In Gertrudes room Hamlet goes to speak to his mother as polonius hides behind the curtain. His mom was scared so she cried out for help and Polonius repeated after her so Hamlet said " How now, a rat? Dead for a ducat, dead." and he kills Polonius thinking it was Claudius.
In the middle of talking to his mother rather madly the ghost appears again but Hamlet is the only one who can see him and the ghost reminds hamlet that his purpose is to avenge his death. Gertrude is scared for she can not see this ghost.
The ghost has left and Hamlet goes on to tell his mother to leave his uncle fathers bed. He tells her all about Claudius's plan to send Hamlet to England and that the trip to England is a set up and that Hamlet is planning to act violently. Hamlet proceeds to exit dragging Polonius away.