The woman on the ledge

A woman falls to her death from a London bank's twenty-fifth floor roof terrace. You're arrested for her murder. You tell the police that you only met the victim the previous night at your office party. She was threatening to jump from the roof, but you talked her down. You've got nothing to do with the tragedy. You're clearly being framed. So why do the police keep picking holes in your story? Even your lawyer doesn't seem to believe you. It soon becomes obvious that you're keeping secrets. But who are you trying to protect? And why?