On behalf of Homer's infamous ignorance, Grampa Simpson falls victim to a textbook kidney blowout. But when Homer steps up to offer a donor kidney, he quickly finds himself suffering from a much more common phenomenon: cold feet.
On their way to Bloodbath Gulch, the Simpsons' car breaks down in front of Springfield Retirement Castle. Grampa is pleasantly surprised, having assumed the family had forgotten his birthday (which of course they had). He makes it to the car and they grudgingly take him with them. After a day of roaming the ghost town the family heads back to Springfield; Homer wants to make great time, and ignores Abe's pleas to stop at several rest areas and The World's Biggest Toilet. Abe eventually begs Homer to turn around claiming that he can make it his birthday present. But Homer refuses. Upon returning to Springfield, Dr. Hibbert informs Abe that both his kidneys have exploded; he is going to need a kidney transplant or he will die. Homer is an exact match and agrees to take part in the transplant to save his father. Everything in preparation goes well until Homer stops by Moe's and everybody tells him that after the surgery he'll be stuck on the organ donor's sucker list. This doesn't seem to affect anything, until it actually comes time for the surgery and Homer runs away before the operation. Ashamed of himself for just leaving his father to die a painful death, Homer becomes a sailor on a ship of lost souls. When the other lost souls hear Homer's tale of deceit they toss him overboard in disgust. The second attempt to extract Homer's kidney does not go well either; he runs out of the hospital, into the street and is crushed by a car, which fell from a car-carrying truck driven by Hans Moleman. When he awakens he apologizes to the family for not saving Abe's life, but is shocked to learn that while Dr. Hibbert was saving his life the kidney transplant was done against his will. Abe is better than ever, and Homer realizes that Bart has a kidney to spare for his father, but keeps this to himself.