When Billy's dad leaves to buy underwear, Billy wears his dad's "lucky pants", despite being told not to. Billy accidentally gets Grim's scythe stuck in the pants, and he tries to get help fixing them before his dad gets home. He calls "Ed, Edd, n Eddy's Pesky Problem Fixers", but Billy doesn't have any money, so Eddy tells him to call the Kids Next Door since "they're cheap."
The KND soon arrive (Billy is confused by this, stating that he tried to call the Powerpuff Girls"), and they find they need 2x4 Technology, so Numbuh 1 dresses up as Billy to stall Billy's dad, while the rest of Sector V take Billy to the KND Deep Sea Base to fix the pants. Mandy comes to Billy's house, and knows that Numbuh 1 is not Billy. Mandy captures Numbuh 1, and tortures him until Numbuh 1 tells Mandy everything about the KND.
Meanwhile, the rest of Sector V are flying Billy to the Deep Sea Base, and they get an urgent message that the Delightful Children from Down the Lane are using a "delightfulization machine" to make more children into their delightful slaves. Sector V reluctantly takes Billy on the mission, where he mistakes the delightful machine for a bathroom, fusing him, the lucky pants, Grim's scythe and the Delightful Children (who only ran inside the machine to pull Billy out) into a being called the "Delightful Reaper".
Meanwhile, Grim comes to Billy's house, and mistakes Numbuh 1 for Billy. In a sudden show of pain, Grim senses that his scythe has been fused with evil and strangely delightful children, and Numbuh 1 realizes that he is talking about the Delightful Children from Down the Lane. The Delightful Reaper begins fusing kids into it, including Numbuh 2.
Sector V returns to the Moonbase, where they find Mandy disguised as Numbuh 1. Only Numbuh 5 and later, Numbuh 362 knew that she's an impostor, but since nobody else does, Mandy convinces everyone that Numbuh 5 is losing her mind. Soon Mandy becomes ruler of the KND, renaming it the MND (Mandy New Dictator) and told Numbuh 5 that Numbuh 362 caught the same virus she did and had her quarantined (Numbuh 362 was likely unhappy with Mandy's proposed changes to the KND, and when Mandy refused to see reason, she realized Numbuh 5 was telling the truth). On the way to the Moonbase, Numbuh 1 and Grim are attacked by Numbuh 60 in which they are in restricted airspace but doesn't realize its Numbuh 1, Numbuh 1 and Grim arrive at the Moonbase, where Grim finds out that Numbuh 1 isn't Billy, and Mandy imprisons them.
The Delightful Reaper continues fusing with children, so after escaping, Numbuh 1 and Grim fuse together with the Bone of Barnacles to create a Giant Skeleton Samurai to battle the Delightful Reaper. A giant M.A.N.D.R.O.B.O.T. piloted by Mandy, Numbuh 3, and Numbuh 4, comes into the battle. Mandy's robot begins to lose until Numbuh 1 and Grim show up and fight the Delightful Reaper, though they begin to lose as well because Billy's dad's lucky pants are impervious to lasers, supernatural energy, and mustard.
When the Delightful Reaper is distracted by Numbuh 1 and Grim, Mandy purposely fuses herself with the Delightful Reaper, giving her control of it. However, Billy's dad comes, removes the lucky pants and takes them away, being really, really angry that someone is wearing his pants. Without the lucky pants, the Delightful Reaper is defenseless and the Skeleton Samurai finishes it off, freeing the other kids inside it. Mandy is taken away by the KND, but eventually she escapes, vowing revenge. Everything goes back to the way it was, except for Numbuh 1, mistaken by Numbuh 3 and Billy's dad for Billy, is taking away by Billy's Dad who plans on to punishing him for taking his lucky pants. Meanwhile, Billy, who is dressed like Numbuh 1, plans to make changes to the KND only for the KND to throw various items at him in response to prevent what happened that day.