Mr. Burns is on his deathbed, suffering from a low blood level. His blood type is rare (00-) and only Bart can help him. Driven by greed, Homer orders Bart to donate his blood. Marge only wants Bart to do it because it's the right thing to do.
After his transfusion, Mr. Burns decides to send a thank you card to the Simpson family. This angers Homer because he felt he deserved riches for helping Mr. Burns. He writes a sarcastic and insulting letter to Mr. Burns but doesn't send it when Marge tells him to sleep on it first. Homer wakes up the next morning, thankful for Marge's intervention. What he doesn't know is Bart took it upon himself to send the letter himself.
Homer attempts to get the letter back before Mr. Burns gets it, to no avail. At first, Mr. Burns is angry and wants Homer beaten. Smithers refuses and reminds Mr. Burns Homer was responsible for saving his life. Mr. Burns has a change of heart and buys the Simpsons a giant Olmec Indian Head. Homer is disappointed because he's still not rich, but all is well with the Simpson family and Homer's relationship with Mr. Burns is good and his job is safe.