https://www.imdb.com/title/tt1314193/

The Grim Adventures of the Kids Next Door is a half-hour TV special featuring a crossover between Codename: Kids Next Door and The Grim Adventures of Billy & Mandy. Cartoon Network officially announced it on February 16, 2007, although staff confirmed it on December 8, 2006. The crossover aired on November 11, 2007. This special was created for entertainment purposes and not meant to be part of either shows' continuity.

Synopsis

When Harold 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 gets 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."

When the KND arrive to fix the pants, 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 into a being called the "Delightful Reaper".

Meanwhile, Grim comes to Billy's house, and mistakes Numbuh 1 for Billy. Grim then says 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 knows that she's an imposter, but since nobody else does, Mandy convinces everyone that Number 5 is losing her mind. Soon Mandy becomes ruler of the KND, renaming it the MND (Mandy New Dictator). When Number 362 finds out the truth, Mandy has her quarantined as well. 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 making children fuse with it, 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 and Mandy. 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 Harold'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 and is about to destroy Number One and Grim. However, Harold 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 (even though she never comes back for revenge), and everything goes back to the way it was, except for Numbuh 1 (who gets dragged away by Harold, believing him to be Billy, to be punished) and Billy (who pretends to be Numbuh 1, only for everyone to not be fooled, and throw things at him).

http://grimadventures.wikia.com/wiki/The_Grim_Adventures_of_the_Kids_Next_Door