Operation Z.E.R.O. is the first made-for-TV movie for the animated television series Codename: Kids Next Door. It originally aired on August 11, 2006 on Cartoon Network. A widescreen DVD release is planned. As of August 2008, the release date of the DVD has not been assigned. Z.E.R.O. stands for "Zero Explanation Reveals Origins." Chronologically, the movie takes place between Seasons 5 and 6.
In the present day, Numbuh 1 tells the story, claiming that if he ever found the Book of KND, he would write only five words: "I am Kids Next Door". He is then given an order by Numbuh 86 to deliver a high priority package to the Kids Next Door Seriously Cool Museum of Artifacts and Stuff. Much to Numbuh 1's frustration, it turns out to be an ice cream cone for its tour guide, Numbuh 101, who greatly admires Sector V. Midway into the tour, a coalition of KND villains simultaneously attack both the KND Museum and Moonbase. Sector V manages to repel both attacks, but Numbuh One draws flak from Numbuh 362 for disobeying orders by going to the museum first. When it is discovered that the Recommissioning Module (a device capable of restoring the memories of anyone, even someone who has been decommissioned for many years) has been stolen, Numbuh 1 goes to retrieve the module from the villains, but falls into a trap. Father captures Numbuh 1 and, using one of his boogers to activate the module, restores the memories of his father, Grandfather. Grandfather quickly usurps Father's position as leader of the KND villains, and begins to turn everybody into obedient Senior Citizombies, including the villains and adults. His control spreads across the globe, and transforms KND tree houses into tapioca-making factories quickly.
At the Moonbase, operatives are battling invading Senior Citizombies. Numbuh 1, angry at himself, stays at the convention center while Cree turns Numbuh 5 into an evil Senior Citizombie. Numbuhs 362, 86, 2, 3, and 4 are trying to figure out what to do when Numbuh 2 decides to remodel the birthday suits so that they protect the person who wears one from Senior Citizombification. However, Numbuh 2 needs a device back at Sector V's treehouse, so Numbuh 3 volunteers to go with Numbuh 4 to get it. The treehouse is quickly ambushed by Senior Citizombies, and Numbuh 3 and Numbuh 4 are turned into them. Numbuh 362 tells him Numbuh 2 have to finish without the device and Numbuh 86 guards him. He works in a closet and finishes four of them. Numbuhs 362, 86, and 2 are soon transformed, and Numbuh 1 remains the last un-citizombified operative.
In desperation, Numbuh One uses a DNA tracker (the KNDNA tracker, given to him by Numbuh 101) and the Recommissioning Module to track down Numbuh Zero in hopes of recommissioning him. When the signal leads him to his own home, Numbuh 1 realizes that his father is Numbuh Zero, and that the Book of KND has been in a secret chamber (the Treehouse of Coolness) under his house. After a chase with zombie hamsters, Numbuh 1 recommissions his father, and the restored Numbuh Zero takes him to Father's house, where it is revealed that Father is Numbuh Zero's brother, making Father Numbuh One's uncle and Grandfather his actual grandfather. It is also revealed, through the use of the Recommissioning Module, that the Delightful Children from Down the Lane are actually the long-lost KND Sector Z, who were kidnapped and permanently brainwashed in a "delightfulization" experiment by Father gone wrong. The module restores their memories, but will only have a temporary effect. Meanwhile, Grandfather is determined to personally destroy the Book of KND, which contains the only weapon capable of defeating him: hope. Numbuh 1 and Sector Z make their way to the Moonbase while Numbuh Zero and Father stays behind to face Grandfather with the Book of KND.
On the Moonbase, Numbuh 1 and Sector Z manage to defeat the zombified Sector V and activate the Moonbase's most powerful weapon. Unfortunately, Sector Z reverse back to their evil selves and attack Numbuh 1. Numbuh One manages to defeat the DCFDTL by sending them into deep space, but not before the targeting system is destroyed. Numbuh 1 switches to manual targeting, but it requires four other members to operate. Fortunately, Numbuh 2's suits are able to partially reverse Grandfather's aging effects, and Sector V uses the weapon to literally drop the entire Moonbase on top of Grandfather. Grandfather easily survives the impact, but is actually lured into the Decommissioning Chamber by Numbuh Zero, which results in the both of them getting their memories erased. With Grandfather defeated, the world turns back to normal. When Numbuh 1 attempts to Recommission Numbuh Zero again, he comes across a recorded message from Numbuh Zero telling him that he has destroyed the Recommissioning Module and that he would rather not have his memories restored so that he could complete his greatest mission of all: being a good father. Monty comes along and reassures a tearful Numbuh 1, who hugs his father.
One month later, the existence of Numbuh Zero is public knowledge and considered fact. The Moonbase has been completely rebuilt and is larger and improved from the previous one. It now contains a massive statue honoring Numbuh Zero. Numbuh 1, who now has possession of the Book of KND, writes his entry into the book: "We are Kids Next Door."