The villains of the Kids Next Door, lead 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. However, Father disgraced him that he can't even try to destroy the KND and the Villains were quickly betrayed when they are turned into Senior Citizombies, creatures that are immortal and can transform any living creature into one of them and slaves who are forced to make Tapioca to refuel Grandfather so he can find and destroy the Book of K.N.D.

A crossover with The Grim Adventures of Billy and Mandy and Codename: Kids Next Door. While Billy's Dad is underwear shopping, Billy wears his dad's lucky pants, and accidentely rips them with Grim's scythe. He calls Ed, Edd n Eddy, and they recommend Kids Next Door for charity. Numbuh 1 disguises as Billy while Billy is taken to the Deep Sea Lab. Mandy arrives at Billy's house and knows Numbuh 1 is not Billy, and tortures him into revealing the truth. Meanwhile, the Delightful Children From Down the Lane and Billy accidentally fuse with the scythes power to create the Delightful Reaper, and Mandy takes over the KND by fooling Numbuh 362 and the rest of the operatives. Now its up to Numbuh 1 and the Grim Reaper to save KND and stop Mandy and the Delightful Reaper.

In a series of interviews with the KND after they're all grown up, the former operatives are temporarily brought out of their decommissioned haze to discuss what happened during their very last mission with Numbuh one - where he disappeared and was never heard from again! It all started with a routine mission to get the Delightful Children's birthday cake, but when a mysterious boy steals it instead, he makes it available to whoever wants it, IF they can solve his 'simple' scavenger hunt first. Soon it's a mad, mad, mad, mad chase around the world as the KND, their friends, and virtually all of their enemies compete to win the yummiest cake ever