Macbeth (still at Dunsinane) insists that banners be hung outside the castle.
Many of his former forces are now fighting against him on the English side, making it difficult for him to meet the army in a glorious blaze.
He's still feeling pretty good, since Dunsinane is so fortified that he imagines the enemy army will die of hunger and sickness before he ever even needs to leave the castle.
In the meantime, a shrieking of women tells Macbeth that his wife is dead—it's suicide.
Macbeth here launches into one of Shakespeare's (and literature's) best known and oft-quoted speeches, beginning "She should have died hereafter," meaning one of two things: she would've died eventually so she might as well have died today or, she should have died later because I'm super busy defending the castle right now.
He also gets to say the super famous line, "Life's but a walking shadow […] a tale told by an idiot full of sound and fury," which is not only an early, maybe the earliest occurrence of Existentialist thought in literature—it's also the basis of William Faulkner's famous work, The Sound and the Fury.
Macbeth is quickly distracted by the news that a "grove" of trees seem to be moving towards Dunsinane, which is all around bad news, since said "grove" is likely Birnam Wood.
Macbeth finally realizes that the prophecy was as twisted as the prophets, but he's going to face the army anyway. If you have to go down, you might as well go down fighting.