The apparent normality of the play is suggested by Claudius when he says to Hamlet that all men lose fathers and that the death of Hamlet's father was a natural event. However, when Hamlet sees the ghost of his father, the ghost tells him that he was murdered by his brother. The fact that a ghost would even be seen and that Hamlet would even speak to it also disturbs apparent normality. That's why the first line of the play, "Who's there?" is so telling. In this play, we really must learn the difference between normality and abnormality. We have no idea who is really "there".