Johnny Carson – the man who defined the Tonight show – is nearing retirement. But NBC has yet to settle on an heir. And that sets the stage for one of the most painful moments for the late night rivals battling for the throne. It will prove enormously embarrassing for Carson, Leno and Letterman. And they will vent their sentiments on stage. Meanwhile, Arensio Hall, a young, black comedian, is blowing past all of them, and posing the first serious challenge to Carson.