The plot is easy to sum up: Shanghai in the early 1940s. The resistance persuade a young girl to infiltrate upper class society to help them get to the collaborationists who are aiding the Japanese fascist invaders. She has to target the chief of police, Mr Yee, and become his mistress. While not softening his character, she falls in love with him. Will she betray him or her country?
The film is very long, and seems to me to be a vivid and immersing picture of a particular time and place in Chinese history. When the sex comes it is pretty explicit, quite violent, and goes on for some time. It wasn't all that sexy to me. Well, sometimes. But Tony Leung always gets cast as dark characters yet comes across to me as a nice person who is just pretending. I think he reminds me of my brother (in a funny way). And there's a person at work who looks quite like him, and is similarly nice and brotherly. So, there's a bit of dissonance for me. I know a lot of women totally go for Tony Leung, so it's just me I guess.
(ETA - Andy Lau would have been a better choice IMHO)