Both a journalist and a documentary filmmaker chase the story of a murder and its prime suspect.
In one scene, Thomas, the filmmaker, has this dream or fantasy of him stabbing his ex-partner in the back while she is having sex with her lover. She screams in pain at first, but she looks at him and laughs, like her stab wound doesn't hurt at all. She and her lover laugh at Thomas while they go on kissing each other.
Now, it's clear that Thomas did not really stab his ex-partner because she is alive and well as shown in the latter part of the movie. But the cheating part might be real and the reason why they separated.
Did Kate Beckinsale cheat on her husband?
Simone (Kate Beckinsale) is the journalist that is helping Thomas get information about the murder case. When they first meet, she tells him that she has a husband. But later in the movie, she tells him about how someone spread the rumors that she and a co-worker named Joe was having an affair. That rumor caused her separation from her husband. After she tells him this, Thomas kisses her. Through a flashback scene a little later in the movie, we see them have sex that night (headshots only, with Kate Beckinsale moaning lustfully).
Later, Kate sees Thomas in his hotel room. Before they have sex, he asks her if she is fucking Joe. She replies, "Not tonight." Meaning, she is currently fucking Joe, but just not tonight. That means she is cheating on Joe. And that might also mean that the rumors were true and she may have had an affair with Joe while she was still with her husband.