During a field trip, Lisa discovers desert water that makes usually combative creatures get along; after getting kicked out of the retirement home, Grampa comes to live with the family.

When Flanders' parents prove to prefer Homer's company to his, he jealously responds by giving Homer a shiner, and then tries desperately to reconcile with him. Meanwhile, Lisa is at a loss to understand why a new substitute teacher is bullying her for no obvious reason.

During a research trip, Lisa learns that Sideshow Bob is the chief scientist at a big chemical engineering company; Marge tries to teach a teen church group about healthy sexual practices.