During a trip to "Satan's Anvil", the students from Springfield Elementary stumble into bizarre people and situations: Otto runs over "realistic" versions of Coyote and Road Runner; Martin meets an eccentric and grumpy hermit artist that the government tried to hire for years but never succeeded; Bart, Nelson and Milhouse find old French postcards; and Lisa is almost attacked by scorpions, but they quickly become passive after Lisa passes through a field of silvertongue flowers in Springfield's desert, prompting her to take the scorpions and some flowers for further experiments. When they return home, the Simpsons family is forced to let Grampa live at their house, after he was kicked out of the old folks home for being too grumpy. After Lisa confirms that the flower has a powerful chemical agent that nullifies all the negative feelings of any living being, Homer sneaks some into his father's coffee, curing his usual crankiness. Grampa decides to do this daily, acknowledging that a drug that stimulates happiness is the best thing that ever happened in his bitter life.
Unfortunately, despite the positive effects, Lisa refuses to give more of this drug, or to tell them the composition. While they complain about it at Moe's Tavern, a drug industry employee from Hottenhoffer Pharmaceuticals named Walter Hotenhoffer (formerly known as Augustus Gloop) manages to duplicate the liquid's effects using a sample of Grampa's perspiration. Hottenhoffer produces "MusBeNys" pills, but since the product has not been properly tested, only Grampa is allowed to use them. Unfortunately, Bart decides to sell some of the pills to everyone who has a problem with grumpy old people, making all the elderly people in Springfield becoming happy and carefree.
Lisa eventually realizes that Grampa is still using the drug, but she also admits that the pills are rather helpful to him. However, the drug's side effect quickly arises: it makes everyone's eyes become so lubricated that they pop out of their sockets. The elderly people do not mind this rather gruesome aspect of the drug, but upon seeing Homer's thoughtless antics with his friends and his car, Grampa convinces the old people to stop using the pills, saying that the Baby Boomer generation still needs their guidance, and this is only possible with their constant nagging. In the end, everything turns back to normal (except Hotenhoffer, who still has nightmares regarding what happened at Wonka's factory).