As Hamlet is speaking to his mother, the ghost of his father appears. Hamlet can see him and is confused as to why his mother cannot. His mother really believes Hamlet has gone insane. Hamlet believes his father's ghost has come back to scold him from straying from his mission (Page 81).
Hamlet hears Polonius and discovers that he was spying on his conversation with his mother. He goes over to Polonius and stabs him. This scene contributes to Hamlets master plan of getting revenge for his fathers murder (Page 87).