In the royal palace of Forres, Banquo states his suspicion that Macbeth fulfilled the witches' prophecy by foul play. But he notes that since the prophecy came true for Macbeth, perhaps it will come true for him as well.
Banquo suspects Macbeth, but it is his own ambition—the possibility that the prophecy might be true for him too—that occupies his mind.
Macbeth enters, with other thanes and Lady Macbeth. He asks Banquo to attend a feast that evening. Banquo says he will, but that meanwhile he has to ride somewhere on business. Macbeth asks if Fleance will be riding with him. Banquo says yes, then departs. Once he's alone, Macbeth sends a servant to summon two men. As he waits for them to arrive, he muses if the witches prophecy is true, then Banquo's descendants will be king, and he'll have murdered Duncan for nothing.
Macbeth wants to kill Banquo because he resents Banquo's honor and because the prophecy makes Banquo a threat. Also, Macbeth's guilt at murdering Duncan makes him want that murder to be "worthwhile." Macbeth's guilt about one crime pushes him to commit another.
The two men (identified in the stage directions as "murderers") enter. Macbeth tells them it's Banquo's fault they're poor, then questions their manhood for bearing such offenses. The murderers agree to kill Banquo and Fleance.
Macbeth uses the same methods to get the murderers to kill Banquo and Fleance that Lady Macbeth used against Macbeth: he questions their manhood.