Shatter Thriller

A naked woman in red high-heeled shoes is perched on the edge of Clifton Suspension Bridge with her back pressed to the safety fence, weeping into a mobile phone. Clinical psychologist Joseph O'Loughlin is only feet away, desperately trying to talk her down. She whispers, 'you don't understand,' and jumps.
Later, Joe has a visitor - the woman's teenage daughter, a runaway from boarding school. She refuses to believe that her mother would have jumped off the bridge - not only would she not commit suicide, she is terrified of heights.
Joe wants to believe her, but what would drive a woman to such a desperate act? Whose voice? What evil?