Burns falls ill and it's found out he has leukemia, and he has a very rare blood type. Homer asks around for his family to give blood, and Marge tells him Bart has the same blood type. Bart gives the blood, and Homer expect a reward. After simply getting a thank-you letter, Homer writes a letter back about how disgusted he is with Burns' lack of appreciation of Bart saving his life. Homer decides it shouldn't be mailed. Bart, however, mails it anyway. When Mr. Burns reads it, he thinks it's merely a joke, and gives the family a $32,000 Olmec Indian head, which can be seen in several episodes in the future.
I love this episode. The episode is slow in some spots, but the whole scene with Homer "watering the mail" is the funniest scene in the whole season in my opinion. Definitely one of the best season finales in the Simpson's history.
And that's the end of the season. Off to reviewing season 3!