Mr. Burns is diagnosed with hypohemia, an affliction that leaves a person without enough blood to sustain life. Desperate to save his boss, Smithers issues a plea to the plant's employees for a double-O negative blood donor. No one at the plant comes forward except Homer, who thinks that Burns will reward him richly. However, Homer does not have the right blood type, and instead volunteers Bart, who has the right match.

After the blood transfusion, Burns is more alive than ever, and sends Bart a thank-you note. Homer is outraged by Burns's inadequate response. He immediately writes Burns a vindictive letter, but Marge stops him from mailing it. The next morning, however, when Homer goes to tear up the letter, he discovers that Bart has already mailed the letter.

When Burns receives the letter, he vows to make Homer's life miserable. He orders Smithers to have Homer beaten to a bloody pulp, but Smithers refuses, explaining that he is unable to harm the man who saved his boss's life. Coming to his senses, Burns realizes the good deed the Simpsons have done, and buys them a rare Olmec Indian head statue. Bart thinks it's a cool gift, but Homer insists that it's a cheap reward for saving a man's life.