong Kong star Tony Leung Chiu-wai) takes the woman (newcomer Tang Wei) to bed, and Ang Lee’s Lust, Caution becomes a different movie. In three startling sex scenes, the two actors mime first a brutal seduction, then a sadomasochistic pas de deux, then the flexing of the woman’s wiles until she has achieved erotic control of her prey.