Mr. Burns falls ill with hypohemia (a fictional condition in which the body naturally runs out of blood, though it is akin to a real condition called hypovolemia) and needs a blood transfusion. His blood type, double O negative, is very rare, however, and none of the employees at Springfield Nuclear Power Plant have it. Homer discovers that Bart has double O negative blood and urges his son to donate, promising that he will be handsomely rewarded. Bart reluctantly agrees and his blood donation saves Mr. Burns' life. Burns is rejuvenated by the blood and he sends the Simpson family a thank you card. Enraged at Burns' paltry gesture, Homer writes an insulting reply, but Marge convinces him at the last minute not to send it. The next morning, Homer discovers that the letter is gone as Bart has mailed it.
Bart explains that he knew Homer would probably change his mind, and decided to send the letter before that could happen. Homer desperately tries to prevent the letter from reaching Burns, but fails. Mr. Burns becomes furious and demands that Homer be beaten. However, his assistant Waylon Smithers calls off the beating on the grounds that that is no way to thank the man who saved Mr. Burns's life. Smithers convinces Burns to instead reward the Simpson family. The Simpsons receive an antique Xtapolapocetl, an Olmec head (a massive, Tiki-god-like affair) that Bart, the blood donor, likes, and which Homer hates. At the end, as the family stare at the head, the Simpsons debate on what the moral of this whole story is. It cannot be 'A good deed is its own reward' as Bart got a reward he likes, but at the same time it is not 'No good deed goes unrewarded' as they never would have received anything if Homer had not written the angry letter. Homer decides that there is not a lesson to be learned from this, as it's "just a bunch of stuff that happened". However, Marge says that it "certainly was a memorable few days". Homer then says "Amen to that!" Bart and Lisa then laugh.