On a dark and stormy night, the three witches are hanging out in a cave roasting marshmallows and chanting spells around a boiling cauldron, into which they cast all sorts of nasty bits, from lizard's leg to the finger of a "birth-strangled babe." Hecate enters, announcing "something wicked this way comes." Not surprisingly, Macbeth promptly follows. (So does a Ray Bradbury novel and cinematic adaptation, but not for another few centuries.)
Macbeth gives the witches some props for being able to control the weather and conjure crazy winds that batter churches, cause huge ocean waves to "swallow" ships, destroy crops, topple castles, and so on. (Hmm...this reminds us of Act I, scene iii, where the witches say they're going to punish a sailor's wife by whipping up a nasty little storm for her husband, who is at sea.)
Macbeth says he has some more questions about his future and he wants some answers from the weird sisters, pronto.
The witches add some more ingredients to the cauldron, and then apparitions begin to appear, each addressing Macbeth.
First, an armed head warns him to beware of Macduff. Second, a bloody child promises, "None of woman born shall harm Macbeth." Macbeth welcomes this good news and, assuming Macduff was born the natural way, Macbeth thinks he has nothing to fear.
Though he has no need to kill Macduff now, he pledges to do it anyway – you know, just in case.
The third apparition is a child wearing a crown with a tree in his hand. The child promises that Macbeth won't be conquered until Birnam Wood marches to Dunsinane. This seems about as unlikely as Macduff not being born of a woman.
Given all of this, Macbeth feels safe that he won't be conquered in the upcoming war. But again, to be on the safe side, he still asks if Banquo's children will ever rule the kingdom.
He is warned to ask no more questions.
He demands to be answered anyway.
Macbeth is not pleased when he's shown a line of eight kings, the last of which holds a mirror that reflects on many more such kings. One of the kings in the mirror happens to be holding two orbs.
Time for a History Snack: King James I of England (a.k.a. King James VI of Scotland) traced his lineage back to Banquo and, at his coronation ceremony in England (1603) James held two orbs (one representing England and one representing Scotland). Quite a coincidence, don't you think?
The apparitions disappear and the witches tease Macbeth for looking horrible when he saw his future destruction. The witches do yet another song and dance routine and they vanish.
Enter Lennox to find a perplexed Macbeth. Lennox tells Macbeth the news that Macduff has definitely run away to England, presumably to get some help for a rebellion.
Get your highlighter out because this next bit is important: Macbeth says that from now on, he's going to act immediately on whatever thought enters his mind: "From this moment / The very firstlings of my heart shall be / The firstlings of my hand." In other words, no more thinking and contemplating about the pros and cons of being bad – he's just to do whatever the heck he feels like doing.
Starting with… wiping out Macduff's entire family, especially his kids, since Macbeth doesn't ever want to see any little Macduffs running around.