Ending

I'm the one to tell you, because I wrote the movie. SPOILER ALERT. We are led to believe that it might be the sister who's been threatening her. Someone comes into the house late at night, the husband goes downstairs with a gun while Susan waits upstairs. She hears a shot. We think that he's killed the intruder, the crazy girlfriend, but it turns out that the husband has been making the threats all along. He was pretending to be a jealous ex so as to create a non-existent person to murder his wife.

Perhaps I've been pampered too much with Nat's beauty in her sexier movies, but I got the impression she was not so cute here.