Macduff is the Thane of Fife, the nobleman who discovers Duncan's body, flees to England, loses his family in a massacre ordered by Macbeth, and finally kills the tyrant.
In Act II, scene 3, he goes to wake the king as ordered, and returns shouting: "O horror, horror, horror!" He does not go to Scone for Macbeth's coronation and heads to Fife (scene 4). Later he misses the new king's banquet, and in Act III, scene 6, a lord tells that he went to England to ask King Edward for support for Malcolm.
In Act IV, scene 1, the first apparition of the witches warns Macbeth: beware Macduff, beware the Thane of Fife. When he learns that Macduff has fled, Macbeth decides to take his castle by surprise and put his family to the sword. In Fife, Macduff's wife accuses him of abandoning them ("He does not love us"), and murderers kill their son (scene 2). In England, Malcolm tests Macduff's loyalty, and Ross tells him that his castle was taken by surprise and his wife and children brutally murdered. Malcolm tells him to let his grief turn into anger.
In Act V, scene 7, he confronts Macbeth, who says that, of all men, he has avoided him. Macbeth claims that no man born of woman can kill him, and Macduff answers that he was torn from his mother's womb before his time. After the duel, he returns with Macbeth's head and greets Malcolm as king.