It does seem pretty confusing from the plot descriptions on Wikipedia and IMDB, and even the trailer seems confusing. But, Wikipedia says the movie is really a remake of a French movie named Love Crime. The plot description of Love Crime on Wikipedia seems a lot more detailed and reads as follows:
Set in and around Paris, France, the story revolves around two women who work in a multinational corporation. Christine (Scott Thomas) is a cold, ruthless business executive who enjoys using and betraying people. Isabelle (Sagnier) is Christine's young, ambitious and intelligent administrative assistant who regularly suffers from professional and personal humiliations at the hands of Christine.
A subplot revolves around Christine's boyfriend, Philippe, who is cheating on her with Isabelle. When Christine discovers accounting irregularties in Philippe's business accounts with the firm, Christine threatens him instead of reporting him, hoping to use him to her own advantage.
Midway through the movie, Isabelle, after suffering one-too-many implied threats and humiliations by Christine, murders her by staging a break-in at Christine's suburban house and stabs her to death. Isabelle then implicates herself in the crime, a seeming sign of a nervous breakdown, when she allows herself to get arrested and incarcerated. At this point, the imprisoned Isabelle embarks on a quest to prove her innocence of a murder that she actually committed, but her defense may prove to be even more difficult than she anticipated due to her history with Christine and of the circumstantual evidence that Isabelle herself planted.
So based on what I've seen and read so far, the movie is not a dream.