In this scene Polonius requests that he and Claudius conceal themselves so that they can watch Ophelia and Hamlet, and consider what Hamlet says to her in order to ascertain whether his madness is a result of his love for her or not. Polonius makes this request and says that often actions that are made with the best intentions actually cause harm. Polonius, in an aside, sees the truth of this with great irony:
The harlot's cheek, beautied with plast'ring art,
Is not more ugly to the thing that helps it
Than is my deed to my most painted word.
This is actually the first clear indication of the guilt of Claudius in the play. He compares his guilt and what he has done to the "harlot's cheek" that is ugly in reality but has the appearance of beauty because of the heavy make up that covers it. His deed--crime of regicide--cannot be covered up by even the most "painted word."