WHAT'S SCARIER THAN the Sassafras Siblings and 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 has 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 leads him to a realization about the scariest Old Person at Shady Acres . . .