Cartoon Network Universe: FusionFall was a free browser MMORPG featuring redesigned, Anime-style characters and other elements from nearly all of Cartoon Network's original series, even obscure and/or long-since cancelled ones. Codename: Kids Next Door was one of the most prominently represented series in FusionFall. The game lasted from January 14, 2009 to August 29, 2013. FusionFall has now been unofficially revived by a group of fans. The game has been renamed FusionFall Retro:
Numbuh 1 (located by Townsville park), Numbuh 2 (located by the Treehouse in Sector V), Numbuh 3 (behind her "house" at Mt. Blackhead), Numbuh 4 (near the hill entrance at Mt. Blackhead), Numbuh 5 (in Genius Grove in front of Dexter's house) , Father (near the entrance to the cardboard fortress leading to the cul-de-sac), Toiletnator (in Camp Kidney) and Stickybeard (in the village in Candy Cove) are all featured in the game as NPCs. Numbuhs 1-5 are also obtainable as Nanos, and Fusions of the five and villains appear as well. Several 2x4 Technology items, such as S.P.I.C.E.R., T.H.U.M.P.E.R., S.H.A.V.E.R.A.M.A., S.C.A.M.P.E.R., and F.L.A.P.P.U.H., are featured as well, mostly as usable weapons.
- Numbuhs 2 & 5 also appear in the manga prequel "Worlds Collide", where the treehouse is overrun by Fuse's minions and they're rescued by the Powerpuff Girls. Numbuh 2 then joins the team to steal the electro-beam from Mojo Jojo's hideout, while Numbuh 5 leads the team to Mandark's fortress to get the optimised phase-pulse-o-tron.
- It's shown that the KND have a training area.
- The other Cartoon Network characters seem just as annoyed at Numbuh 2's constant bad jokes as the rest of Sector V usually is. When he jokes about "monkey buisness" while going into Mojo's layer, Blossom groans, and when he comments on how freeze rays are "cool", Ben 10 asks "Man, does he ever stop?" to which Numbuh 5 replies "I wish."
- The official site on Cartoon Network said Numbuh 3 is the stealth tactics officer, rather than Numbuh 5. Either Numbuh 5 gave Numbuh 3 that position, or (more likely) the site was incorrect (before the game was shut down).
- Perhaps the most noticable design change for the characters is Numbuh 1's new vest.
- It's revealed in "Worlds Collide" that the KND also have a Plutobase, a Saturnbase, a Jupiterbase, and a Marsbase.
- It's assumed this was non-canon to the series, as Numbuh 1 is still on Earth. But, it is possible that Operation: I.N.T.E.R.V.I.E.W.S. was just a cover up or not the entire story.