The enemies of the Kids Next Door, led by Father, join forces to resurrect the Ultimate Evil, Grandfather, a tyrant who once ruled the world many years ago when most of the villains were themselves kids. When Grandfather uses his reality-warping powers to reconquer the world and transform every human on the planet into his obedient "Senior Citi-Zombies" slaves who are forced to make Tapioca to refuel Grandfather so he can find and destroy the Book of K.N.D. The Kids Next Door are quickly overrun and transformed.

It falls upon Numbuh 1 to locate the KND's only hope for salvation, the legendary hero Numbuh 0. In the process, Numbuh 1 learns some startling revelations about the origins of the KND, his arch-enemies Father and the Delightful Children From Down The Lane, and even his own family history.