“Step right up, folks, and witness the magnificent medicinal miracle of Simpson & Son's patented revitalizing toniiic. [deep breath] Put some ardor in your larder with our energizing, moisturizing, tantalizing, romanticizing, surprising, her-prizing, revitalizing tonic.”
Homer and Marge are having a troublesome sex life until Grampa introduces Homer to a home remedy love tonic. They soon go on the road together, trying to make some money off Grampa's potion. All the while, all the adults in Springfield are taking advantage of their new love lives, leaving the children thinking that UFO's are to blame.
When Homer's and Marge's marriage comes under threat due to their fading sex lives, Grampa pieces together a tonic that is guaranteed to help the bad situation. They begin selling it at a mall, and then in Spittle County. They begin visiting multiple towns to cure frigidness and become quite successful. However, while all the parents of Springfield spend more time with each other, the children are banished from the house. Bart and the other kids believe their parents are acting peculiar, under the influence of aliens or vampires. They are unaware of Homer and Grampa selling their tonic, so they continue to believe that their parents have turned into reverse vampires: they have to get home before dark, which in the children's view explains the adults' behavior of running inside and closing the blinds as soon as they get home.
During one sales trip, Homer and Grampa visit the farmhouse where the family used to live. Grampa reminisces about the time the family lived in the house, but Homer asks why his father never gave him any encouragement when he was growing up. They keep talking as they drive away, and the conversation turns into an argument. Grampa says he never encouraged Homer because he was a screw-up, and Homer says he's had enough of Grampa and the tonic. Grampa then becomes angry and says that Homer was an accident and wouldn't even exist if it weren't for the tonic, and adds that he wishes Homer had never been born. Furious with his father, Homer quits selling the tonic and abandons Grampa on the roadside.
Homer goes back home and tries to be a better father to his kids, but fails. Grampa, for his part, tries to keep selling his tonic (with Barney Gumble standing in for Homer), but it doesn't work. The two of them return to the old farmhouse, each unaware of the other, and both of them manage to accidentally start fires which quickly spread through the house. As the house burns, Grampa and Homer agree that they're both screw-ups, and they forgive each other.