In the opening scene of Macbeth (Act I, Scene I), there is a raging storm going on. There is lightning, thunder, and heavy rain. On the abandoned Scottish heath, it is dark and foggy and eerie. The three witches (or weird sisters) introduce the grotesque tone of the play. They make a reference to the battle going on between Scotland and Norway, and say that they will meet again when the battle is over. This already sets up a strange thing happening--the evil and foreboding events will happen AFTER the actual war. Thus, it implies that the tragedies that will happen are secretive ones. King Duncan believes that he has defeated his enemies, without realizing that his true enemies are his own people, whom he trusts and loves.
The witches plan on meeting Macbeth once this is over, signaling the name of the titular character and thus alerting the audience to the importance of this scene. The scene sets up Macbeth's tragic fate--these evil figures are planning his doom already. His end is already in sight, even before he makes any decision at all.
Despite being very short, the opening scene deals with many of the major themes of the play--evil, ambition, fate, inversion, appearances vs. reality, the supernatural, violence, and madness.
The opening scene of the play sets the tone and atmosphere of the drama ahead. The setting is a lonely, barren heath, and its characters three strange apparitions who appear to be witches. Their rhythmic chanting helps to set the tone of foreboding and evil. By the mention of Macbeth's name, he is connected to these evil and portentous forces.
We are alerted to the duplicity within the play as well as its evil forces by the ambiguous nature of the witches’ language, in their signature line-
Fair is foul, and foul is fair.