In Japanese-occupied Shanghai in the 1940s, a well-dressed, attractive young Chinese woman named "Mrs. Mak" (Tang Wei) is sitting in a caf in a posh neighbourhood. When she makes a call to a male friend, her innocuous comments are a coded signal that prompts a cell of young resistance agents to load their weapons and spring into action. The film then goes back in time to the events in 1938 that led up to the transformation of the shy, inexperienced university student Wong Chia Chi into the glamorously-dressed and seemingly well-to-do Mrs. Mak, her cover role in the Chinese resistance. Chia Chi had been left behind in China by her father, who escaped to England. While in her first year at university in Hong Kong, a male student named Kuang Yu Min (Wang Lee-Hom) invites her to join his patriotic drama theatre group. Chia Chi becomes a leading lady, inspiring both her audience and her new-found friend Kuang. Fired up from the drama troupe's patriotic plays, Kuang urges the group to make a more concrete contribution to the war against Japan. He devises a plan to assassinate Mr. Yee (Tony Leung Chiu-Wai), who is widely denounced as a national traitor. The attractive Chia Chi is chosen to penetrate the security surrounding the Yee family. In the guise of the elegant Mrs. Mak, she insinuates herself in the social circle of Mrs. Yee (Joan Chen). She catches the eye of Mr. Yee and tries to lure him into a location where he can be assassinated. When Mr. and Mrs. Yee move away, the plotters are almost exposed, and they are forced to go into hiding when they kill a collaborationist government agent. In Shanghai, three years later, Chia Chi again encounters Kuang, who is now part of an organized underground resistance group seeking to overturn the Japanese occupation. He enlists her into a renewed conspiracy to kill Yee. By this time, Mr. Yee has become the head of the Nanjing regime secret police working for the occupation to hunt down Kuomintang resistance agents. Eventually, Chia Chi becomes the mistress of Mr. Yee. During their first encounter Yee is sadistic and violent, but over the weeks that follow their sexual relationship becomes very passionate and deeply emotional, but also very conflicted for both of them, especially for Chia Chi, who is setting her lover up for assassination. When Chia Chi reports to her superior officer in the Chinese resistance, she exhorts him to carry out the assassination soon, so that she will not have to continue her sexual liaisons with the brutal Yee, but the officer argues that the assassination needs to be delayed for strategic reasons. Chia Chi describes the inhuman emotional conflict she is in, on one hand sexually and emotionally bound to Mr. Yee and on the other hand part of a plot to kill him. When Mr. Yee sends Chia Chi to a jewellery store with a sealed envelope, she is surprised to discover that he has purchased a large and extremely rare 6 carat pink diamond for her, to be mounted in a ring. This provides the Chinese resistance with a chance to get at Mr. Yee when he is not accompanied by his bodyguards. The next time Chia Chi and Mr. Yee meet, she asks him to go to the jewelry store with her to collect the diamond ring. As they enter the shop, she notices several resistance agents waiting to spring the trap. But when she sees the magnificent ring, and experiences Mr. Yee's love for her, she is overcome by emotion and breaks down and urges him to flee. Mr. Yee runs out of the shop and is rushed away by his driver, and escapes the assassination attempt. By the end of the day most of the resistance group including Kuang and Chia Chi herself are captured. It is revealed that Mr. Yee's deputy has been aware of the resistance cell, but did not inform Mr. Yee, both because of Mr. Yee's relationship with Chia Chi and because the deputy had hoped to use this opportunity to catch the resistance cell leader. Mr. Yee, emotionally in turmoil, signs their death warrants and the resistance group members, including Chia Chi, are led out to a quarry and executed. In the last scene, Mr. Yee sits on Chia Chi's empty bed in the family guest room, and informs his wife that their house guest is gone, and that she should not ask any questions.