The main characters of the series are five 10-year-old children who operate from their high-tech tree house. Their mission is to fight "crimes" against children (such as homework and flossing) committed by adults, senior citizens, teenagers, and other evil children. They make up what is known as Sector V of a worldwide organization called the Kids Next Door. The show was also part of Cartoon Network's series, Cartoon Cartoons, and is the 13th Cartoon of the series.
On April 13, 2012, this series returned to Cartoon Network in re-runs on the revived block, "Cartoon Planet".
Mr. Warburton created a pilot episode for another show, Kenny and the Chimp. Originally, there was a group of children who called themselves "The Kids Next Door" among the recurring characters, and would often get Kenny into trouble. The plot-line was then changed to focus on the group of kids alone, and later, the kids battling adulthood. In 2001, the show's pilot episode, "No P in the OOL", won a Cartoon Network viewer's poll, and as a result, Codename: Kids Next Door was greenlit to become a series.
The episodes are titled as the Kids Next Door's missions, denoted as "Operation:" followed by an acronym which often gives viewers clues as to what the episode is about. The episodes have little continuity, although occasionally missions make a reference to earlier episodes, or lead to consequences in another. Mostly in the earlier episodes, the stories were often about typical childhood problems, but magnified and exaggerated; as the series progressed, a more complex storyline developed.
The Kids Next Door Organization is a worldwide group comprising thousands of kids joined in mutual struggle against adult tyranny. They fight villains that embody specific menaces to children overall (such as dental hygiene or school lunch). For every member of Kids Next Door, upon following a period of training, they then choose a number or alphanumerical code (deliberately spelled "numbuh,") and sent to a "sector", that is, a tree house of ludicrous proportions. The main headquarters of the Kids Next Door is a tree house Moon Base. Kids follow their oath of protecting other kids as well as battling adulthood until the age of 13, when they are "decommissioned", a process of wiping their minds of memories of any past KND activity, as they are then teenagers, and a threat to KND. Such a practice has inevitably led to the creation of many KND villains (notably Cree, Numbuh 5's older sister, and Chad, formerly numbuh 274 and supreme commander of the Kids Next Door). The practice of strict decommissioning at age 13 was later on revealed to be subject to some exceptions: kids who have proved to be exceptional agents are offered the chance to carry on in KND as spies infiltrating the teen organization. Decommissioning has also proved reversible due to a "re-commissioning" module used twice in the series.
KND agents utilize a vast array of mechanical, electronic gadgets, and machinery; collectively referred to as 2x4 (two-by-four) technology. The KND 2x4 technology was originally conceived as an assortment of common household objects, being put to use in combat situation; a theme that their weapons continued to adhere to. However, as the series progressed, the complexity of their defensive weaponry and transportation advanced to such absurd levels as to be considered science fiction, despite all of it still being constructed with nothing but household materials.
A common theme of the show is the introduction of a new never-before-seen device, complete with its own acronym, in a sidebar screen that temporarily interrupts the storyline. It is interesting to note that many of these devices require military-level driving or piloting skills, in order for them to operate effectively. However not all of the technology is made of other items.
- Numbuh 1 / Nigel Uno (voiced by Benjamin Diskin) - The leader of Sector V, a bald English boy who is rarely seen without his trademark sunglasses, and does not put complete trust in anyone but his teammates. He is a workaholic who consistently puts business before himself. His uncommon baldness is due to an unexplained event involving the Delightful Children from Down the Lane, and Numbuh 5's shirking of duties (as mentioned in episode "F.O.U.N.T.A.I.N."). Throughout the series, his girlfriend is Lizzie Devine, but their relationship came to an end when she explained that she was tired of his always putting KND priorities over her. He is the only operative on his team without a sibling. Before a fight, he shouts his signature war cry: "Kids Next Door... battle stations!".
- Numbuh 2 / Hogarth "Hoagie" Pennywhistle Gilligan Jr. (voiced by Benjamin Diskin) - An inventor and pilot with a passion for flight, who wears an old fashioned aviator hat. Numbuh 2 builds all the technology used by his sector. He is an overweight Swiss American boy who enjoys cracking puns at every chance possible. Hoagie is also Numbuh 1's best friend. His interests besides flight include trading cards and being a detective. It has been implied that he had feelings for Numbuh 5, and in Operation: I.N.T.E.R.V.I.E.W.S., it was shown that they had gotten married.
- Numbuh 3 / Kuki Sanban (voiced by Lauren Tom) - An unusually happy Japanese girl whose love for every creature almost always interferes with her responsibilities. She is a great fan of Rainbow Monkeys, a stuffed monkey toy series, and will often irrationally defend them, occasionally allowing her insanely aggressive side to show. She and Numbuh 4 share feelings for each other. Her last name, "Sanban" is Japanese for "Number Three". In contrast with her innocent, gentle and happy-go-lucky personality, she is the pilot of a giant robotic rabbit named H.I.P.P.I.E.-H.O.P. (Hare Inspired Piston Powered Ingenious Exoskeleton Hops Over Parents) which is thoroughly armed with missiles and gets destroyed in every episode it appears in, with Numbah 3 parachuting out afterwards.
- Numbuh 4 / Wallabee "Wally" Beatles (voiced by Dee Bradley Baker) - A short, Australian boy known for acting without thinking. Numbuh 4 wears an orange hooded sweater, blue jeans, and has blonde hair. He is a jock and a bully on occasion, as he is the most capable when it comes to physical fighting; however, he is arguably the least intelligent KND operative, which leads to him being easily fooled and most usually selected for dangerous parts in missions. He is barely literate and is ignorant in basic knowledge of English and mathematics, reflected by his grades in school. It is also clearly apparent he has a crush on Numbuh 3, which runs throughout the series, including in Operation: I.N.T.E.R.V.I.E.W.S., where it was shown that they are married.
- Numbuh 5 / Abigail "Abby" Lincoln (voiced by Cree Summer) - An intelligent and laid back tomboy of French and African-American descent; she keeps her head cool even in the worst situations. Abigail rarely overreacts in their missions, and sometimes tries to calm her friends, and even friends that have become enemies. She talks in third person much of the time, and loves sweets - most notably ice cream. Her dream was to taste the legendary Fourth Flavor of ice-cream, which she accomplished in "Operation F.L.A.V.O.R.". She is the oldest operative in Sector V. In I.N.T.E.R.V.I.E.W.S it is shown that she takes over sector V in wake of Numbuh 1's departure, and eventually, she became the supreme leader of KND.
- The Delightful Children From Down the Lane (voiced by Benjamin Diskin, Dee Bradley Baker, and Cree Summer) - Five well-behaved snooty children who move and talk in perfect synchronization. They obey all adults, especially their father: they and Father are the most recurring villains of the series. Rather than interfere themselves, the Delightful Children usually stand back and let a giant robot or super-powered minion do their work. In Operation: Z.E.R.O., it's revealed that they're the lost KND operatives of Sector Z, having been "delightfulized" by Father. Their names are Bruce (short, blonde boy), Lenny (helmet boy), David (tall, brunette boy), Ashley (tall, blonde girl) and Ogie (brunette girl)
- Father / Benedict Uno (voiced by Maurice LaMarche in Father mode and Jeff Bennett in Benedict mode) - An adult who constantly wears a suit of shadows, and is the KND's greatest enemy. He first appeared in the season 1 finale: Operation: G.R.O.W.-U.P.. His powers include pyrokinesis, levitation, mind control, and shape-shifting. In Operation: Z.E.R.O., it is revealed that he's Numbuh 1's paternal uncle.
- Mr. Boss (voiced by Jeff Bennett) - A child-hating corporate executive. He often holds gatherings for the other villains of the series. It is revealed that he is also Numbuh 86's father.
- Count Spankulot (voiced by Daran Norris) - A spoof of Count Dracula, he is a vampire who spanks naughty children. The count made his brief debut in the episode Operation: C.A.N.N.O.N to spank the KND for not returning their library books on time and has become a recurring character ever since. By removing one of his gloves, he can turn people into spank-happy vampires like himself, and they can only change back to normal if he himself is spanked.
- Cree Lincoln (voiced by Cree Summer) - Numbuh 5's teenage sister and Father's apprentice. She first appeared in the season 2 episode Operation: S.U.P.P.O.R.T.. She is formerly a KND operative: Numbuh 11, leader and 2x4 tech officer of Sector V three years prior to the start of the series. She was the one who gave Numbah 5 her trademark hat. Numbah 2 has a mild crush on her, despite her constantly hurting and offending him, but in Operation: K.I.S.S. when he accidentaly turned into a teenager, she fell in love with him, and even kissed him(but totally snubbed him once she found the truth.
- Grandma Stuffum (voiced by Grey DeLisle) - A fat old German woman who creates gross armies of living food to fatten kids up to obese proportions so they won't cause trouble.
- Henrietta "Heinrich" Von Marzipan (voiced by Dee Bradley Baker and Candi Milo) - He is Numbuh 5's chubby German candy-hunting rival with blonde hair, blue eyes and a monocle. He speaks with a German accent and loves candy, just like his rival Numbuh 5. He is based on the Nazis from Indiana Jones films. In Operation: C.A.R.A.M.E.L. it is revealed that Heinrich is actually a girl. In the episode, magic Caramels can be created by ritual that takes away one's most cherished quality. In turn for this sacrifice, the caramels become delicious. Henrietta created the caramels using her femininity, and then ate what she believed to be all of them. Numbuh 5 kept the last remaining caramel that contained Heinrich's femininity and was able to reverse the ritual.
- Knightbrace/Mr. Jelly (voiced by Tom Kenny) - A villain obsessed with dental and oral hygiene.
- Stickybeard (voiced by Mark Hamill) - A candy-hunting pirate who sails the suburbs in his ship, The Sweet Revenge, with his crew of pirates, robbing kids of their candy.
- Toiletnator / Lou Pottingsworth III (voiced by Dee Bradley Baker) - The most pathetic villain in the series, considered an embarrassment by the other villains, to whom he's constantly trying to prove himself. He has control over toilets and toiletries. He's also the self-proclaimed arch-nemesis of Numbuh 4, who's later revealed to be his nephew in the comic story Operation: H.I.S.T.O.R.I.E..
- Common Cold (voiced by Tom Kenny) - An enemy of the kids who always tries to get them sick. He is always covered in snot and has a niece named Hanna who is a schoolmate of the show's five main kid characters. The Common Cold doesn't like the KND and even teamed-up once with Numbah Two's little brother Tommy, but The Common Cold betrayed Tommy in the end.
There are several other villains in the series as well who do well against the KND.
- Numbuh 6 / Bradley / R.O.B.O.B.R.A.D.L.E.Y. (voiced by Dee Bradley Baker) - A skunk found and adopted by Numbuhs 2 and 3 in Operation: C.A.M.P.. Though Numbuh 1 originally disliked him due to his being a distraction to the others, he apologized after Bradley and his real parents saved the team from an evil camp counselor. Numbuh 1 then granted Bradley the status of honorary operative Numbuh Six. He returned in Operation: H.O.S.P.I.T.A.L., where he's rebuilt as a cyborg called "R.O.B.O.B.R.A.D.L.E.Y." (Robotic Operative's Ballistic Odor-Blasting Rocket Armed Derrier Launches Extreme Yuckiness) due to near-fatal injuries from a spying mission. He made one last appearance in Operation: S.I.X..
- Numbuh 60 / Patton Drilovsky(voiced by Matt Levin) - a high-ranking Canadian operative and the drill sergeant stationed at the Kids Next Door Arctic Base who's in charge of training new recruits to the Kids Next Door Organization. He is occasionally seen in various episodes in either at the Kids Next Door Moonbase or Kids Next Door Arctic Base.
- Numbuh 86 / Fanny Fulbright (voiced by Jennifer Hale) - The Kids Next Door's head-of-decommissioning. She speaks with a Scottish accent and yells a lot. She is also known to underestimate boys, often calling them "stupid" and believes girls are far superior. It is revealed in "Operation: END" she has a small crush on Numbuh 4. Numbuh 5 never liked her because of her dislike for boys and aggressive attitude.
- Numbuh 274 / Chad Dickson (voiced by Jason Harris) - In season 2, it's shown that he's the KND's supreme commander, however, in the season 2 finale Operation: E.N.D. he betrayed his fellow operatives by hiding that he turned 13 years old. In the following seasons, he joined the Teens and became a recurring villain. But in the season 6 finale Operation: T.R.E.A.T.Y., it's revealed that he was working undercover for the KND. He is also the son of villains Mega-Mom and Destructo-Dad.
- Numbuh 362 / Rachel T. McKenzie (voiced by Rachael MacFarlane) - Introduced in the season 2 episode Operation: F.U.G.I.T.I.V.E. as the KND's top spy, it was revealed in the season 3 episode Operation: G.R.A.D.U.A.T.E.S. that she'd taken the position of supreme leader after Numbuh 274's "betrayal." It is later suggested in the series that she has a crush on Numbuh 1. Also, she's one of the only characters to regularly change her wardrobe.
- Lizzie Devine/Numbuh 49-Temporary (voiced by Grey DeLisle) - She is Numbuh 1's chubby American girlfriend with ginger hair and glasses. She is very possessive over him and will often go out of her way to try to get him to spend more time with her. She gets angry when he puts his KND work before her and his tendency to jilt her or break off dates just to go on missions. Even though she continued to tolerate it, she eventually breaks up with him after realizing that his job is what he loves the most and not her.
- Tommy Gilligan (voiced by Dee Bradley Baker) - Numbuh 2's little brother, first appearing in Operation: T.O.M.M.Y. He originally sought to join the KND as "Numbuh T" (despite, as was continuously pointed out to him, T not being a number), which the others agreed to after his helping them in the season 2 finale Operation: E.N.D.. However, he was forced to quit to save them from Father in Operation: G.R.A.D.U.A.T.E.S. and, due to a computer safeguard, was unable to rejoin. As such, he continued to defend kids as a vigilante, "The Tommy".
There are six seasons, each with 13 episodes (two half-length episodes counting as one), adding up to 78 episodes altogether. There have been two TV movies, the first taking place between the second and third episodes of Season 6 (Operation: Z.E.R.O.), and the second at the end of Season 6 (Operation: I.N.T.E.R.V.I.E.W.S.).