The Guilty Party

On a night out, four friends lose each other in the crowd - and each witness a stranger in trouble. One by one, they each decide to do nothing to help. Later, a body washes up on the banks of the Thames - and the group realises that ignoring the victim 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? Is it possible the victim was not really a stranger at all?