What's scarier that the Sassafras Siblings and Slovakian Rot-Toothed Hopping Monkey Demons? The answer, my friends, is OLD PEOPLE. And Billy is forced to face his fears when his family picks up his great-aunt Tilley at Shady Acres to bring her home for the long weekend.

Meanwhile, in Monstros City, a mysterious villain unleashes a legendarily destructive monster: the enormous Zis-Boom-Bah. Zis-Boom-Bah eats boulders for breakfast, destroys cities with one step, and could make Monstros City disappear faster than you can say, "Godzilla." He's got the inhabitants of Monstros so riled up that the military wants to use the OMG ("oh my gosh!") device, which will destroy Zis-Boom-Bah but take half of Monstros with him. But Billy has a revolutionary idea . . . which takes him a realization about the scariest Old Person at Shady Acres.