Macbeth is a Scottish nobleman who supports the king -- King Duncan -- and fights hard for him in battle. But then some witches prophesy that he will become king.
When his wife hears this, she eggs him on (not that he's unwilling) to kill Duncan so Macbeth can become king in his place. Macbeth does this and becomes king.
Once he is king, Macbeth is pretty brutal. He kills various people because he worries that they will replace him (or his sons) as king.
In the end, Macbeth makes everyone angry at him. The son of King Duncan leads a rebellion and Macbeth is killed (soon after his wife kills herself).