Macbeth enters the stage alone and says he refuses to "play the Roman fool" (someone who would choose noble suicide in the face of defeat, like, ahem, Antony).
Macduff enters and calls Macbeth a "hell-hound" and Macbeth talks a little trash in return: I already killed your family so you best be steppin' back now unless you want me to have your blood on my hands, too. Macduff is having none of it.
They fight, and Macbeth continues to be cocky. He says Macduff hasn't got a chance since he, Macbeth, can't be killed by anyone "of woman born."
That's funny, says Macduff, because "Macduff was from his mother's womb / Untimely ripped." In other words: He was delivered, prematurely, via Cesarean section. And apparently that means he wasn't "born." Don't anyone tell Macduff's mom. Recovering from a medieval C-section was probably no fun. (Well, to be honest, she probably died, like the lady whose son, Robert II, may have been an inspiration for Macduff.)
Macbeth curses the "juggling fiends" and their twisted prophesy. Now that he knows he's not invulnerable, he doesn't want to fight Macduff anymore—but he also doesn't want to yield. Since he has to pick one, he decides to keep fighting...right until Macduff kills him.
Malcolm and Siward (the father of the young man Macbeth recently killed, so we guess "Old Siward") run across the stage looking for Macbeth.
Siward takes time out to exposit for the audience: there's a lot of fighting going on at the castle, the thanes are fighting exceptionally well, and Malcolm's pretty close to victory.
Malcolm, Siward, Ross, the thanes, and the soldiers all assess what's been going down during the battle at the castle.
It looks like Siward's son (Young Siward) and Macduff are missing.
Ross delivers the news that Young Siward was slain by Macbeth. That's okay, says Young Siward's dad; his wounds were on his front, which means he stood and fought, and that means he died honorably. You know, "like a man."
Things seriously improve when Macduff shows up waving Macbeth's severed head. Everyone turns to Malcolm and yells "Hail, King of Scotland." Hmm. Isn't that how the witches greeted Macbeth back at the beginning of the play?
Malcolm just can't wait to be king. When he is, all the Scottish thanes will be made earls, as in the English system, making them the first earls in Scottish history. Together with Malcolm they'll call home everyone who had to flee the country because of Macbeth's tyranny and punish all of the people who helped the Macbeths.
In his speech, Malcolm also suggests that Lady MacB took her own life, which we didn't really know till now.) But enough death-talk. It's time to party down at the coronation ceremony at Scone. (Mmm...scones.)