The Grim Adventures of the Kids Next Door is a special crossover episode between The Grim Adventures of Billy & Mandy and Codename: Kids Next Door that aired on November 11, 2007. In addition to the two titular shows, several characters from other shows cameoed, including the Eds, who had the most screentime of any non-Billy & Mandy or KND character. The show is the last episode of The Grim Adventures of Billy & Mandy to air before the Halloween special "Underfist" and aired between the Kids Next Door episodes "Operation: G.I.R.L.F.R.I.E.N.D." and "Operation: C.A.R.A.M.E.L."
Grim's scythe is once again being used for something it wasn't intended to. This time, it's being used for golfing by Billy, who is shooting apples in his living room. However, when his father walks in, he swiftly gets beaned by an apple, putting Billy's game to a halt as Harold tells him that he's off to go underwear shopping and won't be back for a few hours. Upon hearing this, Billy is naturally excited, and asks his father if he can wear his lucky pants. His dad angrily tells him no, and proceeds to drive the point home repeatedly.
However, as soon as his father leaves, Billy ignores him and goes to try on the pants. All seems well until he slips on a Sassy Cat doll, falls over a banister, and ends up getting Grim's scythe caught in his dad's lucky pants. Upon realizing the magnitude of his problem, Billy calls the only people who can help him: the Eds. Unfortunately, Billy is broke, so Ed, Edd n Eddy's Pesky Problem Fixers refuse to help, with Eddy instead directing him to call the Kids Next Door.
When the KND get there, they find that Billy was instead expecting the Powerpuff Girls. However, soon enough they get past this block and to the crux of the problem, which is that Billy's pants won't come off. After a few hefty pulls refuse to do the trick, Numbuh 1 orders that they take Billy with them to the KND Deep Sea Lab to try and get the pants off while he stays in Endsville disguised as Billy.
As soon as the rest of Sector V leaves, there comes a knock on the door. Steadying himself, Numbuh 1 goes to answer it and finds himself face to face with Mandy, who is not taken in by his disguise. As soon as it becomes apparent that he won't be able to fool her, Numbuh 1 stops acting like Billy, but still refuses to tell her who he really is.
Back in Endsville, Numbuh 1 is being tortured by Mandy. Mandy has managed to get Fred Fredburger into the TV, and she is forcing him to watch as he babbles about the types of TV shows he likes. Soon enough, Numbuh 1 caves and agrees to reveal everything.
Numbuh 1 has spilled the beans to Mandy, who gets an idea upon realizing that he belongs to a group of kids with a large private army and large amounts of weaponry. She leaves him tied up and puts on his sunglasses, with some dark sinister plot in mind. However, as soon as she's gone, Numbuh 1 is able to break free from his bonds, as Mandy didn't tie him up very tightly, and he sets off to find his team. Before he can get very far, however, a tall, dark, shadowy figure stops him, angrily demanding his scythe.
In Sector V, meanwhile, the KND have snuck into the Delightful's mansion, where they have found them making new modifications to the Delightfulization Chamber. Before they can intervene, however, Billy runs into the room and asks to use the bathroom. The Delightful Children politely agree until they realize that he's mistaken their chamber for a restroom and rush in to try and stop him. Realizing that things are going wrong, the four KND Operatives run in to try and make sure things don't get worse, but before they can do anything Billy activates the chamber, and rumblings come from within. The Operatives turn tail when they hear the rumblings, but only a few seconds later the chamber explodes.
In Endsville, Grim has indeed fallen for Numbuh 1's now mostly ruined disguise, and Numbuh 1 is being tortured for the second time today. Grim's method of choice isn't Fred Fredburger, however, but rather the mystical Bone of Barnacles, which he plans to use to make "Billy" tell the truth. Before he can truly start to use it, however, he collapses, as a wave of great horror has just rushed through the underworld. Children have apparently melded with his scythe–evil, yet strangely delightful children.
Inside the mansion, the chamber has well and truly been destroyed. All the members of Sector V are okay, but the same cannot be said of the Delightful Children and Billy. These six have assimilated into one giant superbeing: a reaper wearing the lucky pants. Upon seeing this, the KND operatives head for the exit, but before they can get there Numbuh 2 is cut down by the reaper's cold iron blade and joins the Delightful Reaper, which grows with its newest addition.
The three remaining members of Sector V get out of the mansion and head to KND Moon Base Headquarters, where they inform Numbuh 362 of the horrible things going on. Numbuh 362 is duly shocked, and asks for Numbuh 1. At this moment, Mandy shows up dressed like the leader, fooling everyone but Numbuh 5, whom she suggests be taken to the med lab, as she is obviously tired after her last mission. As soon as Numbuh 5 is gone, Mandy begins talking to the Supreme Leader, and convinces her that there is a virus planted by the adults causing kids to lose their minds.
While Mandy's newest plot to rule the world unfolds on the moon, the Delightful Reaper stalks the streets of Endsville. It walks through the city, cutting down all it sees and growing larger with each addition.
At the Moon Base, however, Mandy's plan continues to unfold. She puts Numbuh 362 in quarantine for having been "infected" with the fake virus, and takes over. In addition, she demands that she be called "Mandy," renames the KND the MND (for Mandy, New Dictator), and essentially remakes the organization in her own image. This goes on to the point where the Moon Base is rebuilt to look like her head when approached from the normal direction, which Numbuh 1 and Grim find out the hard way when they fly towards it. As they haven't been informed of the changes, they are taken into custody and thrown before the new dictator, who in turn puts them in jail.
As soon as Numbuh 1 and Grim are taken away, however, Mandy receives a call from one of the Treehouses. It has come under attack by a giant version of the Grim Reaper. As Mandy watches from an aerial view, the reaper suddenly turns and grabs the plane as it speaks in Billy's voice. Seeing this, Mandy angrily realizes that Billy is somehow involved and sets out to stop him.
In jail, Numbuh 1 watches as the business of the MND goes on without him. He then angrily chastises Grim for having made this a horrible Sunday for him and calls him a freak in a Halloween costume. When Grim protests that it's not a costume, Numbuh 1 calls him out and suggests that he use his powers to open the cell door just as the door swings open. However, the open door is not the work of Grim; rather, it is Numbuh 5, who is busting her commanding officer out. Before they can leave, though, Grim asks them to wait, as he can help. Numbuh 1 agrees, and Grim pulls out the Bone of Barnacles again, ready to use it for good rather than evil this time.
Back down there on Earth, the Delightful Reaper is cutting a swath through a suburb when the KND approaches and drops a golden sphere from one of their aerial assault vehicles. The sphere lands heavily in a field, causing a huge explosion. When the smoke clears, a gigantic robot in the shape of Mandy is revealed. It stands up and begins to battle the Delightful Reaper. Its attacks are largely ineffective, however, in part because the lasers that Mandy ordered were sidetracked as a shipment to a lab, leaving the MND to outfit the robot with mustard blasters.
Soon, the reaper manages to gain the upper hand in the battle, and is ready to finish off the M.A.N.D.R.O.B.O.T. when a voice from behind it gets its attention. The voice is that of Numbuh 1, who is piloting a supernatural vehicle best described as a gigantic skeleton samurai. The reaper turns around fully and faces off against it. They charge each other, and the samurai's sword meets the reaper's scythe. The reaper wins the battle, however, and knocks the sword away before bringing its' scythe down upon the samurai. Thinking quickly, Numbuh 1 blocks with his shield, which is really Grim's head grown to gigantic proportions. After a few moments of disconcertion, Numbuh 1 regains his composure, rolls out of the way of the scythe, picks up the sword, and unleashes the "Cone of Bone" attack, which shoots a barrage of bones at the reaper. Thinking quickly, the Delightful Reaper bends over and lets the bones bounce off of the pants. Confused, Grim looks at the pants, and realizes he knows them from somewhere.
Grim flashes back to when he was sitting on a couch with Billy's dad, who is indeed wearing the lucky pants. Harold explains that the pants are lucky, and that he was wearing them the day he married Gladys. However, the real reason for their luck is that they're immune to lasers, supernatural energies, and mustard.
Once again, the Delightful Reaper goes on the attack, and this time comes perilously close to defeating the Skeleton Samurai. However, before he can, the M.A.N.D.R.O.B.O.T. comes up behind him and taps him on the shoulder before just standing there, stock still. The reaper takes the opportunity to slice it in half, and assimilates those inside, including Mandy. Of course, Mandy's personality is so forceful that she takes control of the reaper instantly, turning it into the Mandy Reaper.
Before Mandy can truly use her power, however, Harold arrives on the scene and pulls down the reaper's pants. This provides the samurai with the opportunity it needs, and it unleashes the "Super Skeleton Sword Strike." This defeats the reaper, and as it lies on the ground, defeated, Harold carries his pants away–pants which are now far too big for him. The reaper begins to shake, and soon its' victims pop out of it, flying away and destroying the reaper. The scythe flies away from the reaper as well, and the samurai catches it, reuniting Grim with his most necessary tool.
Finally, everything has gone back to normal. Numbuh 1 and Grim are congratulated by Numbuh 362 at the moonbase and Numbuh 1 is reunited with his team, while Mandy is taken into custody by the Kids Next Door. However, Mandy isn't in custody for long, as she escapes using rocket blasters she stored in her hair, and Numbuh 1 isn't reunited with his team for long, as Harold believes he's Billy and drags him off to Endsville for punishment. As soon as he's gone, Numbuhs 3 and 362 reveal that they too thought he was Billy, and ask where Numbuh 1 is, only to be answered by the real Billy, who is dressed up as Numbuh 1 and is impersonating him much like Mandy did. However, nobody falls for it this time, and several characters throw things at him as the episode ends.
To promote this crossover episode, Cartoon Network made a game called "Operation: Z.E.R.O.: Out-Mandy'd!" The game is essentially a remake of Operation: Z.E.R.O.: Out-Numbuh'd!" which was made to promote the KND movie "Operation: Z.E.R.O." The only difference in the game is that you fight as Mandy rather than as Numbuh 1 and that the intro and ending sequences are different, and that all references to Numbuh 1 are replaced with "Mandy."
[Billy has just accidentally torn a hole in his father's pants with Grim's scythe.] Billy: [horrified] "Dad's pants! What do I do? Dad's gonna sell my organs if he finds out I ripped his lucky pants!" [determined] "There's only one group of kids who can help me now." [At Ed, Edd n Eddy's Pesky Problem Fixers, the phone rings.] Ed: "Hello!" [Eddy picks up the phone.] Eddy: "Ed, Edd n Eddy's Pesky Problem Fixers! Got a problem that's pesky? We'll fix 'er, for just one lousy quarter!" [Muffled gobbling comes from the telephone. Eddy gives it a look.] Eddy:[angry] "WHADDYA MEAN YOU'RE BROKE? Hey! If you're looking for charity, call Kids Next Door. They're cheap." [Eddy angrily hangs up the phone.]
Mandy: [to Numbuh 5] "You're obviously sicker than I thought. And whatever it is you've got, it's spreading. So in order to contain it, my first order as leader is that I will now be referred to as...Mandy." Numbuh 5: "Mandy?!? That's the stupidest thing that–" Mandy: "And not only that, but until a cure can be found, the KND will be renamed the MND." Numbuh 3: "The Mean Nasty Doggies?" Numbuh 4: "The Big Rubber Toilets?"
When the Delightful Reaper explodes, characters from several cartoon series are seen coming out of it. These include:
During the end credits, five posters for fake crossovers are shown.
Ed, Edd n Mandy(crossover between Ed, Edd n Eddy and The Grim Adventures of Billy & Mandy): In this poster, Ed and Edd are running in fear as Mandy waves around Plank, who has a nail driven through his forehead. This poster suggests that in the show, Mandy will have taken over as the resident megalomaniac and acts far more violent than Eddy.
Evil Camp Carne(crossover between Camp Lazlo and Evil Con Carne): In this poster, Lazlo's body is seen having been taken over by Hector Con Carne, with Lazlo's friends Raj and Clam stepping into the roles of General Skarr and Major Doctor Ghastly, respectively.
Class of Numbuh 3000(crossover between Codename: Kids Next Door and Class of 3000): The poster for the show has Sunny Bridges shooting mustard from a KND M.U.S.K.E.T. to spell out the title of the show. He is dressed as Numbuh 3. This poster suggests that in the show, Sunny Bridges will join the Kids Next Door in order to help fight adult tyranny.
My Gym Partner's A Mandark(crossover between My Gym Partner's a Monkey and Dexter's Laboratory): In this poster, Jake Spidermonkey and Mandark Astronomonov are seen swinging on a vine, presumably headed to class. The poster implies that Mandark will have taken Adam Lyon's place in the show.
According to Numbuh 1, this episode took place on a Sunday. Coincidentally, it also premiered on a Sunday.
A blog post by Tom Warburton about the creation of the crossover revealed many things, including:
The first plot for the crossover involved an army of pants-eating ants that Mandy would use to take over the world. It was rejected by Cartoon Network.
Originally, the torture Numbuh 1 undertook was going to be Mandy forcing him to watch Sheep in the Big City. After this was rejected, Evil Con Carne was suggested, only to be rejected as well. This lead to Fred Fredburger being used in the scene.
The Moon Base is known as Moon Base Zero, which in Codename: Kids Next Door continuity was only built after the events of "Operation: Z.E.R.O."
After the theme song plays, the "created by" credit is a mashup of the two creators' names: Tom Warburton, usually referred to as Mr. Warburton, is credited as Maxwell Warburton, while Maxwell Atoms is credited as Mr. Atoms.