Blood Feud

"Blood Feud" is the last episode of the second season of The Simpsons. However, according to The Simpsons Archive,"Blood Feud" technically does not belong to any season, as it aired after the formal end of the second season and before the beginning of the third season.

Synopsis
When Mr. Burns falls ill and desperately needs a blood transfusion, Homer discovers Bart has Mr. Burns' rare blood type, and Homer urges his son to donate, promising that they will be handsomely rewarded. However, Mr. Burns knows nothing of this promise, or even of the Simpsons' desire for a reward, and having received the blood, all he does is send the family a card. Enraged, Homer writes an insulting reply, but Marge convinces him at the last minute not to send it. Later the letter goes missing, as Bart has mailed it, still thinking its message is called for.

Homer and Bart's attempts to stop the mail delivery end in failure, and Mr. Burns is furious at the insult and demands that Homer be beaten. Waylon Smithers calls off the beating, however, on the grounds that this action is no way to thank the man who saved Mr. Burns' life. Just as Marge had convinced Homer to refrain from sending the letter, Smithers has a calming influence on Mr. Burns, and the two men decide instead to buy the family a present. The Simpsons receive an antique Xt'Tapalatakettle, an Olmec head (a massive, Tiki-god-like affair) that Bart, the blood donor, likes, and in which Homer hates.