The Guilty Party

On a night out, four friends witness a stranger in trouble. They decide to do nothing to help. Later, a body washes up on the banks of the Thames - and the group realises that ignoring the woman has left blood on their hands. But why did each of them refuse to step in? Why did none of them want to be noticed that night? Who is really responsible? And is it possible that the victim was not really a stranger at all