In Shakespeare's Macbeth, one of the witches' first utterances in the play sets the tone for the rest of the play. Once the witches become involved, the theme of "fair is foul and foul is fair" is seen in every corner of the drama. Shakespeare uses this theme to cast doubt on even the most noble of characters, especially in that Macbeth, a seemingly noble and devoted subject (and cousin and friend) to Duncan, is soon blamed for killing the King and of other terrible acts. The witches seem to call up this dark image, perhaps like a spell, and "command" that it hover through the air, where nothing is left untouched.
Fair is foul, and foul is fair.
Hover through the fog and filthy air. (I.i.11-12)
When Macbeth enters the scene from the battlefield, he echoes the same words, perhaps showing how closely aligned he actually is to what the witches have planned for him: perhaps he is more willing than even he knows, seen especially in how quickly he accepts their prophecies.
So foul and fair a day I have not seen. (I.iii.39)
Macbeth himself reflects this theme. He seems so "fair" in that he is such a hero, supportive of Duncan, and loved by Duncan. No one would, at the place's start, question Macbeth's loyalty, yet this quickly changes. He represents what seems fair ("good") and what is really foul ("evil").
When Duncan enters the courtyard of Inverness (Macbeth's castle), he comments upon how lovely it is. He has no way of knowing that what seems so fair will actually be the last place he lives to see. No one would suspect it for the castle and the air are so fine.
This castle hath a pleasant seat; the air
Nimbly and sweetly recommends itself
Unto our gentle senses. (I.iv.1-3)
When Macbeth hires the men to kill Banquo, he presents Banquo as "foul," blaming the murderers' disappointments in life on Banquo. The truth is that Banquo is the good one and Macbeth has caused their hardships, but the truth becomes cloudy, as if the air truly is "foggy and filthy."
When Macduff has had enough of Macbeth's treachery, he travels to England, where Duncan's son Malcolm, true heir to the throne, has taken refuge in the court of Edward the Confessor. However, because Macduff has left his family unguarded at home and comes from Macbeth's court, Malcolm distrusts Macduff, who is actually a very good man. Because of Macbeth's behavior, Macduff seems "evil," while he is actually a "faithful" and dedicated citizen of Scotland.
Malcolm, in order to test Macduff, presents himself as an evil man who would destroy Scotland, being a worse King even then Macbeth. In doing so, he pretends to be a threat to his homeland. When Macduff hears Malcolm's words, he believes that Malcolm is not fit to live, let alone to govern Scotland. As Macduff mourns the fate of Scotland, Malcolm realizes that Macduff is a good man. Very soon after, news comes that Macduff's family has been slaughtered at Macbeth's order, and Malcolm is further convinced that the man before him is as just and noble as he claims.
Of course, one would believe that being King and Queen would be wonderful for Macbeth and Lady Macbeth, but we soon see that their lives have deteriorated terribly, supporting this theme.
Once the words "fair is foul and foul is fair" are uttered by the witches, this theme works its way throughout the play, and it is not put to rest until the Macbeths are dead and Malcolm returns to the throne, where "God" has "ordained" he should be.