A short summary of the plot, Part 2 :-)

One day, when Ben leaves the house to go to work, Christine's phone rings.
She hears a familiar voice, and she notice soon that she is talking to her psychiatrist. He tells her that she is writing a diary for some time and that she should hide it from Ben.

She opens the diary and the first sentence which she read is:
"Don't trust Ben!!!"

So Christine knows that somebody is playing a very bad game with her...