Here, Polonius is instructing Gertrude on what to say and how to act when Hamlet enters the room.
Do you know what Claudius did?!
Polonius then runs and hides behind a tapestry as Hamlet enters the room. Hamlet confronts his mother about his father's death, and Gertrude cries for help.
He really is insane!
I know I can't be forgiven!
Leave your mother alone, Hamlet.
Polonius then calls for help too from behind the tapestry. Hamlet hears this, and then stabs through the tapestry thinking it is Claudius listening to their conversation.
I know you hired spies!!!
Hamlet then pulls back the tapestry, and realizes that he has killed Polonius. Gertrude is hysterical, and Hamlet continues criticiizing his mother for marrying Claudius, and wonders if she knew about the King's death.
Gertrude admits that what she has done cannot be forgiven. The ghost then appears, and tells Hamlet to leave his mother alone. As Hamlet speaks with the ghost, Gertrude believes that Hamlet has truly gone mad, as she cannot see the ghost.
Hamlet also scolds his mother for ordering Rosencrantz and Guildenstern to spy on him while he is banished to England. He then exits, taking Polonius' lifeless body along with him.