Read this short summary (slightly abridged from sparknotes.com).
The witches vanish, and Macbeth and Banquo treat their prophecies skeptically until some of King Duncan’s men come to thank the two generals for their victories in battle and to tell Macbeth that he has indeed been named thane of Cawdor. Macbeth is intrigued by the possibility that the remainder of the witches’ prophecy—that he will be crowned king—might be true, but he is uncertain what to expect. He visits with King Duncan, and they plan to dine together at Inverness, Macbeth’s castle, that night. Macbeth writes a letter to his wife, Lady Macbeth, telling her all that has happened.
When Macbeth arrives at Inverness, he and his wife decide to help fate a litte.
They come up with a plan ...