Yin is a lonely woman whose husband Keung physically abuses and rapes her. She wants to kill him but never has the courage. He is always traveling to Shenzhen for whoring and once raped her best friend Hong, who is cohabiting with her boyfriend Long. After a night of karaoke with the live-in couple, Yin has a one-night stand with Long. Long decides to kill Keung. Hong and Yin share their tragic past, with Yin confessing to Hong about her fling with Long.
45:50 - The wife has sex with her best friend's boyfriend after having a night of karaoke with the couple. Non-nude.
The husband's supposed rape of the wife's best friend is not shown. Maybe it's just mentioned in the dialog.