When Mr. Burns learns he only has a few weeks to live, he goes to commit suicide, but fails.  After the traumatizing shock, Mr. Burns is rendered like a small child mentally, and the whole town elects to take turns getting revenge on him.  In the end, Lisa takes Mr. Burns back to his mansion and he's mind returns.  He goes to get revenge against the town but instead almost has a change of heart until he realizes it's his hate that keeps him alive.