Three weird sisters (witches) meet up in a deserted place on a stormy night, and plan to meet Macbeth.
Act 1, Scene 4
Let not light see my black and deep desires
King Duncan, in his Forres camp, learns about Macbeth's bravery and the thane of Cawdor's betrayal in the battle against thee rebels through the army's sergeant,. He then gives the title of Cawdor to Macbeth.
Act 1, Scene 5
The three witches appear in front of Macbeth and his friend, Banquo, and tell him he shall be Thane of Cawdor and King. At first not believing them, Macbeth is proven wrong when the Thane of Ross announces it.
Act 1, Scene 6&7
And if we should fail?
King Duncan, in his castle, praises loyal Macbeth for his bravery, and proudly calls him by his title of Thane of Cawdor. However, Macbeth knows he will be King one way or another and take Duncan's place.
Lady Macbeth receives a letter from her husband telling her about the witches prophecy. She swears that he will become King. She then starts preparing for the King's and Macbeth's arrival at her castle.
... and shalt be what thou art promised
Duncan and his troop arrive at the castle and are greeted by Lady Macbeth. Macbeth starts having second thoughts about the witch's prophecy... his wife's plan to kill Duncan doesn't help him either.