Well, in a nutshell, an honorable man falls from grace and is punished for it.
Macbeth, recognized for his military skill and integrity, is given a prophecy by three witches. They tell him that he will receive two titles other than the one he has--one of those is "King". He does get the other title, and dreams of becoming King of Scotland. He tells his wife, and everything else is history. Together, the two plot to make the prophecy come true. They kill King Duncan, frame his sons, and rise to the throne. Macbeth also kills his best friend and attempts to kill his son, Fleance, as a result of the prophecy. To ensure his throne, he has others murdered. Lady Macbeth can't take it and kills herself. Macbeth vows to fight the good fight until the end, and he is beheaded. Duncan's son, Malcolm, comes to the throne and Scotland is again a peaceful realm.
There is much detail that I have left out of this extremely succinct rendition. I suggest you fill in the blanks with the detailed analysis here at enotes.