Banquo is one of King Duncan's commanders, Macbeth's companion in arms, and the father of Fleance. He hears the witches together with Macbeth, receives from them a prophecy only for himself, that his sons will be kings, and ends up murdered at his friend's command.
On the heath (Act I, scene 3), the witches tell him that his sons will be kings, though he will not be. When Ross and Angus confirm that Macbeth has become Thane of Cawdor, Banquo warns that Satan's instruments tell us truths and then betray us. Later, in Act II, scene 1, Macbeth tells him that if he follows his advice when the time comes, he will receive great honor, and Banquo agrees only if he does not lose his honor, if his heart stays free of guilt, and if he remains a loyal subject.
In Act III, scene 1, with Macbeth already king, Banquo says to himself that the idea of his having played foul frightens him, but he hopes the prophecy about his sons as kings will also come true. Macbeth, for his part, says: "Our fears about Banquo run deep." In scene 3, Banquo is attacked on the road by murderers and dies after crying: "Run, good Fleance, run, run, run! You can avenge me." Fleance escapes.
Banquo remains in the play after death. At the banquet (Act III, scene 4), Macbeth sees his ghost sitting in Macbeth's own place, and the guests do not see it. In Act IV, scene 1, the witches show Macbeth eight kings, the last one holding a mirror, and Banquo, bloodied, smiles and points to his descendants.