The Simpson family goes on a long trip, but Grandpa Simpson has to go to the bathroom. Homer doesn’t pull the car over, and eventually when they get back to Springfield, Grandpa Simpson’s kidneys explode.

Homer decided to donate his kidney, but kept getting scared and would run away from the hospital. Then a bad accident later and he’s in the hospital sans kidney - they took it out of him. At the end Bart hugs him and Homer is squeezing him checking for kidneys to harvest from his son.