On the heath the witches appear. They call themselves the "weird sisters" (1.3.30) and brag of their dread and magical deeds such as killing swine and cursing a sailor to waste away.
The witches are established as both wicked and magically powerful.
Macbeth and Banquo enter. The witches hail Macbeth as Thane of Glamis, Thane of Cawdor, and "king hereafter" (1.3.47). Banquo asks Macbeth why he seems to fear this good news, then questions the witches about his own future. They say that Banquo is "lesser than Macbeth and greater" (1.3.63) because though he'll never be king, his descendants will.
Does the fear Banquo notes in Macbeth signal that Macbeth's doomed struggle against his ambition starts the instant he hears the prophecy?
Macbeth asks how the witches know this information. But the witches vanish, making the two men wonder if they could have imagined the whole thing. Just then, Ross and Angus enter. They tell Macbeth that the old Thane of Cawdor was a traitor and that Duncan has made Macbeth the new Thane of Cawdor.
The prophecy is fulfilled and the witches' power is proved to be genuine. The traitorous old Thane of Cawdor is replaced by Macbeth.
Macbeth and Banquo are shocked. Macbeth asks Banquo if he now thinks that his children will be king. Banquo seems unsure, and comments that "instruments of darkness" sometimes tell half truths to bring men to ruin.
Banquo guesses the witches' plot exactly. This means that when Macbeth chooses to believe the witches and act, he knows the risks.
As Banquo talks with Ross and Angus, Macbeth ponders the prophecy. If it's evil, why would it truly predict his being made Thane of Cawdor? If it's good, why would he already be contemplating murder, a thought that makes "my seated heart knock at my ribs" (1.3.134-136)? Macbeth feels that he's losing himself, and hopes that if fate says he'll become king, he won't have to act to make it happen.
Macbeth is already thinking about killing Duncan, but the thought terrifies him: he's struggling against his ambition. His thoughts about fate are classic: does fate happen no matter what, or must one act?
Ross and Angus think Macbeth's reverie is caused by becoming Thane of Cawdor. Macbeth and Banquo agree to speak about the witches' prophecy later.
This exchange with Banquo is the last time Macbeth is honest in the play.