After a field trip to the desert, Lisa discovers that the desert water mysteriously makes typically combative creatures get along, so she brings some of the water home as a souvenir.
When Grampa moves in with the family after getting kicked out of the retirement home, Homer secretly tests the desert water on Grampa, who is instantly cured of his crankiness.

A pharmaceutical representative (guest voice Herzog) gets word of Homer’s discovery and quickly tries to replicate the water into a drug, using Grampa as the primary guinea pig.
But when the pills get in the wrong hands and are sold on the black market, unexpected side effects of the new drug take an eye-popping toll on the elderly citizens of Springfield, and the Simpsons learn that their new medical cure comes with major consequences.

Source: FOX