She had trusted the nanny with her secrets and to take care of her kids, which is why the nanny had the therapist role in her head. Except.what if a girl gave her son AIDS, what if she saw the girl who went missing (newspaper headline in her cart at the end) and didn't save her, like she didn't save her brother. Her mental illness takes a turn, and basically, most of the movie is her delusions of what life would have been like if things had gone on-and happily. She catches her husband with the nanny, possibly kills him, drives off with her kids and gets in an accident (if it is an accident and not an intentional red-light run, which would explain the bomb on he school bus delusion). She hears the bank robbery vaguely in the background on the phone when her boss calls her before she hangs up.
We know she has some mental health issues. She became an obsessively overprotective mom as a result.
If you're confused: The facts: Danika did lose her brother as a child.
I actually am surprised at how much of an impact it had on me now and how much I thoroughly understand what I saw. I went back and watched it today, remembering the ending and certain scenes but not much else and I didn't hold out much hope to get it. I saw this movie when I was 17 and had no idea what the ending meant, back then we didn't have IMDB and there weren't really explanations on google.
