Here's the summary without all my notes.

 When a girl commits suicide, detectives, Charlie Jacobs and John Carson, search for evidence as to what exactly occurred. On their investigation, they discover unopened letters.

Charlie begins to read her letters and soon falls in love with the young woman who has written them. Nobody knew her name, nobody knew who she was; all Charlie knew about her was that her initials were Z.H. - which she signed her letters with.

As Charlie continues to read these letters, secrets begin to unfold, and more is discovered about what exactly happened to the woman who died. She hadn't committed suicide. She was murdered, and the letters she left behind could lead to the one responsible.