In the start of this scene Hamlet is leading off from the cliffhanger of scene IV in which the specter/ghost declares its identity as his father.

Hamlet feels quite emotional at the sight of his late father. He is somehow reassured that it is not just any Ghost but his fathers as it explains the circumstances of his death. Soon after he is then quick to believe the Ghost as it had happened the way Hamlet ha most feared. It had come to be that his Uncle, Claudius poured poison into his Fathers ear whilst he had been sleeping. At this point Hamlet is enraged and confides with his father in finding a way to pursue his revenge.